Implement checkpoint sync (#2244)

## Issue Addressed

Closes #1891
Closes #1784

## Proposed Changes

Implement checkpoint sync for Lighthouse, enabling it to start from a weak subjectivity checkpoint.

## Additional Info

- [x] Return unavailable status for out-of-range blocks requested by peers (#2561)
- [x] Implement sync daemon for fetching historical blocks (#2561)
- [x] Verify chain hashes (either in `historical_blocks.rs` or the calling module)
- [x] Consistency check for initial block + state
- [x] Fetch the initial state and block from a beacon node HTTP endpoint
- [x] Don't crash fetching beacon states by slot from the API
- [x] Background service for state reconstruction, triggered by CLI flag or API call.

Considered out of scope for this PR:

- Drop the requirement to provide the `--checkpoint-block` (this would require some pretty heavy refactoring of block verification)


Co-authored-by: Diva M <divma@protonmail.com>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -14,6 +14,7 @@ use crate::errors::{BeaconChainError as Error, BlockProductionError};
 use crate::eth1_chain::{Eth1Chain, Eth1ChainBackend};
 use crate::events::ServerSentEventHandler;
 use crate::head_tracker::HeadTracker;
+use crate::historical_blocks::HistoricalBlockError;
 use crate::migrate::BackgroundMigrator;
 use crate::naive_aggregation_pool::{
     AggregatedAttestationMap, Error as NaiveAggregationError, NaiveAggregationPool,
@@ -431,10 +432,23 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     /// - Skipped slots contain the root of the closest prior
     ///     non-skipped slot (identical to the way they are stored in `state.block_roots`).
     /// - Iterator returns `(Hash256, Slot)`.
+    ///
+    /// Will return a `BlockOutOfRange` error if the requested start slot is before the period of
+    /// history for which we have blocks stored. See `get_oldest_block_slot`.
     pub fn forwards_iter_block_roots(
         &self,
         start_slot: Slot,
     ) -> Result<impl Iterator<Item = Result<(Hash256, Slot), Error>>, Error> {
+        let oldest_block_slot = self.store.get_oldest_block_slot();
+        if start_slot < oldest_block_slot {
+            return Err(Error::HistoricalBlockError(
+                HistoricalBlockError::BlockOutOfRange {
+                    slot: start_slot,
+                    oldest_block_slot,
+                },
+            ));
+        }
+
         let local_head = self.head()?;
 
         let iter = HotColdDB::forwards_block_roots_iterator(
@@ -620,6 +634,12 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             return Ok(Some(self.genesis_state_root));
         }
 
+        // Check limits w.r.t historic state bounds.
+        let (historic_lower_limit, historic_upper_limit) = self.store.get_historic_state_limits();
+        if request_slot > historic_lower_limit && request_slot < historic_upper_limit {
+            return Ok(None);
+        }
+
         // Try an optimized path of reading the root directly from the head state.
         let fast_lookup: Option<Hash256> = self.with_head(|head| {
             if head.beacon_block.slot() <= request_slot {
@@ -657,7 +677,8 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     /// ## Notes
     ///
     /// - Use the `skips` parameter to define the behaviour when `request_slot` is a skipped slot.
-    /// - Returns `Ok(None)` for any slot higher than the current wall-clock slot.
+    /// - Returns `Ok(None)` for any slot higher than the current wall-clock slot, or less than
+    ///   the oldest known block slot.
     pub fn block_root_at_slot(
         &self,
         request_slot: Slot,
@@ -667,6 +688,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             WhenSlotSkipped::None => self.block_root_at_slot_skips_none(request_slot),
             WhenSlotSkipped::Prev => self.block_root_at_slot_skips_prev(request_slot),
         }
+        .or_else(|e| match e {
+            Error::HistoricalBlockError(_) => Ok(None),
+            e => Err(e),
+        })
     }
 
     /// Returns the block root at the given slot, if any. Only returns roots in the canonical chain.

beacon_node/beacon_chain/src/block_verification.rs

@@ -501,6 +501,9 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
 
         let block_root = get_block_root(&block);
 
+        // Disallow blocks that conflict with the anchor (weak subjectivity checkpoint), if any.
+        check_block_against_anchor_slot(block.message(), chain)?;
+
         // Do not gossip a block from a finalized slot.
         check_block_against_finalized_slot(block.message(), chain)?;
 
@@ -708,6 +711,9 @@ impl<T: BeaconChainTypes> SignatureVerifiedBlock<T> {
             .fork_name(&chain.spec)
             .map_err(BlockError::InconsistentFork)?;
 
+        // Check the anchor slot before loading the parent, to avoid spurious lookups.
+        check_block_against_anchor_slot(block.message(), chain)?;
+
         let (mut parent, block) = load_parent(block, chain)?;
 
         // Reject any block that exceeds our limit on skipped slots.
@@ -1115,6 +1121,19 @@ fn check_block_skip_slots<T: BeaconChainTypes>(
     Ok(())
 }
 
+/// Returns `Ok(())` if the block's slot is greater than the anchor block's slot (if any).
+fn check_block_against_anchor_slot<T: BeaconChainTypes>(
+    block: BeaconBlockRef<'_, T::EthSpec>,
+    chain: &BeaconChain<T>,
+) -> Result<(), BlockError<T::EthSpec>> {
+    if let Some(anchor_slot) = chain.store.get_anchor_slot() {
+        if block.slot() <= anchor_slot {
+            return Err(BlockError::WeakSubjectivityConflict);
+        }
+    }
+    Ok(())
+}
+
 /// Returns `Ok(())` if the block is later than the finalized slot on `chain`.
 ///
 /// Returns an error if the block is earlier or equal to the finalized slot, or there was an error

beacon_node/beacon_chain/src/builder.rs

@@ -28,8 +28,8 @@ use std::time::Duration;
 use store::{Error as StoreError, HotColdDB, ItemStore};
 use task_executor::ShutdownReason;
 use types::{
-    BeaconBlock, BeaconState, ChainSpec, EthSpec, Graffiti, Hash256, PublicKeyBytes, Signature,
-    SignedBeaconBlock, Slot,
+    BeaconBlock, BeaconState, ChainSpec, Checkpoint, EthSpec, Graffiti, Hash256, PublicKeyBytes,
+    Signature, SignedBeaconBlock, Slot,
 };
 
 /// An empty struct used to "witness" all the `BeaconChainTypes` traits. It has no user-facing
@@ -282,12 +282,19 @@ where
         Ok(self)
     }
 
-    /// Starts a new chain from a genesis state.
-    pub fn genesis_state(
+    /// Store the genesis state & block in the DB.
+    ///
+    /// Do *not* initialize fork choice, or do anything that assumes starting from genesis.
+    ///
+    /// Return the `BeaconSnapshot` representing genesis as well as the mutated builder.
+    fn set_genesis_state(
         mut self,
         mut beacon_state: BeaconState<TEthSpec>,
-    ) -> Result<Self, String> {
-        let store = self.store.clone().ok_or("genesis_state requires a store")?;
+    ) -> Result<(BeaconSnapshot<TEthSpec>, Self), String> {
+        let store = self
+            .store
+            .clone()
+            .ok_or("set_genesis_state requires a store")?;
 
         let beacon_block = genesis_block(&mut beacon_state, &self.spec)?;
 
@@ -298,9 +305,6 @@ where
         let beacon_state_root = beacon_block.message().state_root();
         let beacon_block_root = beacon_block.canonical_root();
 
-        self.genesis_state_root = Some(beacon_state_root);
-        self.genesis_block_root = Some(beacon_block_root);
-
         store
             .put_state(&beacon_state_root, &beacon_state)
             .map_err(|e| format!("Failed to store genesis state: {:?}", e))?;
@@ -318,11 +322,26 @@ where
                 )
             })?;
 
-        let genesis = BeaconSnapshot {
-            beacon_block,
-            beacon_block_root,
-            beacon_state,
-        };
+        self.genesis_state_root = Some(beacon_state_root);
+        self.genesis_block_root = Some(beacon_block_root);
+        self.genesis_time = Some(beacon_state.genesis_time());
+
+        Ok((
+            BeaconSnapshot {
+                beacon_block_root,
+                beacon_block,
+                beacon_state,
+            },
+            self,
+        ))
+    }
+
+    /// Starts a new chain from a genesis state.
+    pub fn genesis_state(mut self, beacon_state: BeaconState<TEthSpec>) -> Result<Self, String> {
+        let store = self.store.clone().ok_or("genesis_state requires a store")?;
+
+        let (genesis, updated_builder) = self.set_genesis_state(beacon_state)?;
+        self = updated_builder;
 
         let fc_store = BeaconForkChoiceStore::get_forkchoice_store(store, &genesis);
 
@@ -332,10 +351,115 @@ where
             &genesis.beacon_block,
             &genesis.beacon_state,
         )
-        .map_err(|e| format!("Unable to build initialize ForkChoice: {:?}", e))?;
+        .map_err(|e| format!("Unable to initialize ForkChoice: {:?}", e))?;
+
+        self.fork_choice = Some(fork_choice);
+
+        Ok(self.empty_op_pool())
+    }
+
+    /// Start the chain from a weak subjectivity state.
+    pub fn weak_subjectivity_state(
+        mut self,
+        mut weak_subj_state: BeaconState<TEthSpec>,
+        weak_subj_block: SignedBeaconBlock<TEthSpec>,
+        genesis_state: BeaconState<TEthSpec>,
+    ) -> Result<Self, String> {
+        let store = self.store.clone().ok_or("genesis_state requires a store")?;
+
+        let weak_subj_slot = weak_subj_state.slot();
+        let weak_subj_block_root = weak_subj_block.canonical_root();
+        let weak_subj_state_root = weak_subj_block.state_root();
+
+        // Check that the given block lies on an epoch boundary. Due to the database only storing
+        // full states on epoch boundaries and at restore points it would be difficult to support
+        // starting from a mid-epoch state.
+        if weak_subj_slot % TEthSpec::slots_per_epoch() != 0 {
+            return Err(format!(
+                "Checkpoint block at slot {} is not aligned to epoch start. \
+                 Please supply an aligned checkpoint with block.slot % 32 == 0",
+                weak_subj_block.slot(),
+            ));
+        }
+
+        // Check that the block and state have consistent slots and state roots.
+        if weak_subj_state.slot() != weak_subj_block.slot() {
+            return Err(format!(
+                "Slot of snapshot block ({}) does not match snapshot state ({})",
+                weak_subj_block.slot(),
+                weak_subj_state.slot(),
+            ));
+        }
+
+        let computed_state_root = weak_subj_state
+            .update_tree_hash_cache()
+            .map_err(|e| format!("Error computing checkpoint state root: {:?}", e))?;
+
+        if weak_subj_state_root != computed_state_root {
+            return Err(format!(
+                "Snapshot state root does not match block, expected: {:?}, got: {:?}",
+                weak_subj_state_root, computed_state_root
+            ));
+        }
+
+        // Check that the checkpoint state is for the same network as the genesis state.
+        // This check doesn't do much for security but should prevent mistakes.
+        if weak_subj_state.genesis_validators_root() != genesis_state.genesis_validators_root() {
+            return Err(format!(
+                "Snapshot state appears to be from the wrong network. Genesis validators root \
+                 is {:?} but should be {:?}",
+                weak_subj_state.genesis_validators_root(),
+                genesis_state.genesis_validators_root()
+            ));
+        }
+
+        // Set the store's split point *before* storing genesis so that genesis is stored
+        // immediately in the freezer DB.
+        store.set_split(weak_subj_slot, weak_subj_state_root);
+        store
+            .store_split()
+            .map_err(|e| format!("Error storing DB split point: {:?}", e))?;
+
+        let (_, updated_builder) = self.set_genesis_state(genesis_state)?;
+        self = updated_builder;
+
+        store
+            .put_state(&weak_subj_state_root, &weak_subj_state)
+            .map_err(|e| format!("Failed to store weak subjectivity state: {:?}", e))?;
+        store
+            .put_block(&weak_subj_block_root, weak_subj_block.clone())
+            .map_err(|e| format!("Failed to store weak subjectivity block: {:?}", e))?;
+
+        // Store anchor info (context for weak subj sync).
+        store
+            .init_anchor_info(weak_subj_block.message())
+            .map_err(|e| format!("Failed to initialize anchor info: {:?}", e))?;
+
+        // Store pruning checkpoint to prevent attempting to prune before the anchor state.
+        store
+            .store_pruning_checkpoint(Checkpoint {
+                root: weak_subj_block_root,
+                epoch: weak_subj_state.slot().epoch(TEthSpec::slots_per_epoch()),
+            })
+            .map_err(|e| format!("Failed to write pruning checkpoint: {:?}", e))?;
+
+        let snapshot = BeaconSnapshot {
+            beacon_block_root: weak_subj_block_root,
+            beacon_block: weak_subj_block,
+            beacon_state: weak_subj_state,
+        };
+
+        let fc_store = BeaconForkChoiceStore::get_forkchoice_store(store, &snapshot);
+
+        let fork_choice = ForkChoice::from_anchor(
+            fc_store,
+            snapshot.beacon_block_root,
+            &snapshot.beacon_block,
+            &snapshot.beacon_state,
+        )
+        .map_err(|e| format!("Unable to initialize ForkChoice: {:?}", e))?;
 
         self.fork_choice = Some(fork_choice);
-        self.genesis_time = Some(genesis.beacon_state.genesis_time());
 
         Ok(self.empty_op_pool())
     }
@@ -520,12 +644,13 @@ where
         let fc_finalized = fork_choice.finalized_checkpoint();
         let head_finalized = canonical_head.beacon_state.finalized_checkpoint();
         if fc_finalized != head_finalized {
-            if head_finalized.root == Hash256::zero()
+            let is_genesis = head_finalized.root.is_zero()
                 && head_finalized.epoch == fc_finalized.epoch
-                && fc_finalized.root == genesis_block_root
-            {
-                // This is a legal edge-case encountered during genesis.
-            } else {
+                && fc_finalized.root == genesis_block_root;
+            let is_wss = store.get_anchor_slot().map_or(false, |anchor_slot| {
+                fc_finalized.epoch == anchor_slot.epoch(TEthSpec::slots_per_epoch())
+            });
+            if !is_genesis && !is_wss {
                 return Err(format!(
                     "Database corrupt: fork choice is finalized at {:?} whilst head is finalized at \
                     {:?}",
@@ -654,6 +779,11 @@ where
             "head_slot" => format!("{}", head.beacon_block.slot()),
         );
 
+        // Check for states to reconstruct (in the background).
+        if beacon_chain.config.reconstruct_historic_states {
+            beacon_chain.store_migrator.process_reconstruction();
+        }
+
         Ok(beacon_chain)
     }
 }

beacon_node/beacon_chain/src/chain_config.rs

@@ -12,6 +12,8 @@ pub struct ChainConfig {
     ///
     /// If `None`, there is no weak subjectivity verification.
     pub weak_subjectivity_checkpoint: Option<Checkpoint>,
+    /// Determine whether to reconstruct historic states, usually after a checkpoint sync.
+    pub reconstruct_historic_states: bool,
 }
 
 impl Default for ChainConfig {
@@ -19,6 +21,7 @@ impl Default for ChainConfig {
         Self {
             import_max_skip_slots: None,
             weak_subjectivity_checkpoint: None,
+            reconstruct_historic_states: false,
         }
     }
 }

beacon_node/beacon_chain/src/errors.rs

@@ -2,6 +2,7 @@ use crate::attester_cache::Error as AttesterCacheError;
 use crate::beacon_chain::ForkChoiceError;
 use crate::beacon_fork_choice_store::Error as ForkChoiceStoreError;
 use crate::eth1_chain::Error as Eth1ChainError;
+use crate::historical_blocks::HistoricalBlockError;
 use crate::migrate::PruningError;
 use crate::naive_aggregation_pool::Error as NaiveAggregationError;
 use crate::observed_aggregates::Error as ObservedAttestationsError;
@@ -117,6 +118,7 @@ pub enum BeaconChainError {
         block_slot: Slot,
         state_slot: Slot,
     },
+    HistoricalBlockError(HistoricalBlockError),
     InvalidStateForShuffling {
         state_epoch: Epoch,
         shuffling_epoch: Epoch,
@@ -150,6 +152,7 @@ easy_from_to!(BlockSignatureVerifierError, BeaconChainError);
 easy_from_to!(PruningError, BeaconChainError);
 easy_from_to!(ArithError, BeaconChainError);
 easy_from_to!(ForkChoiceStoreError, BeaconChainError);
+easy_from_to!(HistoricalBlockError, BeaconChainError);
 easy_from_to!(StateAdvanceError, BeaconChainError);
 
 #[derive(Debug)]

beacon_node/beacon_chain/src/historical_blocks.rs

@@ -0,0 +1,195 @@
+use crate::{errors::BeaconChainError as Error, BeaconChain, BeaconChainTypes};
+use itertools::Itertools;
+use slog::debug;
+use state_processing::{
+    per_block_processing::ParallelSignatureSets,
+    signature_sets::{block_proposal_signature_set_from_parts, Error as SignatureSetError},
+};
+use std::borrow::Cow;
+use std::iter;
+use std::time::Duration;
+use store::{chunked_vector::BlockRoots, AnchorInfo, ChunkWriter, KeyValueStore};
+use types::{Hash256, SignedBeaconBlock, Slot};
+
+/// Use a longer timeout on the pubkey cache.
+///
+/// It's ok if historical sync is stalled due to writes from forwards block processing.
+const PUBKEY_CACHE_LOCK_TIMEOUT: Duration = Duration::from_secs(30);
+
+#[derive(Debug)]
+pub enum HistoricalBlockError {
+    /// Block is not available (only returned when fetching historic blocks).
+    BlockOutOfRange { slot: Slot, oldest_block_slot: Slot },
+    /// Block root mismatch, caller should retry with different blocks.
+    MismatchedBlockRoot {
+        block_root: Hash256,
+        expected_block_root: Hash256,
+    },
+    /// Bad signature, caller should retry with different blocks.
+    SignatureSet(SignatureSetError),
+    /// Bad signature, caller should retry with different blocks.
+    InvalidSignature,
+    /// Transitory error, caller should retry with the same blocks.
+    ValidatorPubkeyCacheTimeout,
+    /// No historical sync needed.
+    NoAnchorInfo,
+    /// Logic error: should never occur.
+    IndexOutOfBounds,
+}
+
+impl<T: BeaconChainTypes> BeaconChain<T> {
+    /// Store a batch of historical blocks in the database.
+    ///
+    /// The `blocks` should be given in slot-ascending order. One of the blocks should have a block
+    /// root corresponding to the `oldest_block_parent` from the store's `AnchorInfo`.
+    ///
+    /// The block roots and proposer signatures are verified. If any block doesn't match the parent
+    /// root listed in its successor, then the whole batch will be discarded and
+    /// `MismatchedBlockRoot` will be returned. If any proposer signature is invalid then
+    /// `SignatureSetError` or `InvalidSignature` will be returned.
+    ///
+    /// To align with sync we allow some excess blocks with slots greater than or equal to
+    /// `oldest_block_slot` to be provided. They will be ignored without being checked.
+    ///
+    /// This function should not be called concurrently with any other function that mutates
+    /// the anchor info (including this function itself). If a concurrent mutation occurs that
+    /// would violate consistency then an `AnchorInfoConcurrentMutation` error will be returned.
+    ///
+    /// Return the number of blocks successfully imported.
+    pub fn import_historical_block_batch(
+        &self,
+        blocks: &[SignedBeaconBlock<T::EthSpec>],
+    ) -> Result<usize, Error> {
+        let anchor_info = self
+            .store
+            .get_anchor_info()
+            .ok_or(HistoricalBlockError::NoAnchorInfo)?;
+
+        // Take all blocks with slots less than the oldest block slot.
+        let num_relevant =
+            blocks.partition_point(|block| block.slot() < anchor_info.oldest_block_slot);
+        let blocks_to_import = &blocks
+            .get(..num_relevant)
+            .ok_or(HistoricalBlockError::IndexOutOfBounds)?;
+
+        if blocks_to_import.len() != blocks.len() {
+            debug!(
+                self.log,
+                "Ignoring some historic blocks";
+                "oldest_block_slot" => anchor_info.oldest_block_slot,
+                "total_blocks" => blocks.len(),
+                "ignored" => blocks.len().saturating_sub(blocks_to_import.len()),
+            );
+        }
+
+        if blocks_to_import.is_empty() {
+            return Ok(0);
+        }
+
+        let mut expected_block_root = anchor_info.oldest_block_parent;
+        let mut prev_block_slot = anchor_info.oldest_block_slot;
+        let mut chunk_writer =
+            ChunkWriter::<BlockRoots, _, _>::new(&self.store.cold_db, prev_block_slot.as_usize())?;
+
+        let mut cold_batch = Vec::with_capacity(blocks.len());
+        let mut hot_batch = Vec::with_capacity(blocks.len());
+
+        for block in blocks_to_import.iter().rev() {
+            // Check chain integrity.
+            let block_root = block.canonical_root();
+
+            if block_root != expected_block_root {
+                return Err(HistoricalBlockError::MismatchedBlockRoot {
+                    block_root,
+                    expected_block_root,
+                }
+                .into());
+            }
+
+            // Store block in the hot database.
+            hot_batch.push(self.store.block_as_kv_store_op(&block_root, block));
+
+            // Store block roots, including at all skip slots in the freezer DB.
+            for slot in (block.slot().as_usize()..prev_block_slot.as_usize()).rev() {
+                chunk_writer.set(slot, block_root, &mut cold_batch)?;
+            }
+
+            prev_block_slot = block.slot();
+            expected_block_root = block.message().parent_root();
+
+            // If we've reached genesis, add the genesis block root to the batch and set the
+            // anchor slot to 0 to indicate completion.
+            if expected_block_root == self.genesis_block_root {
+                let genesis_slot = self.spec.genesis_slot;
+                chunk_writer.set(
+                    genesis_slot.as_usize(),
+                    self.genesis_block_root,
+                    &mut cold_batch,
+                )?;
+                prev_block_slot = genesis_slot;
+                expected_block_root = Hash256::zero();
+                break;
+            }
+        }
+        chunk_writer.write(&mut cold_batch)?;
+
+        // Verify signatures in one batch, holding the pubkey cache lock for the shortest duration
+        // possible. For each block fetch the parent root from its successor. Slicing from index 1
+        // is safe because we've already checked that `blocks_to_import` is non-empty.
+        let pubkey_cache = self
+            .validator_pubkey_cache
+            .try_read_for(PUBKEY_CACHE_LOCK_TIMEOUT)
+            .ok_or(HistoricalBlockError::ValidatorPubkeyCacheTimeout)?;
+        let block_roots = blocks_to_import
+            .get(1..)
+            .ok_or(HistoricalBlockError::IndexOutOfBounds)?
+            .iter()
+            .map(|block| block.parent_root())
+            .chain(iter::once(anchor_info.oldest_block_parent));
+        let signature_set = blocks_to_import
+            .iter()
+            .zip_eq(block_roots)
+            .map(|(block, block_root)| {
+                block_proposal_signature_set_from_parts(
+                    block,
+                    Some(block_root),
+                    block.message().proposer_index(),
+                    &self.spec.fork_at_epoch(block.message().epoch()),
+                    self.genesis_validators_root,
+                    |validator_index| pubkey_cache.get(validator_index).map(Cow::Borrowed),
+                    &self.spec,
+                )
+            })
+            .collect::<Result<Vec<_>, _>>()
+            .map_err(HistoricalBlockError::SignatureSet)
+            .map(ParallelSignatureSets::from)?;
+        if !signature_set.verify() {
+            return Err(HistoricalBlockError::InvalidSignature.into());
+        }
+        drop(pubkey_cache);
+
+        // Write the I/O batches to disk, writing the blocks themselves first, as it's better
+        // for the hot DB to contain extra blocks than for the cold DB to point to blocks that
+        // do not exist.
+        self.store.hot_db.do_atomically(hot_batch)?;
+        self.store.cold_db.do_atomically(cold_batch)?;
+
+        // Update the anchor.
+        let new_anchor = AnchorInfo {
+            oldest_block_slot: prev_block_slot,
+            oldest_block_parent: expected_block_root,
+            ..anchor_info
+        };
+        let backfill_complete = new_anchor.block_backfill_complete();
+        self.store
+            .compare_and_set_anchor_info(Some(anchor_info), Some(new_anchor))?;
+
+        // If backfill has completed and the chain is configured to reconstruct historic states,
+        // send a message to the background migrator instructing it to begin reconstruction.
+        if backfill_complete && self.config.reconstruct_historic_states {
+            self.store_migrator.process_reconstruction();
+        }
+
+        Ok(blocks_to_import.len())
+    }
+}

beacon_node/beacon_chain/src/lib.rs

@@ -13,6 +13,7 @@ pub mod eth1_chain;
 pub mod events;
 pub mod fork_revert;
 mod head_tracker;
+pub mod historical_blocks;
 mod metrics;
 pub mod migrate;
 mod naive_aggregation_pool;
@@ -39,6 +40,7 @@ pub use self::beacon_chain::{
 pub use self::beacon_snapshot::BeaconSnapshot;
 pub use self::chain_config::ChainConfig;
 pub use self::errors::{BeaconChainError, BlockProductionError};
+pub use self::historical_blocks::HistoricalBlockError;
 pub use attestation_verification::Error as AttestationError;
 pub use beacon_fork_choice_store::{BeaconForkChoiceStore, Error as ForkChoiceStoreError};
 pub use block_verification::{BlockError, GossipVerifiedBlock};

beacon_node/beacon_chain/src/migrate.rs

@@ -30,7 +30,7 @@ const COMPACTION_FINALITY_DISTANCE: u64 = 1024;
 pub struct BackgroundMigrator<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> {
     db: Arc<HotColdDB<E, Hot, Cold>>,
     #[allow(clippy::type_complexity)]
-    tx_thread: Option<Mutex<(mpsc::Sender<MigrationNotification>, thread::JoinHandle<()>)>>,
+    tx_thread: Option<Mutex<(mpsc::Sender<Notification>, thread::JoinHandle<()>)>>,
     /// Genesis block root, for persisting the `PersistedBeaconChain`.
     genesis_block_root: Hash256,
     log: Logger,
@@ -73,7 +73,12 @@ pub enum PruningError {
 }
 
 /// Message sent to the migration thread containing the information it needs to run.
-pub struct MigrationNotification {
+pub enum Notification {
+    Finalization(FinalizationNotification),
+    Reconstruction,
+}
+
+pub struct FinalizationNotification {
     finalized_state_root: BeaconStateHash,
     finalized_checkpoint: Checkpoint,
     head_tracker: Arc<HeadTracker>,
@@ -112,13 +117,46 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> BackgroundMigrator<E, Ho
         finalized_checkpoint: Checkpoint,
         head_tracker: Arc<HeadTracker>,
     ) -> Result<(), BeaconChainError> {
-        let notif = MigrationNotification {
+        let notif = FinalizationNotification {
             finalized_state_root,
             finalized_checkpoint,
             head_tracker,
             genesis_block_root: self.genesis_block_root,
         };
 
+        // Send to background thread if configured, otherwise run in foreground.
+        if let Some(Notification::Finalization(notif)) =
+            self.send_background_notification(Notification::Finalization(notif))
+        {
+            Self::run_migration(self.db.clone(), notif, &self.log);
+        }
+
+        Ok(())
+    }
+
+    pub fn process_reconstruction(&self) {
+        if let Some(Notification::Reconstruction) =
+            self.send_background_notification(Notification::Reconstruction)
+        {
+            Self::run_reconstruction(self.db.clone(), &self.log);
+        }
+    }
+
+    pub fn run_reconstruction(db: Arc<HotColdDB<E, Hot, Cold>>, log: &Logger) {
+        if let Err(e) = db.reconstruct_historic_states() {
+            error!(
+                log,
+                "State reconstruction failed";
+                "error" => ?e,
+            );
+        }
+    }
+
+    /// If configured to run in the background, send `notif` to the background thread.
+    ///
+    /// Return `None` if the message was sent to the background thread, `Some(notif)` otherwise.
+    #[must_use = "Message is not processed when this function returns `Some`"]
+    fn send_background_notification(&self, notif: Notification) -> Option<Notification> {
         // Async path, on the background thread.
         if let Some(tx_thread) = &self.tx_thread {
             let (ref mut tx, ref mut thread) = *tx_thread.lock();
@@ -143,17 +181,21 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> BackgroundMigrator<E, Ho
                 // Retry at most once, we could recurse but that would risk overflowing the stack.
                 let _ = tx.send(tx_err.0);
             }
-        }
+            None
         // Synchronous path, on the current thread.
-        else {
-            Self::run_migration(self.db.clone(), notif, &self.log)
+        } else {
+            Some(notif)
         }
-
-        Ok(())
     }
 
     /// Perform the actual work of `process_finalization`.
-    fn run_migration(db: Arc<HotColdDB<E, Hot, Cold>>, notif: MigrationNotification, log: &Logger) {
+    fn run_migration(
+        db: Arc<HotColdDB<E, Hot, Cold>>,
+        notif: FinalizationNotification,
+        log: &Logger,
+    ) {
+        debug!(log, "Database consolidation started");
+
         let finalized_state_root = notif.finalized_state_root;
 
         let finalized_state = match db.get_state(&finalized_state_root.into(), None) {
@@ -223,31 +265,44 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> BackgroundMigrator<E, Ho
         ) {
             warn!(log, "Database compaction failed"; "error" => format!("{:?}", e));
         }
+
+        debug!(log, "Database consolidation complete");
     }
 
     /// Spawn a new child thread to run the migration process.
     ///
-    /// Return a channel handle for sending new finalized states to the thread.
+    /// Return a channel handle for sending requests to the thread.
     fn spawn_thread(
         db: Arc<HotColdDB<E, Hot, Cold>>,
         log: Logger,
-    ) -> (mpsc::Sender<MigrationNotification>, thread::JoinHandle<()>) {
+    ) -> (mpsc::Sender<Notification>, thread::JoinHandle<()>) {
         let (tx, rx) = mpsc::channel();
         let thread = thread::spawn(move || {
             while let Ok(notif) = rx.recv() {
-                // Read the rest of the messages in the channel, ultimately choosing the `notif`
-                // with the highest finalized epoch.
-                let notif = rx
-                    .try_iter()
-                    .fold(notif, |best, other: MigrationNotification| {
-                        if other.finalized_checkpoint.epoch > best.finalized_checkpoint.epoch {
-                            other
-                        } else {
-                            best
-                        }
-                    });
+                // Read the rest of the messages in the channel, preferring any reconstruction
+                // notification, or the finalization notification with the greatest finalized epoch.
+                let notif =
+                    rx.try_iter()
+                        .fold(notif, |best, other: Notification| match (&best, &other) {
+                            (Notification::Reconstruction, _)
+                            | (_, Notification::Reconstruction) => Notification::Reconstruction,
+                            (
+                                Notification::Finalization(fin1),
+                                Notification::Finalization(fin2),
+                            ) => {
+                                if fin2.finalized_checkpoint.epoch > fin1.finalized_checkpoint.epoch
+                                {
+                                    other
+                                } else {
+                                    best
+                                }
+                            }
+                        });
 
-                Self::run_migration(db.clone(), notif, &log);
+                match notif {
+                    Notification::Reconstruction => Self::run_reconstruction(db.clone(), &log),
+                    Notification::Finalization(fin) => Self::run_migration(db.clone(), fin, &log),
+                }
             }
         });
         (tx, thread)

beacon_node/beacon_chain/src/schema_change.rs

@@ -2,12 +2,15 @@
 use crate::beacon_chain::{BeaconChainTypes, OP_POOL_DB_KEY};
 use crate::validator_pubkey_cache::ValidatorPubkeyCache;
 use operation_pool::{PersistedOperationPool, PersistedOperationPoolBase};
+use ssz::{Decode, Encode};
+use ssz_derive::{Decode, Encode};
 use std::fs;
 use std::path::Path;
 use std::sync::Arc;
+use store::config::OnDiskStoreConfig;
 use store::hot_cold_store::{HotColdDB, HotColdDBError};
-use store::metadata::{SchemaVersion, CURRENT_SCHEMA_VERSION};
-use store::Error as StoreError;
+use store::metadata::{SchemaVersion, CONFIG_KEY, CURRENT_SCHEMA_VERSION};
+use store::{DBColumn, Error as StoreError, ItemStore, StoreItem};
 
 const PUBKEY_CACHE_FILENAME: &str = "pubkey_cache.ssz";
 
@@ -73,6 +76,23 @@ pub fn migrate_schema<T: BeaconChainTypes>(
 
             Ok(())
         }
+        // Migration for weak subjectivity sync support and clean up of `OnDiskStoreConfig` (#1784).
+        (SchemaVersion(4), SchemaVersion(5)) => {
+            if let Some(OnDiskStoreConfigV4 {
+                slots_per_restore_point,
+                ..
+            }) = db.hot_db.get(&CONFIG_KEY)?
+            {
+                let new_config = OnDiskStoreConfig {
+                    slots_per_restore_point,
+                };
+                db.hot_db.put(&CONFIG_KEY, &new_config)?;
+            }
+
+            db.store_schema_version(to)?;
+
+            Ok(())
+        }
         // Anything else is an error.
         (_, _) => Err(HotColdDBError::UnsupportedSchemaVersion {
             target_version: to,
@@ -81,3 +101,24 @@ pub fn migrate_schema<T: BeaconChainTypes>(
         .into()),
     }
 }
+
+// Store config used in v4 schema and earlier.
+#[derive(Debug, Clone, PartialEq, Eq, Encode, Decode)]
+pub struct OnDiskStoreConfigV4 {
+    pub slots_per_restore_point: u64,
+    pub _block_cache_size: usize,
+}
+
+impl StoreItem for OnDiskStoreConfigV4 {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconMeta
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, StoreError> {
+        Ok(Self::from_ssz_bytes(bytes)?)
+    }
+}

beacon_node/beacon_chain/src/test_utils.rs

@@ -12,7 +12,7 @@ use crate::{
 };
 use bls::get_withdrawal_credentials;
 use futures::channel::mpsc::Receiver;
-use genesis::interop_genesis_state;
+pub use genesis::interop_genesis_state;
 use int_to_bytes::int_to_bytes32;
 use merkle_proof::MerkleTree;
 use parking_lot::Mutex;

beacon_node/beacon_chain/tests/store_tests.rs

@@ -1,11 +1,16 @@
 #![cfg(not(debug_assertions))]
 
 use beacon_chain::attestation_verification::Error as AttnError;
+use beacon_chain::builder::BeaconChainBuilder;
 use beacon_chain::test_utils::{
     test_logger, test_spec, AttestationStrategy, BeaconChainHarness, BlockStrategy,
     DiskHarnessType, HARNESS_SLOT_TIME,
 };
-use beacon_chain::{BeaconChain, BeaconChainTypes, BeaconSnapshot, ChainConfig};
+use beacon_chain::{
+    historical_blocks::HistoricalBlockError, migrate::MigratorConfig, BeaconChain,
+    BeaconChainError, BeaconChainTypes, BeaconSnapshot, ChainConfig, ServerSentEventHandler,
+    WhenSlotSkipped,
+};
 use lazy_static::lazy_static;
 use maplit::hashset;
 use rand::Rng;
@@ -558,7 +563,7 @@ fn multiple_attestations_per_block() {
     let harness = get_harness(store, HIGH_VALIDATOR_COUNT);
 
     harness.extend_chain(
-        MainnetEthSpec::slots_per_epoch() as usize * 3,
+        E::slots_per_epoch() as usize * 3,
         BlockStrategy::OnCanonicalHead,
         AttestationStrategy::AllValidators,
     );
@@ -1741,6 +1746,173 @@ fn garbage_collect_temp_states_from_failed_block() {
     assert_eq!(store.iter_temporary_state_roots().count(), 0);
 }
 
+#[test]
+fn weak_subjectivity_sync() {
+    // Build an initial chain on one harness, representing a synced node with full history.
+    let num_initial_blocks = E::slots_per_epoch() * 11;
+    let num_final_blocks = E::slots_per_epoch() * 2;
+
+    let temp1 = tempdir().unwrap();
+    let full_store = get_store(&temp1);
+    let harness = get_harness(full_store.clone(), LOW_VALIDATOR_COUNT);
+
+    harness.extend_chain(
+        num_initial_blocks as usize,
+        BlockStrategy::OnCanonicalHead,
+        AttestationStrategy::AllValidators,
+    );
+
+    let genesis_state = full_store
+        .get_state(&harness.chain.genesis_state_root, Some(Slot::new(0)))
+        .unwrap()
+        .unwrap();
+    let wss_checkpoint = harness.chain.head_info().unwrap().finalized_checkpoint;
+    let wss_block = harness.get_block(wss_checkpoint.root.into()).unwrap();
+    let wss_state = full_store
+        .get_state(&wss_block.state_root(), None)
+        .unwrap()
+        .unwrap();
+    let wss_slot = wss_block.slot();
+
+    // Add more blocks that advance finalization further.
+    harness.advance_slot();
+    harness.extend_chain(
+        num_final_blocks as usize,
+        BlockStrategy::OnCanonicalHead,
+        AttestationStrategy::AllValidators,
+    );
+
+    let (shutdown_tx, _shutdown_rx) = futures::channel::mpsc::channel(1);
+    let log = test_logger();
+    let temp2 = tempdir().unwrap();
+    let store = get_store(&temp2);
+
+    // Initialise a new beacon chain from the finalized checkpoint
+    let beacon_chain = BeaconChainBuilder::new(MinimalEthSpec)
+        .store(store.clone())
+        .custom_spec(test_spec::<E>())
+        .weak_subjectivity_state(wss_state, wss_block.clone(), genesis_state)
+        .unwrap()
+        .logger(log.clone())
+        .store_migrator_config(MigratorConfig::default().blocking())
+        .dummy_eth1_backend()
+        .expect("should build dummy backend")
+        .testing_slot_clock(HARNESS_SLOT_TIME)
+        .expect("should configure testing slot clock")
+        .shutdown_sender(shutdown_tx)
+        .chain_config(ChainConfig::default())
+        .event_handler(Some(ServerSentEventHandler::new_with_capacity(
+            log.clone(),
+            1,
+        )))
+        .monitor_validators(true, vec![], log)
+        .build()
+        .expect("should build");
+
+    // Apply blocks forward to reach head.
+    let chain_dump = harness.chain.chain_dump().unwrap();
+    let new_blocks = &chain_dump[wss_slot.as_usize() + 1..];
+
+    assert_eq!(new_blocks[0].beacon_block.slot(), wss_slot + 1);
+
+    for snapshot in new_blocks {
+        let block = &snapshot.beacon_block;
+        beacon_chain.slot_clock.set_slot(block.slot().as_u64());
+        beacon_chain.process_block(block.clone()).unwrap();
+        beacon_chain.fork_choice().unwrap();
+
+        // Check that the new block's state can be loaded correctly.
+        let state_root = block.state_root();
+        let mut state = beacon_chain
+            .store
+            .get_state(&state_root, Some(block.slot()))
+            .unwrap()
+            .unwrap();
+        assert_eq!(state.update_tree_hash_cache().unwrap(), state_root);
+    }
+
+    // Forwards iterator from 0 should fail as we lack blocks.
+    assert!(matches!(
+        beacon_chain.forwards_iter_block_roots(Slot::new(0)),
+        Err(BeaconChainError::HistoricalBlockError(
+            HistoricalBlockError::BlockOutOfRange { .. }
+        ))
+    ));
+
+    // Simulate processing of a `StatusMessage` with an older finalized epoch by calling
+    // `block_root_at_slot` with an old slot for which we don't know the block root. It should
+    // return `None` rather than erroring.
+    assert_eq!(
+        beacon_chain
+            .block_root_at_slot(Slot::new(1), WhenSlotSkipped::None)
+            .unwrap(),
+        None
+    );
+
+    // Simulate querying the API for a historic state that is unknown. It should also return
+    // `None` rather than erroring.
+    assert_eq!(beacon_chain.state_root_at_slot(Slot::new(1)).unwrap(), None);
+
+    // Supply blocks backwards to reach genesis. Omit the genesis block to check genesis handling.
+    let historical_blocks = chain_dump[..wss_block.slot().as_usize()]
+        .iter()
+        .filter(|s| s.beacon_block.slot() != 0)
+        .map(|s| s.beacon_block.clone())
+        .collect::<Vec<_>>();
+    beacon_chain
+        .import_historical_block_batch(&historical_blocks)
+        .unwrap();
+    assert_eq!(beacon_chain.store.get_oldest_block_slot(), 0);
+
+    // Resupplying the blocks should not fail, they can be safely ignored.
+    beacon_chain
+        .import_historical_block_batch(&historical_blocks)
+        .unwrap();
+
+    // The forwards iterator should now match the original chain
+    let forwards = beacon_chain
+        .forwards_iter_block_roots(Slot::new(0))
+        .unwrap()
+        .map(Result::unwrap)
+        .collect::<Vec<_>>();
+    let expected = harness
+        .chain
+        .forwards_iter_block_roots(Slot::new(0))
+        .unwrap()
+        .map(Result::unwrap)
+        .collect::<Vec<_>>();
+    assert_eq!(forwards, expected);
+
+    // All blocks can be loaded.
+    for (block_root, slot) in beacon_chain
+        .forwards_iter_block_roots(Slot::new(0))
+        .unwrap()
+        .map(Result::unwrap)
+    {
+        let block = store.get_block(&block_root).unwrap().unwrap();
+        assert_eq!(block.slot(), slot);
+    }
+
+    // All states from the oldest state slot can be loaded.
+    let (_, oldest_state_slot) = store.get_historic_state_limits();
+    for (state_root, slot) in beacon_chain
+        .forwards_iter_state_roots(oldest_state_slot)
+        .unwrap()
+        .map(Result::unwrap)
+    {
+        let state = store.get_state(&state_root, Some(slot)).unwrap().unwrap();
+        assert_eq!(state.slot(), slot);
+        assert_eq!(state.canonical_root(), state_root);
+    }
+
+    // Anchor slot is still set to the starting slot.
+    assert_eq!(store.get_anchor_slot(), Some(wss_slot));
+
+    // Reconstruct states.
+    store.clone().reconstruct_historic_states().unwrap();
+    assert_eq!(store.get_anchor_slot(), None);
+}
+
 #[test]
 fn finalizes_after_resuming_from_db() {
     let validator_count = 16;

beacon_node/client/Cargo.toml

@@ -32,6 +32,8 @@ futures = "0.3.7"
 reqwest = { version = "0.11.0", features = ["native-tls-vendored"] }
 url = "2.1.1"
 eth1 = { path = "../eth1" }
+eth2 = { path = "../../common/eth2" }
+sensitive_url = { path = "../../common/sensitive_url" }
 genesis = { path = "../genesis" }
 task_executor = { path = "../../common/task_executor" }
 environment = { path = "../../lighthouse/environment" }

beacon_node/client/src/builder.rs

@@ -12,6 +12,10 @@ use beacon_chain::{
 };
 use environment::RuntimeContext;
 use eth1::{Config as Eth1Config, Service as Eth1Service};
+use eth2::{
+    types::{BlockId, StateId},
+    BeaconNodeHttpClient, Error as ApiError, Timeouts,
+};
 use eth2_libp2p::NetworkGlobals;
 use genesis::{interop_genesis_state, Eth1GenesisService};
 use monitoring_api::{MonitoringHttpClient, ProcessType};
@@ -25,11 +29,16 @@ use std::sync::Arc;
 use std::time::Duration;
 use timer::spawn_timer;
 use tokio::sync::{mpsc::UnboundedSender, oneshot};
-use types::{test_utils::generate_deterministic_keypairs, BeaconState, ChainSpec, EthSpec};
+use types::{
+    test_utils::generate_deterministic_keypairs, BeaconState, ChainSpec, EthSpec, SignedBeaconBlock,
+};
 
 /// Interval between polling the eth1 node for genesis information.
 pub const ETH1_GENESIS_UPDATE_INTERVAL_MILLIS: u64 = 7_000;
 
+/// Timeout for checkpoint sync HTTP requests.
+pub const CHECKPOINT_SYNC_HTTP_TIMEOUT: Duration = Duration::from_secs(60);
+
 /// Builds a `Client` instance.
 ///
 /// ## Notes
@@ -168,11 +177,22 @@ where
         //
         // Alternatively, if there's a beacon chain in the database then always resume
         // using it.
-        let client_genesis = if client_genesis == ClientGenesis::FromStore && !chain_exists {
+        let client_genesis = if matches!(client_genesis, ClientGenesis::FromStore) && !chain_exists
+        {
             info!(context.log(), "Defaulting to deposit contract genesis");
 
             ClientGenesis::DepositContract
         } else if chain_exists {
+            if matches!(client_genesis, ClientGenesis::WeakSubjSszBytes { .. })
+                || matches!(client_genesis, ClientGenesis::CheckpointSyncUrl { .. })
+            {
+                info!(
+                    context.log(),
+                    "Refusing to checkpoint sync";
+                    "msg" => "database already exists, use --purge-db to force checkpoint sync"
+                );
+            }
+
             ClientGenesis::FromStore
         } else {
             client_genesis
@@ -200,6 +220,103 @@ where
 
                 builder.genesis_state(genesis_state).map(|v| (v, None))?
             }
+            ClientGenesis::WeakSubjSszBytes {
+                anchor_state_bytes,
+                anchor_block_bytes,
+                genesis_state_bytes,
+            } => {
+                info!(context.log(), "Starting checkpoint sync");
+
+                let anchor_state = BeaconState::from_ssz_bytes(&anchor_state_bytes, &spec)
+                    .map_err(|e| format!("Unable to parse weak subj state SSZ: {:?}", e))?;
+                let anchor_block = SignedBeaconBlock::from_ssz_bytes(&anchor_block_bytes, &spec)
+                    .map_err(|e| format!("Unable to parse weak subj block SSZ: {:?}", e))?;
+                let genesis_state = BeaconState::from_ssz_bytes(&genesis_state_bytes, &spec)
+                    .map_err(|e| format!("Unable to parse genesis state SSZ: {:?}", e))?;
+
+                builder
+                    .weak_subjectivity_state(anchor_state, anchor_block, genesis_state)
+                    .map(|v| (v, None))?
+            }
+            ClientGenesis::CheckpointSyncUrl {
+                genesis_state_bytes,
+                url,
+            } => {
+                info!(
+                    context.log(),
+                    "Starting checkpoint sync";
+                    "remote_url" => %url,
+                );
+
+                let remote =
+                    BeaconNodeHttpClient::new(url, Timeouts::set_all(CHECKPOINT_SYNC_HTTP_TIMEOUT));
+                let slots_per_epoch = TEthSpec::slots_per_epoch();
+
+                // Find a suitable finalized block on an epoch boundary.
+                let mut block = remote
+                    .get_beacon_blocks_ssz::<TEthSpec>(BlockId::Finalized, &spec)
+                    .await
+                    .map_err(|e| match e {
+                        ApiError::InvalidSsz(e) => format!(
+                            "Unable to parse SSZ: {:?}. Ensure the checkpoint-sync-url refers to a \
+                            node for the correct network",
+                            e
+                        ),
+                        e => format!("Error fetching finalized block from remote: {:?}", e),
+                    })?
+                    .ok_or("Finalized block missing from remote, it returned 404")?;
+
+                let mut block_slot = block.slot();
+
+                while block.slot() % slots_per_epoch != 0 {
+                    block_slot = (block_slot / slots_per_epoch - 1) * slots_per_epoch;
+
+                    debug!(
+                        context.log(),
+                        "Searching for aligned checkpoint block";
+                        "block_slot" => block_slot,
+                    );
+
+                    if let Some(found_block) = remote
+                        .get_beacon_blocks_ssz::<TEthSpec>(BlockId::Slot(block_slot), &spec)
+                        .await
+                        .map_err(|e| {
+                            format!("Error fetching block at slot {}: {:?}", block_slot, e)
+                        })?
+                    {
+                        block = found_block;
+                    }
+                }
+
+                let state_root = block.state_root();
+                let state = remote
+                    .get_debug_beacon_states_ssz::<TEthSpec>(StateId::Root(state_root), &spec)
+                    .await
+                    .map_err(|e| {
+                        format!(
+                            "Error loading checkpoint state from remote {:?}: {:?}",
+                            state_root, e
+                        )
+                    })?
+                    .ok_or_else(|| {
+                        format!("Checkpoint state missing from remote: {:?}", state_root)
+                    })?;
+
+                let genesis_state = BeaconState::from_ssz_bytes(&genesis_state_bytes, &spec)
+                    .map_err(|e| format!("Unable to parse genesis state SSZ: {:?}", e))?;
+
+                info!(
+                    context.log(),
+                    "Loaded checkpoint block and state";
+                    "slot" => block.slot(),
+                    "block_root" => ?block.canonical_root(),
+                    "state_root" => ?state_root,
+                );
+
+                builder
+                    .weak_subjectivity_state(state, block, genesis_state)
+                    .map(|v| (v, None))?
+            }
             ClientGenesis::DepositContract => {
                 info!(
                     context.log(),

beacon_node/client/src/config.rs

@@ -1,5 +1,6 @@
 use directory::DEFAULT_ROOT_DIR;
 use network::NetworkConfig;
+use sensitive_url::SensitiveUrl;
 use serde_derive::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
@@ -9,7 +10,7 @@ use types::{Graffiti, PublicKeyBytes};
 const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
 
 /// Defines how the client should initialize the `BeaconChain` and other components.
-#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum ClientGenesis {
     /// Creates a genesis state as per the 2019 Canada interop specifications.
     Interop {
@@ -26,6 +27,15 @@ pub enum ClientGenesis {
     /// We include the bytes instead of the `BeaconState<E>` because the `EthSpec` type
     /// parameter would be very annoying.
     SszBytes { genesis_state_bytes: Vec<u8> },
+    WeakSubjSszBytes {
+        genesis_state_bytes: Vec<u8>,
+        anchor_state_bytes: Vec<u8>,
+        anchor_block_bytes: Vec<u8>,
+    },
+    CheckpointSyncUrl {
+        genesis_state_bytes: Vec<u8>,
+        url: SensitiveUrl,
+    },
 }
 
 impl Default for ClientGenesis {

beacon_node/client/src/notifier.rs

@@ -1,6 +1,6 @@
 use crate::metrics;
 use beacon_chain::{BeaconChain, BeaconChainTypes};
-use eth2_libp2p::NetworkGlobals;
+use eth2_libp2p::{types::SyncState, NetworkGlobals};
 use parking_lot::Mutex;
 use slog::{debug, error, info, warn, Logger};
 use slot_clock::SlotClock;
@@ -19,6 +19,9 @@ const MINUTES_PER_HOUR: i64 = 60;
 /// The number of historical observations that should be used to determine the average sync time.
 const SPEEDO_OBSERVATIONS: usize = 4;
 
+/// The number of slots between logs that give detail about backfill process.
+const BACKFILL_LOG_INTERVAL: u64 = 5;
+
 /// Spawns a notifier service which periodically logs information about the node.
 pub fn spawn_notifier<T: BeaconChainTypes>(
     executor: task_executor::TaskExecutor,
@@ -42,6 +45,16 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
     let log = executor.log().clone();
     let mut interval = tokio::time::interval_at(start_instant, interval_duration);
 
+    // Keep track of sync state and reset the speedo on specific sync state changes.
+    // Specifically, if we switch between a sync and a backfill sync, reset the speedo.
+    let mut current_sync_state = network.sync_state();
+
+    // Store info if we are required to do a backfill sync.
+    let original_anchor_slot = beacon_chain
+        .store
+        .get_anchor_info()
+        .map(|ai| ai.oldest_block_slot);
+
     let interval_future = async move {
         // Perform pre-genesis logging.
         loop {
@@ -63,11 +76,30 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
         }
 
         // Perform post-genesis logging.
+        let mut last_backfill_log_slot = None;
         loop {
             interval.tick().await;
             let connected_peer_count = network.connected_peers();
             let sync_state = network.sync_state();
 
+            // Determine if we have switched syncing chains
+            if sync_state != current_sync_state {
+                match (current_sync_state, &sync_state) {
+                    (_, SyncState::BackFillSyncing { .. }) => {
+                        // We have transitioned to a backfill sync. Reset the speedo.
+                        let mut speedo = speedo.lock();
+                        speedo.clear();
+                    }
+                    (SyncState::BackFillSyncing { .. }, _) => {
+                        // We have transitioned from a backfill sync, reset the speedo
+                        let mut speedo = speedo.lock();
+                        speedo.clear();
+                    }
+                    (_, _) => {}
+                }
+                current_sync_state = sync_state;
+            }
+
             let head_info = match beacon_chain.head_info() {
                 Ok(head_info) => head_info,
                 Err(e) => {
@@ -97,17 +129,46 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
             let finalized_root = head_info.finalized_checkpoint.root;
             let head_root = head_info.block_root;
 
-            let mut speedo = speedo.lock();
-            speedo.observe(head_slot, Instant::now());
+            // The default is for regular sync but this gets modified if backfill sync is in
+            // progress.
+            let mut sync_distance = current_slot - head_slot;
 
+            let mut speedo = speedo.lock();
+            match current_sync_state {
+                SyncState::BackFillSyncing { .. } => {
+                    // Observe backfilling sync info.
+                    if let Some(oldest_slot) = original_anchor_slot {
+                        if let Some(current_anchor_slot) = beacon_chain
+                            .store
+                            .get_anchor_info()
+                            .map(|ai| ai.oldest_block_slot)
+                        {
+                            sync_distance = current_anchor_slot;
+                            speedo
+                                // For backfill sync use a fake slot which is the distance we've progressed from the starting `oldest_block_slot`.
+                                .observe(
+                                    oldest_slot.saturating_sub(current_anchor_slot),
+                                    Instant::now(),
+                                );
+                        }
+                    }
+                }
+                SyncState::SyncingFinalized { .. }
+                | SyncState::SyncingHead { .. }
+                | SyncState::SyncTransition => {
+                    speedo.observe(head_slot, Instant::now());
+                }
+                SyncState::Stalled | SyncState::Synced => {}
+            }
+
+            // NOTE: This is going to change based on which sync we are currently performing. A
+            // backfill sync should process slots significantly faster than the other sync
+            // processes.
             metrics::set_gauge(
                 &metrics::SYNC_SLOTS_PER_SECOND,
                 speedo.slots_per_second().unwrap_or(0_f64) as i64,
             );
 
-            // The next two lines take advantage of saturating subtraction on `Slot`.
-            let head_distance = current_slot - head_slot;
-
             if connected_peer_count <= WARN_PEER_COUNT {
                 warn!(log, "Low peer count"; "peer_count" => peer_count_pretty(connected_peer_count));
             }
@@ -121,16 +182,57 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
                 "head_block" => format!("{}", head_root),
                 "head_slot" => head_slot,
                 "current_slot" => current_slot,
-                "sync_state" =>format!("{}", sync_state)
+                "sync_state" =>format!("{}", current_sync_state)
             );
 
+            // Log if we are backfilling.
+            let is_backfilling = matches!(current_sync_state, SyncState::BackFillSyncing { .. });
+            if is_backfilling
+                && last_backfill_log_slot
+                    .map_or(true, |slot| slot + BACKFILL_LOG_INTERVAL <= current_slot)
+            {
+                last_backfill_log_slot = Some(current_slot);
+
+                let distance = format!(
+                    "{} slots ({})",
+                    sync_distance.as_u64(),
+                    slot_distance_pretty(sync_distance, slot_duration)
+                );
+
+                let speed = speedo.slots_per_second();
+                let display_speed = speed.map_or(false, |speed| speed != 0.0);
+
+                if display_speed {
+                    info!(
+                        log,
+                        "Downloading historical blocks";
+                        "distance" => distance,
+                        "speed" => sync_speed_pretty(speed),
+                        "est_time" => estimated_time_pretty(speedo.estimated_time_till_slot(original_anchor_slot.unwrap_or(current_slot))),
+                    );
+                } else {
+                    info!(
+                        log,
+                        "Downloading historical blocks";
+                        "distance" => distance,
+                        "est_time" => estimated_time_pretty(speedo.estimated_time_till_slot(original_anchor_slot.unwrap_or(current_slot))),
+                    );
+                }
+            } else if !is_backfilling && last_backfill_log_slot.is_some() {
+                last_backfill_log_slot = None;
+                info!(
+                    log,
+                    "Historical block download complete";
+                );
+            }
+
             // Log if we are syncing
-            if sync_state.is_syncing() {
+            if current_sync_state.is_syncing() {
                 metrics::set_gauge(&metrics::IS_SYNCED, 0);
                 let distance = format!(
                     "{} slots ({})",
-                    head_distance.as_u64(),
-                    slot_distance_pretty(head_distance, slot_duration)
+                    sync_distance.as_u64(),
+                    slot_distance_pretty(sync_distance, slot_duration)
                 );
 
                 let speed = speedo.slots_per_second();
@@ -154,7 +256,7 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
                         "est_time" => estimated_time_pretty(speedo.estimated_time_till_slot(current_slot)),
                     );
                 }
-            } else if sync_state.is_synced() {
+            } else if current_sync_state.is_synced() {
                 metrics::set_gauge(&metrics::IS_SYNCED, 1);
                 let block_info = if current_slot > head_slot {
                     "   …  empty".to_string()
@@ -397,4 +499,9 @@ impl Speedo {
             None
         }
     }
+
+    /// Clears all past observations to be used for an alternative sync (i.e backfill sync).
+    pub fn clear(&mut self) {
+        self.0.clear()
+    }
 }

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -522,7 +522,7 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
     }
 
     /// Inform the peer that their request produced an error.
-    pub fn _send_error_reponse(
+    pub fn send_error_reponse(
         &mut self,
         peer_id: PeerId,
         id: PeerRequestId,

beacon_node/eth2_libp2p/src/peer_manager/mod.rs

@@ -578,8 +578,17 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                 RPCResponseErrorCode::Unknown => PeerAction::HighToleranceError,
                 RPCResponseErrorCode::ResourceUnavailable => {
                     // NOTE: This error only makes sense for the `BlocksByRange` and `BlocksByRoot`
-                    // protocols. For the time being, there is no reason why a peer should send
-                    // this error.
+                    // protocols.
+                    //
+                    // If we are syncing, there is no point keeping these peers around and
+                    // continually failing to request blocks. We instantly ban them and hope that
+                    // by the time the ban lifts, the peers will have completed their backfill
+                    // sync.
+                    //
+                    // TODO: Potentially a more graceful way of handling such peers, would be to
+                    // implement a new sync type which tracks these peers and prevents the sync
+                    // algorithms from requesting blocks from them (at least for a set period of
+                    // time, multiple failures would then lead to a ban).
                     PeerAction::Fatal
                 }
                 RPCResponseErrorCode::ServerError => PeerAction::MidToleranceError,

beacon_node/eth2_libp2p/src/rpc/codec/base.rs

@@ -211,16 +211,13 @@ mod tests {
         let _ = snappy_buf.split_to(1);
 
         // decode message just as snappy message
-        let snappy_decoded_message = snappy_outbound_codec.decode(&mut snappy_buf).unwrap();
+        let _snappy_decoded_message = snappy_outbound_codec.decode(&mut snappy_buf).unwrap();
 
         // build codecs for entire chunk
         let mut snappy_base_outbound_codec = BaseOutboundCodec::new(snappy_outbound_codec);
 
         // decode message as ssz snappy chunk
-        let snappy_decoded_chunk = snappy_base_outbound_codec.decode(&mut buf).unwrap();
-
-        dbg!(snappy_decoded_message);
-        dbg!(snappy_decoded_chunk);
+        let _snappy_decoded_chunk = snappy_base_outbound_codec.decode(&mut buf).unwrap();
     }
 
     #[test]

beacon_node/eth2_libp2p/src/service.rs

@@ -275,7 +275,7 @@ impl<TSpec: EthSpec> Service<TSpec> {
     ) {
         self.swarm
             .behaviour_mut()
-            ._send_error_reponse(peer_id, id, error, reason);
+            .send_error_reponse(peer_id, id, error, reason);
     }
 
     /// Report a peer's action.

beacon_node/eth2_libp2p/src/types/globals.rs

@@ -1,7 +1,7 @@
 //! A collection of variables that are accessible outside of the network thread itself.
 use crate::peer_manager::PeerDB;
 use crate::rpc::MetaData;
-use crate::types::SyncState;
+use crate::types::{BackFillState, SyncState};
 use crate::Client;
 use crate::EnrExt;
 use crate::{Enr, GossipTopic, Multiaddr, PeerId};
@@ -29,6 +29,8 @@ pub struct NetworkGlobals<TSpec: EthSpec> {
     pub gossipsub_subscriptions: RwLock<HashSet<GossipTopic>>,
     /// The current sync status of the node.
     pub sync_state: RwLock<SyncState>,
+    /// The current state of the backfill sync.
+    pub backfill_state: RwLock<BackFillState>,
 }
 
 impl<TSpec: EthSpec> NetworkGlobals<TSpec> {
@@ -50,6 +52,7 @@ impl<TSpec: EthSpec> NetworkGlobals<TSpec> {
             peers: RwLock::new(PeerDB::new(trusted_peers, log)),
             gossipsub_subscriptions: RwLock::new(HashSet::new()),
             sync_state: RwLock::new(SyncState::Stalled),
+            backfill_state: RwLock::new(BackFillState::NotRequired),
         }
     }
 
@@ -104,6 +107,11 @@ impl<TSpec: EthSpec> NetworkGlobals<TSpec> {
         self.sync_state.read().clone()
     }
 
+    /// Returns the current backfill state.
+    pub fn backfill_state(&self) -> BackFillState {
+        self.backfill_state.read().clone()
+    }
+
     /// Returns a `Client` type if one is known for the `PeerId`.
     pub fn client(&self, peer_id: &PeerId) -> Client {
         self.peers

beacon_node/eth2_libp2p/src/types/mod.rs

@@ -15,5 +15,5 @@ pub type Enr = discv5::enr::Enr<discv5::enr::CombinedKey>;
 pub use globals::NetworkGlobals;
 pub use pubsub::{PubsubMessage, SnappyTransform};
 pub use subnet::{Subnet, SubnetDiscovery};
-pub use sync_state::SyncState;
+pub use sync_state::{BackFillState, SyncState};
 pub use topics::{subnet_from_topic_hash, GossipEncoding, GossipKind, GossipTopic, CORE_TOPICS};

beacon_node/eth2_libp2p/src/types/sync_state.rs

@@ -10,8 +10,13 @@ pub enum SyncState {
     /// The node is performing a long-range (batch) sync over one or many head chains.
     /// In this state parent lookups are disabled.
     SyncingHead { start_slot: Slot, target_slot: Slot },
-    /// The node has identified the need for is sync operations and is transitioning to a syncing
-    /// state.
+    /// The node is undertaking a backfill sync. This occurs when a user has specified a trusted
+    /// state. The node first syncs "forward" by downloading blocks up to the current head as
+    /// specified by its peers. Once completed, the node enters this sync state and attempts to
+    /// download all required historical blocks to complete its chain.
+    BackFillSyncing { completed: usize, remaining: usize },
+    /// The node has completed syncing a finalized chain and is in the process of re-evaluating
+    /// which sync state to progress to.
     SyncTransition,
     /// The node is up to date with all known peers and is connected to at least one
     /// fully synced peer. In this state, parent lookups are enabled.
@@ -21,6 +26,21 @@ pub enum SyncState {
     Stalled,
 }
 
+#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
+/// The state of the backfill sync.
+pub enum BackFillState {
+    /// The sync is partially completed and currently paused.
+    Paused,
+    /// We are currently backfilling.
+    Syncing,
+    /// A backfill sync has completed.
+    Completed,
+    /// A backfill sync is not required.
+    NotRequired,
+    /// Too many failed attempts at backfilling. Consider it failed.
+    Failed,
+}
+
 impl PartialEq for SyncState {
     fn eq(&self, other: &Self) -> bool {
         matches!(
@@ -32,6 +52,10 @@ impl PartialEq for SyncState {
                 | (SyncState::Synced, SyncState::Synced)
                 | (SyncState::Stalled, SyncState::Stalled)
                 | (SyncState::SyncTransition, SyncState::SyncTransition)
+                | (
+                    SyncState::BackFillSyncing { .. },
+                    SyncState::BackFillSyncing { .. }
+                )
         )
     }
 }
@@ -43,14 +67,18 @@ impl SyncState {
             SyncState::SyncingFinalized { .. } => true,
             SyncState::SyncingHead { .. } => true,
             SyncState::SyncTransition => true,
+            // Backfill doesn't effect any logic, we consider this state, not syncing.
+            SyncState::BackFillSyncing { .. } => false,
             SyncState::Synced => false,
             SyncState::Stalled => false,
         }
     }
 
     /// Returns true if the node is synced.
+    ///
+    /// NOTE: We consider the node synced if it is fetching old historical blocks.
     pub fn is_synced(&self) -> bool {
-        matches!(self, SyncState::Synced)
+        matches!(self, SyncState::Synced | SyncState::BackFillSyncing { .. })
     }
 }
 
@@ -61,7 +89,8 @@ impl std::fmt::Display for SyncState {
             SyncState::SyncingHead { .. } => write!(f, "Syncing Head Chain"),
             SyncState::Synced { .. } => write!(f, "Synced"),
             SyncState::Stalled { .. } => write!(f, "Stalled"),
-            SyncState::SyncTransition => write!(f, "Searching syncing peers"),
+            SyncState::SyncTransition => write!(f, "Evaluating known peers"),
+            SyncState::BackFillSyncing { .. } => write!(f, "Syncing Historical Blocks"),
         }
     }
 }

beacon_node/http_api/src/database.rs

@@ -0,0 +1,33 @@
+use beacon_chain::store::{metadata::CURRENT_SCHEMA_VERSION, AnchorInfo};
+use beacon_chain::{BeaconChain, BeaconChainTypes};
+use eth2::lighthouse::DatabaseInfo;
+use std::sync::Arc;
+use types::SignedBeaconBlock;
+
+pub fn info<T: BeaconChainTypes>(
+    chain: Arc<BeaconChain<T>>,
+) -> Result<DatabaseInfo, warp::Rejection> {
+    let store = &chain.store;
+    let split = store.get_split_info();
+    let anchor = store.get_anchor_info();
+
+    Ok(DatabaseInfo {
+        schema_version: CURRENT_SCHEMA_VERSION.as_u64(),
+        split,
+        anchor,
+    })
+}
+
+pub fn historical_blocks<T: BeaconChainTypes>(
+    chain: Arc<BeaconChain<T>>,
+    blocks: Vec<SignedBeaconBlock<T::EthSpec>>,
+) -> Result<AnchorInfo, warp::Rejection> {
+    chain
+        .import_historical_block_batch(&blocks)
+        .map_err(warp_utils::reject::beacon_chain_error)?;
+
+    let anchor = chain.store.get_anchor_info().ok_or_else(|| {
+        warp_utils::reject::custom_bad_request("node is not checkpoint synced".to_string())
+    })?;
+    Ok(anchor)
+}

beacon_node/http_api/src/lib.rs

@@ -7,6 +7,7 @@
 
 mod attester_duties;
 mod block_id;
+mod database;
 mod metrics;
 mod proposer_duties;
 mod state_id;
@@ -349,7 +350,7 @@ pub fn serve<T: BeaconChainTypes>(
                             )))
                         }
                     }
-                    SyncState::SyncingHead { .. } | SyncState::SyncTransition => Ok(()),
+                    SyncState::SyncingHead { .. } | SyncState::SyncTransition | SyncState::BackFillSyncing { .. }  => Ok(()),
                     SyncState::Synced => Ok(()),
                     SyncState::Stalled => Err(warp_utils::reject::not_synced(
                         "sync is stalled".to_string(),
@@ -1579,7 +1580,8 @@ pub fn serve<T: BeaconChainTypes>(
             blocking_task(move || match *network_globals.sync_state.read() {
                 SyncState::SyncingFinalized { .. }
                 | SyncState::SyncingHead { .. }
-                | SyncState::SyncTransition => Ok(warp::reply::with_status(
+                | SyncState::SyncTransition
+                | SyncState::BackFillSyncing { .. } => Ok(warp::reply::with_status(
                     warp::reply(),
                     warp::http::StatusCode::PARTIAL_CONTENT,
                 )),
@@ -2040,7 +2042,7 @@ pub fn serve<T: BeaconChainTypes>(
         .and(warp::path("validator"))
         .and(warp::path("contribution_and_proofs"))
         .and(warp::path::end())
-        .and(not_while_syncing_filter)
+        .and(not_while_syncing_filter.clone())
         .and(chain_filter.clone())
         .and(warp::body::json())
         .and(network_tx_filter.clone())
@@ -2399,6 +2401,49 @@ pub fn serve<T: BeaconChainTypes>(
             })
         });
 
+    let database_path = warp::path("lighthouse").and(warp::path("database"));
+
+    // GET lighthouse/database/info
+    let get_lighthouse_database_info = database_path
+        .and(warp::path("info"))
+        .and(warp::path::end())
+        .and(chain_filter.clone())
+        .and_then(|chain: Arc<BeaconChain<T>>| blocking_json_task(move || database::info(chain)));
+
+    // POST lighthouse/database/reconstruct
+    let post_lighthouse_database_reconstruct = database_path
+        .and(warp::path("reconstruct"))
+        .and(warp::path::end())
+        .and(not_while_syncing_filter)
+        .and(chain_filter.clone())
+        .and_then(|chain: Arc<BeaconChain<T>>| {
+            blocking_json_task(move || {
+                chain.store_migrator.process_reconstruction();
+                Ok("success")
+            })
+        });
+
+    // POST lighthouse/database/historical_blocks
+    let post_lighthouse_database_historical_blocks = database_path
+        .and(warp::path("historical_blocks"))
+        .and(warp::path::end())
+        .and(warp::body::json())
+        .and(chain_filter.clone())
+        .and(log_filter.clone())
+        .and_then(
+            |blocks: Vec<SignedBeaconBlock<T::EthSpec>>,
+             chain: Arc<BeaconChain<T>>,
+             log: Logger| {
+                info!(
+                    log,
+                    "Importing historical blocks";
+                    "count" => blocks.len(),
+                    "source" => "http_api"
+                );
+                blocking_json_task(move || database::historical_blocks(chain, blocks))
+            },
+        );
+
     let get_events = eth1_v1
         .and(warp::path("events"))
         .and(warp::path::end())
@@ -2510,6 +2555,7 @@ pub fn serve<T: BeaconChainTypes>(
                 .or(get_lighthouse_eth1_deposit_cache.boxed())
                 .or(get_lighthouse_beacon_states_ssz.boxed())
                 .or(get_lighthouse_staking.boxed())
+                .or(get_lighthouse_database_info.boxed())
                 .or(get_events.boxed()),
         )
         .or(warp::post().and(
@@ -2526,7 +2572,9 @@ pub fn serve<T: BeaconChainTypes>(
                 .or(post_validator_contribution_and_proofs.boxed())
                 .or(post_validator_beacon_committee_subscriptions.boxed())
                 .or(post_validator_sync_committee_subscriptions.boxed())
-                .or(post_lighthouse_liveness.boxed()),
+                .or(post_lighthouse_liveness.boxed())
+                .or(post_lighthouse_database_reconstruct.boxed())
+                .or(post_lighthouse_database_historical_blocks.boxed()),
         ))
         .recover(warp_utils::reject::handle_rejection)
         .with(slog_logging(log.clone()))

beacon_node/http_api/tests/tests.rs

@@ -2101,6 +2101,29 @@ impl ApiTester {
         self
     }
 
+    pub async fn test_get_lighthouse_database_info(self) -> Self {
+        let info = self.client.get_lighthouse_database_info().await.unwrap();
+
+        assert_eq!(info.anchor, self.chain.store.get_anchor_info());
+        assert_eq!(info.split, self.chain.store.get_split_info());
+        assert_eq!(
+            info.schema_version,
+            store::metadata::CURRENT_SCHEMA_VERSION.as_u64()
+        );
+
+        self
+    }
+
+    pub async fn test_post_lighthouse_database_reconstruct(self) -> Self {
+        let response = self
+            .client
+            .post_lighthouse_database_reconstruct()
+            .await
+            .unwrap();
+        assert_eq!(response, "success");
+        self
+    }
+
     pub async fn test_post_lighthouse_liveness(self) -> Self {
         let epoch = self.chain.epoch().unwrap();
         let head_state = self.chain.head_beacon_state().unwrap();
@@ -2653,6 +2676,10 @@ async fn lighthouse_endpoints() {
         .await
         .test_get_lighthouse_staking()
         .await
+        .test_get_lighthouse_database_info()
+        .await
+        .test_post_lighthouse_database_reconstruct()
+        .await
         .test_post_lighthouse_liveness()
         .await;
 }

beacon_node/network/src/beacon_processor/worker/rpc_methods.rs

@@ -2,7 +2,7 @@ use crate::beacon_processor::worker::FUTURE_SLOT_TOLERANCE;
 use crate::service::NetworkMessage;
 use crate::status::ToStatusMessage;
 use crate::sync::SyncMessage;
-use beacon_chain::{BeaconChainError, BeaconChainTypes, WhenSlotSkipped};
+use beacon_chain::{BeaconChainError, BeaconChainTypes, HistoricalBlockError, WhenSlotSkipped};
 use eth2_libp2p::rpc::StatusMessage;
 use eth2_libp2p::rpc::*;
 use eth2_libp2p::{PeerId, PeerRequestId, ReportSource, Response, SyncInfo};
@@ -38,6 +38,21 @@ impl<T: BeaconChainTypes> Worker<T> {
         })
     }
 
+    pub fn send_error_response(
+        &self,
+        peer_id: PeerId,
+        error: RPCResponseErrorCode,
+        reason: String,
+        id: PeerRequestId,
+    ) {
+        self.send_network_message(NetworkMessage::SendErrorResponse {
+            peer_id,
+            error,
+            reason,
+            id,
+        })
+    }
+
     /* Processing functions */
 
     /// Process a `Status` message to determine if a peer is relevant to us. If the peer is
@@ -163,6 +178,20 @@ impl<T: BeaconChainTypes> Worker<T> {
             .forwards_iter_block_roots(Slot::from(req.start_slot))
         {
             Ok(iter) => iter,
+            Err(BeaconChainError::HistoricalBlockError(
+                HistoricalBlockError::BlockOutOfRange {
+                    slot,
+                    oldest_block_slot,
+                },
+            )) => {
+                debug!(self.log, "Range request failed during backfill"; "requested_slot" => slot, "oldest_known_slot" => oldest_block_slot);
+                return self.send_error_response(
+                    peer_id,
+                    RPCResponseErrorCode::ResourceUnavailable,
+                    "Backfilling".into(),
+                    request_id,
+                );
+            }
             Err(e) => return error!(self.log, "Unable to obtain root iter"; "error" => ?e),
         };
 

beacon_node/network/src/beacon_processor/worker/sync_methods.rs

@@ -2,9 +2,11 @@ use super::{super::work_reprocessing_queue::ReprocessQueueMessage, Worker};
 use crate::beacon_processor::worker::FUTURE_SLOT_TOLERANCE;
 use crate::beacon_processor::BlockResultSender;
 use crate::metrics;
-use crate::sync::manager::SyncMessage;
+use crate::sync::manager::{SyncMessage, SyncRequestType};
 use crate::sync::{BatchProcessResult, ChainId};
-use beacon_chain::{BeaconChainTypes, BlockError, ChainSegmentResult};
+use beacon_chain::{
+    BeaconChainError, BeaconChainTypes, BlockError, ChainSegmentResult, HistoricalBlockError,
+};
 use eth2_libp2p::PeerId;
 use slog::{crit, debug, error, info, trace, warn};
 use tokio::sync::mpsc;
@@ -15,6 +17,8 @@ use types::{Epoch, Hash256, SignedBeaconBlock};
 pub enum ProcessId {
     /// Processing Id of a range syncing batch.
     RangeBatchId(ChainId, Epoch),
+    /// Processing ID for a backfill syncing batch.
+    BackSyncBatchId(Epoch),
     /// Processing Id of the parent lookup of a block.
     ParentLookup(PeerId, Hash256),
 }
@@ -99,11 +103,40 @@ impl<T: BeaconChainTypes> Worker<T> {
                     }
                 };
 
-                self.send_sync_message(SyncMessage::BatchProcessed {
-                    chain_id,
-                    epoch,
-                    result,
-                });
+                let sync_type = SyncRequestType::RangeSync(epoch, chain_id);
+
+                self.send_sync_message(SyncMessage::BatchProcessed { sync_type, result });
+            }
+            // this a request from the Backfill sync
+            ProcessId::BackSyncBatchId(epoch) => {
+                let start_slot = downloaded_blocks.first().map(|b| b.slot().as_u64());
+                let end_slot = downloaded_blocks.last().map(|b| b.slot().as_u64());
+                let sent_blocks = downloaded_blocks.len();
+
+                let result = match self.process_backfill_blocks(&downloaded_blocks) {
+                    (_, Ok(_)) => {
+                        debug!(self.log, "Backfill batch processed";
+                            "batch_epoch" => epoch,
+                            "first_block_slot" => start_slot,
+                            "last_block_slot" => end_slot,
+                            "processed_blocks" => sent_blocks,
+                            "service"=> "sync");
+                        BatchProcessResult::Success(sent_blocks > 0)
+                    }
+                    (_, Err(e)) => {
+                        debug!(self.log, "Backfill batch processing failed";
+                            "batch_epoch" => epoch,
+                            "first_block_slot" => start_slot,
+                            "last_block_slot" => end_slot,
+                            "error" => e,
+                            "service" => "sync");
+                        BatchProcessResult::Failed(false)
+                    }
+                };
+
+                let sync_type = SyncRequestType::BackFillSync(epoch);
+
+                self.send_sync_message(SyncMessage::BatchProcessed { sync_type, result });
             }
             // this is a parent lookup request from the sync manager
             ProcessId::ParentLookup(peer_id, chain_head) => {
@@ -160,6 +193,80 @@ impl<T: BeaconChainTypes> Worker<T> {
         }
     }
 
+    /// Helper function to process backfill block batches which only consumes the chain and blocks to process.
+    fn process_backfill_blocks(
+        &self,
+        blocks: &[SignedBeaconBlock<T::EthSpec>],
+    ) -> (usize, Result<(), String>) {
+        match self.chain.import_historical_block_batch(blocks) {
+            Ok(imported_blocks) => {
+                metrics::inc_counter(
+                    &metrics::BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_SUCCESS_TOTAL,
+                );
+
+                (imported_blocks, Ok(()))
+            }
+            Err(error) => {
+                metrics::inc_counter(
+                    &metrics::BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_FAILED_TOTAL,
+                );
+                let err = match error {
+                    // Handle the historical block errors specifically
+                    BeaconChainError::HistoricalBlockError(e) => match e {
+                        HistoricalBlockError::MismatchedBlockRoot {
+                            block_root,
+                            expected_block_root,
+                        } => {
+                            debug!(
+                                self.log,
+                                "Backfill batch processing error";
+                                "error" => "mismatched_block_root",
+                                "block_root" => ?block_root,
+                                "expected_root" => ?expected_block_root
+                            );
+                            String::from("mismatched_block_root")
+                        }
+                        HistoricalBlockError::InvalidSignature
+                        | HistoricalBlockError::SignatureSet(_) => {
+                            warn!(
+                                self.log,
+                                "Backfill batch processing error";
+                                "error" => ?e
+                            );
+                            "invalid_signature".into()
+                        }
+                        HistoricalBlockError::ValidatorPubkeyCacheTimeout => {
+                            warn!(
+                                self.log,
+                                "Backfill batch processing error";
+                                "error" => "pubkey_cache_timeout"
+                            );
+                            "pubkey_cache_timeout".into()
+                        }
+                        HistoricalBlockError::NoAnchorInfo => {
+                            warn!(self.log, "Backfill not required");
+                            String::from("no_anchor_info")
+                        }
+                        HistoricalBlockError::IndexOutOfBounds
+                        | HistoricalBlockError::BlockOutOfRange { .. } => {
+                            error!(
+                                self.log,
+                                "Backfill batch processing error";
+                                "error" => ?e,
+                            );
+                            String::from("logic_error")
+                        }
+                    },
+                    other => {
+                        warn!(self.log, "Backfill batch processing error"; "error" => ?other);
+                        format!("{:?}", other)
+                    }
+                };
+                (0, Err(err))
+            }
+        }
+    }
+
     /// Runs fork-choice on a given chain. This is used during block processing after one successful
     /// block import.
     fn run_fork_choice(&self) {

beacon_node/network/src/metrics.rs

@@ -338,10 +338,18 @@ lazy_static! {
         "beacon_processor_chain_segment_success_total",
         "Total number of chain segments successfully processed."
     );
+    pub static ref BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_SUCCESS_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_backfill_chain_segment_success_total",
+        "Total number of chain segments successfully processed."
+    );
     pub static ref BEACON_PROCESSOR_CHAIN_SEGMENT_FAILED_TOTAL: Result<IntCounter> = try_create_int_counter(
         "beacon_processor_chain_segment_failed_total",
         "Total number of chain segments that failed processing."
     );
+    pub static ref BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_FAILED_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_backfill_chain_segment_failed_total",
+        "Total number of backfill chain segments that failed processing."
+    );
     // Unaggregated attestations.
     pub static ref BEACON_PROCESSOR_UNAGGREGATED_ATTESTATION_QUEUE_TOTAL: Result<IntGauge> = try_create_int_gauge(
         "beacon_processor_unaggregated_attestation_queue_total",

beacon_node/network/src/router/processor.rs

@@ -418,7 +418,7 @@ impl<T: EthSpec> HandlerNetworkContext<T> {
         error: RPCResponseErrorCode,
         reason: String,
     ) {
-        self.inform_network(NetworkMessage::SendError {
+        self.inform_network(NetworkMessage::SendErrorResponse {
             peer_id,
             error,
             id,

beacon_node/network/src/service.rs

@@ -63,10 +63,8 @@ pub enum NetworkMessage<T: EthSpec> {
         response: Response<T>,
         id: PeerRequestId,
     },
-    /// Respond to a peer's request with an error.
-    SendError {
-        // NOTE: Currently this is never used, we just say goodbye without nicely closing the
-        // stream assigned to the request
+    /// Sends an error response to an RPC request.
+    SendErrorResponse {
         peer_id: PeerId,
         error: RPCResponseErrorCode,
         reason: String,
@@ -386,7 +384,7 @@ fn spawn_service<T: BeaconChainTypes>(
                         NetworkMessage::SendResponse{ peer_id, response, id } => {
                             service.libp2p.send_response(peer_id, id, response);
                         }
-                        NetworkMessage::SendError{ peer_id, error, id, reason } => {
+                        NetworkMessage::SendErrorResponse{ peer_id, error, id, reason } => {
                             service.libp2p.respond_with_error(peer_id, id, error, reason);
                         }
                         NetworkMessage::UPnPMappingEstablished { tcp_socket, udp_socket} => {

beacon_node/network/src/sync/manager.rs

@@ -33,6 +33,7 @@
 //! needs to be searched for (i.e if an attestation references an unknown block) this manager can
 //! search for the block and subsequently search for parents if needed.
 
+use super::backfill_sync::{BackFillSync, ProcessResult, SyncStart};
 use super::network_context::SyncNetworkContext;
 use super::peer_sync_info::{remote_sync_type, PeerSyncType};
 use super::range_sync::{ChainId, RangeSync, RangeSyncType, EPOCHS_PER_BATCH};
@@ -77,14 +78,14 @@ pub enum SyncMessage<T: EthSpec> {
     /// A useful peer has been discovered.
     AddPeer(PeerId, SyncInfo),
 
-    /// A `BlocksByRange` response has been received.
+    /// A [`BlocksByRange`] response has been received.
     BlocksByRangeResponse {
         peer_id: PeerId,
         request_id: RequestId,
         beacon_block: Option<Box<SignedBeaconBlock<T>>>,
     },
 
-    /// A `BlocksByRoot` response has been received.
+    /// A [`BlocksByRoot`] response has been received.
     BlocksByRootResponse {
         peer_id: PeerId,
         request_id: RequestId,
@@ -106,8 +107,7 @@ pub enum SyncMessage<T: EthSpec> {
 
     /// A batch has been processed by the block processor thread.
     BatchProcessed {
-        chain_id: ChainId,
-        epoch: Epoch,
+        sync_type: SyncRequestType,
         result: BatchProcessResult,
     },
 
@@ -120,6 +120,15 @@ pub enum SyncMessage<T: EthSpec> {
     },
 }
 
+/// The type of sync request made
+#[derive(Debug, Clone)]
+pub enum SyncRequestType {
+    /// Request was from the backfill sync algorithm.
+    BackFillSync(Epoch),
+    /// The request was from a chain in the range sync algorithm.
+    RangeSync(Epoch, ChainId),
+}
+
 /// The result of processing a multiple blocks (a chain segment).
 #[derive(Debug)]
 pub enum BatchProcessResult {
@@ -166,6 +175,9 @@ pub struct SyncManager<T: BeaconChainTypes> {
     /// The object handling long-range batch load-balanced syncing.
     range_sync: RangeSync<T>,
 
+    /// Backfill syncing.
+    backfill_sync: BackFillSync<T>,
+
     /// A collection of parent block lookups.
     parent_queue: SmallVec<[ParentRequests<T::EthSpec>; 3]>,
 
@@ -227,6 +239,12 @@ pub fn spawn<T: BeaconChainTypes>(
             beacon_processor_send.clone(),
             log.clone(),
         ),
+        backfill_sync: BackFillSync::new(
+            beacon_chain.clone(),
+            network_globals.clone(),
+            beacon_processor_send.clone(),
+            log.clone(),
+        ),
         network: SyncNetworkContext::new(network_send, network_globals.clone(), log.clone()),
         chain: beacon_chain,
         network_globals,
@@ -576,6 +594,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
         }
     }
 
+    /// Handles RPC errors related to requests that were emitted from the sync manager.
     fn inject_error(&mut self, peer_id: PeerId, request_id: RequestId) {
         trace!(self.log, "Sync manager received a failed RPC");
         // remove any single block lookups
@@ -597,14 +616,16 @@ impl<T: BeaconChainTypes> SyncManager<T> {
             return;
         }
 
-        // otherwise, this is a range sync issue, notify the range sync
-        self.range_sync
-            .inject_error(&mut self.network, peer_id, request_id);
-        self.update_sync_state();
+        // Otherwise this error matches no known request.
+        trace!(self.log, "Response/Error for non registered request"; "request_id" => request_id)
     }
 
     fn peer_disconnect(&mut self, peer_id: &PeerId) {
         self.range_sync.peer_disconnect(&mut self.network, peer_id);
+        // Regardless of the outcome, we update the sync status.
+        let _ = self
+            .backfill_sync
+            .peer_disconnected(peer_id, &mut self.network);
         self.update_sync_state();
     }
 
@@ -624,12 +645,18 @@ impl<T: BeaconChainTypes> SyncManager<T> {
         if let Some(peer_info) = self.network_globals.peers.write().peer_info_mut(peer_id) {
             let new_state = sync_type.as_sync_status(remote_sync_info);
             let rpr = new_state.as_str();
-            let was_updated = peer_info.sync_status.update(new_state);
+            let was_updated = peer_info.sync_status.update(new_state.clone());
             if was_updated {
                 debug!(self.log, "Peer transitioned sync state"; "peer_id" => %peer_id, "new_state" => rpr,
                     "our_head_slot" => local_sync_info.head_slot, "out_finalized_epoch" => local_sync_info.finalized_epoch,
                     "their_head_slot" => remote_sync_info.head_slot, "their_finalized_epoch" => remote_sync_info.finalized_epoch,
                     "is_connected" => peer_info.is_connected());
+
+                // A peer has transitioned its sync state. If the new state is "synced" we
+                // inform the backfill sync that a new synced peer has joined us.
+                if new_state.is_synced() {
+                    self.backfill_sync.fully_synced_peer_joined();
+                }
             }
             peer_info.is_connected()
         } else {
@@ -638,7 +665,17 @@ impl<T: BeaconChainTypes> SyncManager<T> {
         }
     }
 
-    /// Updates the global sync state and logs any changes.
+    /// Updates the global sync state, optionally instigating or pausing a backfill sync as well as
+    /// logging any changes.
+    ///
+    /// The logic for which sync should be running is as follows:
+    /// - If there is a range-sync running (or required) pause any backfill and let range-sync
+    /// complete.
+    /// - If there is no current range sync, check for any requirement to backfill and either
+    /// start/resume a backfill sync if required. The global state will be BackFillSync if a
+    /// backfill sync is running.
+    /// - If there is no range sync and no required backfill and we have synced up to the currently
+    /// known peers, we consider ourselves synced.
     fn update_sync_state(&mut self) {
         let new_state: SyncState = match self.range_sync.state() {
             Err(e) => {
@@ -647,41 +684,75 @@ impl<T: BeaconChainTypes> SyncManager<T> {
             }
             Ok(state) => match state {
                 None => {
-                    // no range sync, decide if we are stalled or synced.
+                    // No range sync, so we decide if we are stalled or synced.
                     // For this we check if there is at least one advanced peer. An advanced peer
                     // with Idle range is possible since a peer's status is updated periodically.
                     // If we synced a peer between status messages, most likely the peer has
                     // advanced and will produce a head chain on re-status. Otherwise it will shift
                     // to being synced
-                    let head = self.chain.best_slot().unwrap_or_else(|_| Slot::new(0));
-                    let current_slot = self.chain.slot().unwrap_or_else(|_| Slot::new(0));
+                    let mut sync_state = {
+                        let head = self.chain.best_slot().unwrap_or_else(|_| Slot::new(0));
+                        let current_slot = self.chain.slot().unwrap_or_else(|_| Slot::new(0));
 
-                    let peers = self.network_globals.peers.read();
-                    if current_slot >= head
-                        && current_slot.sub(head) <= (SLOT_IMPORT_TOLERANCE as u64)
-                        && head > 0
-                    {
-                        SyncState::Synced
-                    } else if peers.advanced_peers().next().is_some() {
-                        SyncState::SyncTransition
-                    } else if peers.synced_peers().next().is_none() {
-                        SyncState::Stalled
-                    } else {
-                        // There are no peers that require syncing and we have at least one synced
-                        // peer
-                        SyncState::Synced
+                        let peers = self.network_globals.peers.read();
+                        if current_slot >= head
+                            && current_slot.sub(head) <= (SLOT_IMPORT_TOLERANCE as u64)
+                            && head > 0
+                        {
+                            SyncState::Synced
+                        } else if peers.advanced_peers().next().is_some() {
+                            SyncState::SyncTransition
+                        } else if peers.synced_peers().next().is_none() {
+                            SyncState::Stalled
+                        } else {
+                            // There are no peers that require syncing and we have at least one synced
+                            // peer
+                            SyncState::Synced
+                        }
+                    };
+
+                    // If we would otherwise be synced, first check if we need to perform or
+                    // complete a backfill sync.
+                    if matches!(sync_state, SyncState::Synced) {
+                        // Determine if we need to start/resume/restart a backfill sync.
+                        match self.backfill_sync.start(&mut self.network) {
+                            Ok(SyncStart::Syncing {
+                                completed,
+                                remaining,
+                            }) => {
+                                sync_state = SyncState::BackFillSyncing {
+                                    completed,
+                                    remaining,
+                                };
+                            }
+                            Ok(SyncStart::NotSyncing) => {} // Ignore updating the state if the backfill sync state didn't start.
+                            Err(e) => {
+                                error!(self.log, "Backfill sync failed to start"; "error" => ?e);
+                            }
+                        }
                     }
+
+                    // Return the sync state if backfilling is not required.
+                    sync_state
                 }
                 Some((RangeSyncType::Finalized, start_slot, target_slot)) => {
+                    // If there is a backfill sync in progress pause it.
+                    self.backfill_sync.pause();
+
                     SyncState::SyncingFinalized {
                         start_slot,
                         target_slot,
                     }
                 }
-                Some((RangeSyncType::Head, start_slot, target_slot)) => SyncState::SyncingHead {
-                    start_slot,
-                    target_slot,
-                },
+                Some((RangeSyncType::Head, start_slot, target_slot)) => {
+                    // If there is a backfill sync in progress pause it.
+                    self.backfill_sync.pause();
+
+                    SyncState::SyncingHead {
+                        start_slot,
+                        target_slot,
+                    }
+                }
             },
         };
 
@@ -690,7 +761,14 @@ impl<T: BeaconChainTypes> SyncManager<T> {
         if !new_state.eq(&old_state) {
             info!(self.log, "Sync state updated"; "old_state" => %old_state, "new_state" => %new_state);
             // If we have become synced - Subscribe to all the core subnet topics
-            if new_state.is_synced() {
+            // We don't need to subscribe if the old state is a state that would have already
+            // invoked this call.
+            if new_state.is_synced()
+                && !matches!(
+                    old_state,
+                    SyncState::Synced { .. } | SyncState::BackFillSyncing { .. }
+                )
+            {
                 self.network.subscribe_core_topics();
             }
         }
@@ -828,14 +906,13 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                 // peer. We don't consider this chain a failure and prevent retries with another
                 // peer.
                 "too many failed attempts"
-            } else {
-                if !parent_request.downloaded_blocks.is_empty() {
-                    self.failed_chains
-                        .insert(parent_request.downloaded_blocks[0].canonical_root());
-                } else {
-                    crit!(self.log, "Parent lookup has no blocks");
-                }
+            } else if !parent_request.downloaded_blocks.is_empty() {
+                self.failed_chains
+                    .insert(parent_request.downloaded_blocks[0].canonical_root());
                 "reached maximum lookup-depth"
+            } else {
+                crit!(self.log, "Parent lookup has no blocks");
+                "no blocks"
             };
 
             debug!(self.log, "Parent import failed";
@@ -887,13 +964,44 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                         request_id,
                         beacon_block,
                     } => {
-                        self.range_sync.blocks_by_range_response(
-                            &mut self.network,
-                            peer_id,
-                            request_id,
-                            beacon_block.map(|b| *b),
-                        );
-                        self.update_sync_state();
+                        let beacon_block = beacon_block.map(|b| *b);
+                        // Obtain which sync requested these blocks and divert accordingly.
+                        match self
+                            .network
+                            .blocks_by_range_response(request_id, beacon_block.is_none())
+                        {
+                            Some(SyncRequestType::RangeSync(batch_id, chain_id)) => {
+                                self.range_sync.blocks_by_range_response(
+                                    &mut self.network,
+                                    peer_id,
+                                    chain_id,
+                                    batch_id,
+                                    request_id,
+                                    beacon_block,
+                                );
+                                self.update_sync_state();
+                            }
+                            Some(SyncRequestType::BackFillSync(batch_id)) => {
+                                match self.backfill_sync.on_block_response(
+                                    &mut self.network,
+                                    batch_id,
+                                    &peer_id,
+                                    request_id,
+                                    beacon_block,
+                                ) {
+                                    Ok(ProcessResult::SyncCompleted) => self.update_sync_state(),
+                                    Ok(ProcessResult::Successful) => {}
+                                    Err(_error) => {
+                                        // The backfill sync has failed, errors are reported
+                                        // within.
+                                        self.update_sync_state();
+                                    }
+                                }
+                            }
+                            None => {
+                                trace!(self.log, "Response/Error for non registered request"; "request_id" => request_id)
+                            }
+                        }
                     }
                     SyncMessage::BlocksByRootResponse {
                         peer_id,
@@ -913,21 +1021,63 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                         self.peer_disconnect(&peer_id);
                     }
                     SyncMessage::RPCError(peer_id, request_id) => {
-                        self.inject_error(peer_id, request_id);
-                    }
-                    SyncMessage::BatchProcessed {
-                        chain_id,
-                        epoch,
-                        result,
-                    } => {
-                        self.range_sync.handle_block_process_result(
-                            &mut self.network,
-                            chain_id,
-                            epoch,
-                            result,
-                        );
-                        self.update_sync_state();
+                        // Redirect to a sync mechanism if the error is related to one of their
+                        // requests.
+                        match self.network.blocks_by_range_response(request_id, true) {
+                            Some(SyncRequestType::RangeSync(batch_id, chain_id)) => {
+                                self.range_sync.inject_error(
+                                    &mut self.network,
+                                    peer_id,
+                                    batch_id,
+                                    chain_id,
+                                    request_id,
+                                );
+                                self.update_sync_state();
+                            }
+                            Some(SyncRequestType::BackFillSync(batch_id)) => {
+                                match self.backfill_sync.inject_error(
+                                    &mut self.network,
+                                    batch_id,
+                                    &peer_id,
+                                    request_id,
+                                ) {
+                                    Ok(_) => {}
+                                    Err(_) => self.update_sync_state(),
+                                }
+                            }
+                            None => {
+                                // This is a request not belonging to a sync algorithm.
+                                // Process internally.
+                                self.inject_error(peer_id, request_id);
+                            }
+                        }
                     }
+                    SyncMessage::BatchProcessed { sync_type, result } => match sync_type {
+                        SyncRequestType::RangeSync(epoch, chain_id) => {
+                            self.range_sync.handle_block_process_result(
+                                &mut self.network,
+                                chain_id,
+                                epoch,
+                                result,
+                            );
+                            self.update_sync_state();
+                        }
+                        SyncRequestType::BackFillSync(epoch) => {
+                            match self.backfill_sync.on_batch_process_result(
+                                &mut self.network,
+                                epoch,
+                                &result,
+                            ) {
+                                Ok(ProcessResult::Successful) => {}
+                                Ok(ProcessResult::SyncCompleted) => self.update_sync_state(),
+                                Err(error) => {
+                                    error!(self.log, "Backfill sync failed"; "error" => ?error);
+                                    // Update the global status
+                                    self.update_sync_state();
+                                }
+                            }
+                        }
+                    },
                     SyncMessage::ParentLookupFailed {
                         chain_head,
                         peer_id,

beacon_node/network/src/sync/mod.rs

@@ -1,6 +1,7 @@
 //! Syncing for lighthouse.
 //!
 //! Stores the various syncing methods for the beacon chain.
+mod backfill_sync;
 pub mod manager;
 mod network_context;
 mod peer_sync_info;

beacon_node/network/src/sync/network_context.rs

@@ -1,6 +1,7 @@
 //! Provides network functionality for the Syncing thread. This fundamentally wraps a network
 //! channel and stores a global RPC ID to perform requests.
 
+use super::manager::SyncRequestType;
 use super::range_sync::{BatchId, ChainId};
 use super::RequestId as SyncRequestId;
 use crate::service::NetworkMessage;
@@ -26,8 +27,8 @@ pub struct SyncNetworkContext<T: EthSpec> {
     /// A sequential ID for all RPC requests.
     request_id: SyncRequestId,
 
-    /// BlocksByRange requests made by range syncing chains.
-    range_requests: FnvHashMap<SyncRequestId, (ChainId, BatchId)>,
+    /// BlocksByRange requests made by syncing algorithms.
+    range_requests: FnvHashMap<SyncRequestId, SyncRequestType>,
 
     /// Logger for the `SyncNetworkContext`.
     log: slog::Logger,
@@ -81,6 +82,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
         }
     }
 
+    /// A blocks by range request for the range sync algorithm.
     pub fn blocks_by_range_request(
         &mut self,
         peer_id: PeerId,
@@ -96,15 +98,37 @@ impl<T: EthSpec> SyncNetworkContext<T> {
             "peer" => %peer_id,
         );
         let req_id = self.send_rpc_request(peer_id, Request::BlocksByRange(request))?;
-        self.range_requests.insert(req_id, (chain_id, batch_id));
+        self.range_requests
+            .insert(req_id, SyncRequestType::RangeSync(batch_id, chain_id));
         Ok(req_id)
     }
 
+    /// A blocks by range request sent by the backfill sync algorithm
+    pub fn backfill_blocks_by_range_request(
+        &mut self,
+        peer_id: PeerId,
+        request: BlocksByRangeRequest,
+        batch_id: BatchId,
+    ) -> Result<SyncRequestId, &'static str> {
+        trace!(
+            self.log,
+            "Sending backfill BlocksByRange Request";
+            "method" => "BlocksByRange",
+            "count" => request.count,
+            "peer" => %peer_id,
+        );
+        let req_id = self.send_rpc_request(peer_id, Request::BlocksByRange(request))?;
+        self.range_requests
+            .insert(req_id, SyncRequestType::BackFillSync(batch_id));
+        Ok(req_id)
+    }
+
+    /// Received a blocks by range response.
     pub fn blocks_by_range_response(
         &mut self,
         request_id: usize,
         remove: bool,
-    ) -> Option<(ChainId, BatchId)> {
+    ) -> Option<SyncRequestType> {
         // NOTE: we can't guarantee that the request must be registered as it could receive more
         // than an error, and be removed after receiving the first one.
         // FIXME: https://github.com/sigp/lighthouse/issues/1634
@@ -115,6 +139,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
         }
     }
 
+    /// Sends a blocks by root request.
     pub fn blocks_by_root_request(
         &mut self,
         peer_id: PeerId,
@@ -130,6 +155,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
         self.send_rpc_request(peer_id, Request::BlocksByRoot(request))
     }
 
+    /// Terminates the connection with the peer and bans them.
     pub fn goodbye_peer(&mut self, peer_id: PeerId, reason: GoodbyeReason) {
         self.network_send
             .send(NetworkMessage::GoodbyePeer {
@@ -142,6 +168,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
             });
     }
 
+    /// Reports to the scoring algorithm the behaviour of a peer.
     pub fn report_peer(&mut self, peer_id: PeerId, action: PeerAction) {
         debug!(self.log, "Sync reporting peer"; "peer_id" => %peer_id, "action" => %action);
         self.network_send
@@ -155,7 +182,8 @@ impl<T: EthSpec> SyncNetworkContext<T> {
             });
     }
 
-    pub fn send_rpc_request(
+    /// Sends an RPC request.
+    fn send_rpc_request(
         &mut self,
         peer_id: PeerId,
         request: Request,
@@ -170,6 +198,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
         Ok(request_id)
     }
 
+    /// Subscribes to core topics.
     pub fn subscribe_core_topics(&mut self) {
         self.network_send
             .send(NetworkMessage::SubscribeCoreTopics)
@@ -178,6 +207,7 @@ impl<T: EthSpec> SyncNetworkContext<T> {
             });
     }
 
+    /// Sends an arbitrary network message.
     fn send_network_msg(&mut self, msg: NetworkMessage<T>) -> Result<(), &'static str> {
         self.network_send.send(msg).map_err(|_| {
             debug!(self.log, "Could not send message to the network service");

beacon_node/network/src/sync/range_sync/batch.rs

@@ -14,15 +14,34 @@ const MAX_BATCH_DOWNLOAD_ATTEMPTS: u8 = 5;
 /// after `MAX_BATCH_PROCESSING_ATTEMPTS` times, it is considered faulty.
 const MAX_BATCH_PROCESSING_ATTEMPTS: u8 = 3;
 
+/// Allows customisation of the above constants used in other sync methods such as BackFillSync.
+pub trait BatchConfig {
+    /// The maximum batch download attempts.
+    fn max_batch_download_attempts() -> u8;
+    /// The max batch processing attempts.
+    fn max_batch_processing_attempts() -> u8;
+}
+
+pub struct RangeSyncBatchConfig {}
+
+impl BatchConfig for RangeSyncBatchConfig {
+    fn max_batch_download_attempts() -> u8 {
+        MAX_BATCH_DOWNLOAD_ATTEMPTS
+    }
+    fn max_batch_processing_attempts() -> u8 {
+        MAX_BATCH_PROCESSING_ATTEMPTS
+    }
+}
+
 /// Error type of a batch in a wrong state.
 // Such errors should never be encountered.
-pub struct WrongState(pub(super) String);
+pub struct WrongState(pub(crate) String);
 
 /// Auxiliary type alias for readability.
 type IsFailed = bool;
 
 /// A segment of a chain.
-pub struct BatchInfo<T: EthSpec> {
+pub struct BatchInfo<T: EthSpec, B: BatchConfig = RangeSyncBatchConfig> {
     /// Start slot of the batch.
     start_slot: Slot,
     /// End slot of the batch.
@@ -33,6 +52,8 @@ pub struct BatchInfo<T: EthSpec> {
     failed_download_attempts: Vec<PeerId>,
     /// State of the batch.
     state: BatchState<T>,
+    /// Pin the generic
+    marker: std::marker::PhantomData<B>,
 }
 
 /// Current state of a batch
@@ -73,7 +94,7 @@ impl<T: EthSpec> BatchState<T> {
     }
 }
 
-impl<T: EthSpec> BatchInfo<T> {
+impl<T: EthSpec, B: BatchConfig> BatchInfo<T, B> {
     /// Batches are downloaded excluding the first block of the epoch assuming it has already been
     /// downloaded.
     ///
@@ -91,6 +112,7 @@ impl<T: EthSpec> BatchInfo<T> {
             failed_processing_attempts: Vec::new(),
             failed_download_attempts: Vec::new(),
             state: BatchState::AwaitingDownload,
+            marker: std::marker::PhantomData,
         }
     }
 
@@ -120,6 +142,7 @@ impl<T: EthSpec> BatchInfo<T> {
         false
     }
 
+    /// Returns the peer that is currently responsible for progressing the state of the batch.
     pub fn current_peer(&self) -> Option<&PeerId> {
         match &self.state {
             BatchState::AwaitingDownload | BatchState::Failed => None,
@@ -131,6 +154,7 @@ impl<T: EthSpec> BatchInfo<T> {
         }
     }
 
+    /// Returns a BlocksByRange request associated with the batch.
     pub fn to_blocks_by_range_request(&self) -> BlocksByRangeRequest {
         BlocksByRangeRequest {
             start_slot: self.start_slot.into(),
@@ -192,7 +216,7 @@ impl<T: EthSpec> BatchInfo<T> {
                         // can be tried again
                         self.failed_download_attempts.push(peer);
                         self.state = if self.failed_download_attempts.len()
-                            >= MAX_BATCH_DOWNLOAD_ATTEMPTS as usize
+                            >= B::max_batch_download_attempts() as usize
                         {
                             BatchState::Failed
                         } else {
@@ -219,14 +243,21 @@ impl<T: EthSpec> BatchInfo<T> {
         }
     }
 
+    /// Mark the batch as failed and return whether we can attempt a re-download.
+    ///
+    /// This can happen if a peer disconnects or some error occurred that was not the peers fault.
+    /// THe `mark_failed` parameter, when set to false, does not increment the failed attempts of
+    /// this batch and register the peer, rather attempts a re-download.
     #[must_use = "Batch may have failed"]
-    pub fn download_failed(&mut self) -> Result<IsFailed, WrongState> {
+    pub fn download_failed(&mut self, mark_failed: bool) -> Result<IsFailed, WrongState> {
         match self.state.poison() {
             BatchState::Downloading(peer, _, _request_id) => {
                 // register the attempt and check if the batch can be tried again
-                self.failed_download_attempts.push(peer);
+                if mark_failed {
+                    self.failed_download_attempts.push(peer);
+                }
                 self.state = if self.failed_download_attempts.len()
-                    >= MAX_BATCH_DOWNLOAD_ATTEMPTS as usize
+                    >= B::max_batch_download_attempts as usize
                 {
                     BatchState::Failed
                 } else {
@@ -294,7 +325,7 @@ impl<T: EthSpec> BatchInfo<T> {
 
                     // check if the batch can be downloaded again
                     if self.failed_processing_attempts.len()
-                        >= MAX_BATCH_PROCESSING_ATTEMPTS as usize
+                        >= B::max_batch_processing_attempts() as usize
                     {
                         BatchState::Failed
                     } else {
@@ -324,7 +355,7 @@ impl<T: EthSpec> BatchInfo<T> {
 
                 // check if the batch can be downloaded again
                 self.state = if self.failed_processing_attempts.len()
-                    >= MAX_BATCH_PROCESSING_ATTEMPTS as usize
+                    >= B::max_batch_processing_attempts() as usize
                 {
                     BatchState::Failed
                 } else {
@@ -365,7 +396,7 @@ impl Attempt {
     }
 }
 
-impl<T: EthSpec> slog::KV for &mut BatchInfo<T> {
+impl<T: EthSpec, B: BatchConfig> slog::KV for &mut BatchInfo<T, B> {
     fn serialize(
         &self,
         record: &slog::Record,
@@ -375,7 +406,7 @@ impl<T: EthSpec> slog::KV for &mut BatchInfo<T> {
     }
 }
 
-impl<T: EthSpec> slog::KV for BatchInfo<T> {
+impl<T: EthSpec, B: BatchConfig> slog::KV for BatchInfo<T, B> {
     fn serialize(
         &self,
         record: &slog::Record,

beacon_node/network/src/sync/range_sync/chain.rs

@@ -181,7 +181,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
             // fail the batches
             for id in batch_ids {
                 if let Some(batch) = self.batches.get_mut(&id) {
-                    if batch.download_failed()? {
+                    if batch.download_failed(true)? {
                         return Err(RemoveChain::ChainFailed(id));
                     }
                     self.retry_batch_download(network, id)?;
@@ -273,7 +273,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
         }
     }
 
-    /// Sends to process the batch with the given id.
+    /// Processes the batch with the given id.
     /// The batch must exist and be ready for processing
     fn process_batch(
         &mut self,
@@ -794,7 +794,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
             if let Some(active_requests) = self.peers.get_mut(peer_id) {
                 active_requests.remove(&batch_id);
             }
-            if batch.download_failed()? {
+            if batch.download_failed(true)? {
                 return Err(RemoveChain::ChainFailed(batch_id));
             }
             self.retry_batch_download(network, batch_id)
@@ -837,7 +837,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
         }
     }
 
-    /// Requests the batch asigned to the given id from a given peer.
+    /// Requests the batch assigned to the given id from a given peer.
     pub fn send_batch(
         &mut self,
         network: &mut SyncNetworkContext<T::EthSpec>,
@@ -883,7 +883,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
                     self.peers
                         .get_mut(&peer)
                         .map(|request| request.remove(&batch_id));
-                    if batch.download_failed()? {
+                    if batch.download_failed(true)? {
                         return Err(RemoveChain::ChainFailed(batch_id));
                     } else {
                         return self.retry_batch_download(network, batch_id);
@@ -990,7 +990,7 @@ impl<T: BeaconChainTypes> SyncingChain<T> {
         // this batch could have been included already being an optimistic batch
         match self.batches.entry(batch_id) {
             Entry::Occupied(_) => {
-                // this batch doesn't need downlading, let this same function decide the next batch
+                // this batch doesn't need downloading, let this same function decide the next batch
                 self.to_be_downloaded += EPOCHS_PER_BATCH;
                 self.include_next_batch()
             }

beacon_node/network/src/sync/range_sync/mod.rs

@@ -7,7 +7,7 @@ mod chain_collection;
 mod range;
 mod sync_type;
 
-pub use batch::BatchInfo;
+pub use batch::{BatchConfig, BatchInfo, BatchState};
 pub use chain::{BatchId, ChainId, EPOCHS_PER_BATCH};
 pub use range::RangeSync;
 pub use sync_type::RangeSyncType;

beacon_node/network/src/sync/range_sync/range.rs

@@ -39,7 +39,7 @@
 //!  Each chain is downloaded in batches of blocks. The batched blocks are processed sequentially
 //!  and further batches are requested as current blocks are being processed.
 
-use super::chain::{ChainId, RemoveChain, SyncingChain};
+use super::chain::{BatchId, ChainId, RemoveChain, SyncingChain};
 use super::chain_collection::ChainCollection;
 use super::sync_type::RangeSyncType;
 use crate::beacon_processor::WorkEvent as BeaconWorkEvent;
@@ -194,34 +194,29 @@ impl<T: BeaconChainTypes> RangeSync<T> {
         &mut self,
         network: &mut SyncNetworkContext<T::EthSpec>,
         peer_id: PeerId,
+        chain_id: ChainId,
+        batch_id: BatchId,
         request_id: RequestId,
         beacon_block: Option<SignedBeaconBlock<T::EthSpec>>,
     ) {
-        // get the chain and batch for which this response belongs
-        if let Some((chain_id, batch_id)) =
-            network.blocks_by_range_response(request_id, beacon_block.is_none())
-        {
-            // check if this chunk removes the chain
-            match self.chains.call_by_id(chain_id, |chain| {
-                chain.on_block_response(network, batch_id, &peer_id, request_id, beacon_block)
-            }) {
-                Ok((removed_chain, sync_type)) => {
-                    if let Some((removed_chain, remove_reason)) = removed_chain {
-                        self.on_chain_removed(
-                            removed_chain,
-                            sync_type,
-                            remove_reason,
-                            network,
-                            "block response",
-                        );
-                    }
-                }
-                Err(_) => {
-                    trace!(self.log, "BlocksByRange response for removed chain"; "chain" => chain_id)
+        // check if this chunk removes the chain
+        match self.chains.call_by_id(chain_id, |chain| {
+            chain.on_block_response(network, batch_id, &peer_id, request_id, beacon_block)
+        }) {
+            Ok((removed_chain, sync_type)) => {
+                if let Some((removed_chain, remove_reason)) = removed_chain {
+                    self.on_chain_removed(
+                        removed_chain,
+                        sync_type,
+                        remove_reason,
+                        network,
+                        "block response",
+                    );
                 }
             }
-        } else {
-            trace!(self.log, "Response/Error for non registered request"; "request_id" => request_id)
+            Err(_) => {
+                trace!(self.log, "BlocksByRange response for removed chain"; "chain" => chain_id)
+            }
         }
     }
 
@@ -298,31 +293,28 @@ impl<T: BeaconChainTypes> RangeSync<T> {
         &mut self,
         network: &mut SyncNetworkContext<T::EthSpec>,
         peer_id: PeerId,
+        batch_id: BatchId,
+        chain_id: ChainId,
         request_id: RequestId,
     ) {
-        // get the chain and batch for which this response belongs
-        if let Some((chain_id, batch_id)) = network.blocks_by_range_response(request_id, true) {
-            // check that this request is pending
-            match self.chains.call_by_id(chain_id, |chain| {
-                chain.inject_error(network, batch_id, &peer_id, request_id)
-            }) {
-                Ok((removed_chain, sync_type)) => {
-                    if let Some((removed_chain, remove_reason)) = removed_chain {
-                        self.on_chain_removed(
-                            removed_chain,
-                            sync_type,
-                            remove_reason,
-                            network,
-                            "RPC error",
-                        );
-                    }
-                }
-                Err(_) => {
-                    trace!(self.log, "BlocksByRange response for removed chain"; "chain" => chain_id)
+        // check that this request is pending
+        match self.chains.call_by_id(chain_id, |chain| {
+            chain.inject_error(network, batch_id, &peer_id, request_id)
+        }) {
+            Ok((removed_chain, sync_type)) => {
+                if let Some((removed_chain, remove_reason)) = removed_chain {
+                    self.on_chain_removed(
+                        removed_chain,
+                        sync_type,
+                        remove_reason,
+                        network,
+                        "RPC error",
+                    );
                 }
             }
-        } else {
-            trace!(self.log, "Response/Error for non registered request"; "request_id" => request_id)
+            Err(_) => {
+                trace!(self.log, "BlocksByRange response for removed chain"; "chain" => chain_id)
+            }
         }
     }
 

beacon_node/src/cli.rs

@@ -54,7 +54,8 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("shutdown-after-sync")
                 .long("shutdown-after-sync")
-                .help("Shutdown beacon node as soon as sync is completed")
+                .help("Shutdown beacon node as soon as sync is completed. Backfill sync will \
+                       not be performed before shutdown.")
                 .takes_value(false),
         )
         .arg(
@@ -479,12 +480,46 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("wss-checkpoint")
                 .long("wss-checkpoint")
                 .help(
-                    "Used to input a Weak Subjectivity State Checkpoint in `block_root:epoch_number` format,\
-                     where block_root is an '0x' prefixed 32-byte hex string and epoch_number is an integer."
+                    "Specify a weak subjectivity checkpoint in `block_root:epoch` format to verify \
+                     the node's sync against. The block root should be 0x-prefixed. Note that this \
+                     flag is for verification only, to perform a checkpoint sync from a recent \
+                     state use --checkpoint-sync-url."
                 )
                 .value_name("WSS_CHECKPOINT")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("checkpoint-state")
+                .long("checkpoint-state")
+                .help("Set a checkpoint state to start syncing from. Must be aligned and match \
+                       --checkpoint-block. Using --checkpoint-sync-url instead is recommended.")
+                .value_name("STATE_SSZ")
+                .takes_value(true)
+                .requires("checkpoint-block")
+        )
+        .arg(
+            Arg::with_name("checkpoint-block")
+                .long("checkpoint-block")
+                .help("Set a checkpoint block to start syncing from. Must be aligned and match \
+                       --checkpoint-state. Using --checkpoint-sync-url instead is recommended.")
+                .value_name("BLOCK_SSZ")
+                .takes_value(true)
+                .requires("checkpoint-state")
+        )
+        .arg(
+            Arg::with_name("checkpoint-sync-url")
+                .long("checkpoint-sync-url")
+                .help("Set the remote beacon node HTTP endpoint to use for checkpoint sync.")
+                .value_name("BEACON_NODE")
+                .takes_value(true)
+                .conflicts_with("checkpoint-state")
+        )
+        .arg(
+            Arg::with_name("reconstruct-historic-states")
+                .long("reconstruct-historic-states")
+                .help("After a checkpoint sync, reconstruct historic states in the database.")
+                .takes_value(false)
+        )
         .arg(
             Arg::with_name("validator-monitor-auto")
                 .long("validator-monitor-auto")

beacon_node/src/config.rs

@@ -292,15 +292,62 @@ pub fn get_config<E: EthSpec>(
         }
     }
 
-    if let Some(genesis_state_bytes) = eth2_network_config.genesis_state_bytes {
-        // Note: re-serializing the genesis state is not so efficient, however it avoids adding
-        // trait bounds to the `ClientGenesis` enum. This would have significant flow-on
-        // effects.
-        client_config.genesis = ClientGenesis::SszBytes {
-            genesis_state_bytes,
-        };
+    client_config.genesis = if let Some(genesis_state_bytes) =
+        eth2_network_config.genesis_state_bytes
+    {
+        // Set up weak subjectivity sync, or start from the hardcoded genesis state.
+        if let (Some(initial_state_path), Some(initial_block_path)) = (
+            cli_args.value_of("checkpoint-state"),
+            cli_args.value_of("checkpoint-block"),
+        ) {
+            let read = |path: &str| {
+                use std::fs::File;
+                use std::io::Read;
+                File::open(Path::new(path))
+                    .and_then(|mut f| {
+                        let mut buffer = vec![];
+                        f.read_to_end(&mut buffer)?;
+                        Ok(buffer)
+                    })
+                    .map_err(|e| format!("Unable to open {}: {:?}", path, e))
+            };
+
+            let anchor_state_bytes = read(initial_state_path)?;
+            let anchor_block_bytes = read(initial_block_path)?;
+
+            ClientGenesis::WeakSubjSszBytes {
+                genesis_state_bytes,
+                anchor_state_bytes,
+                anchor_block_bytes,
+            }
+        } else if let Some(remote_bn_url) = cli_args.value_of("checkpoint-sync-url") {
+            let url = SensitiveUrl::parse(remote_bn_url)
+                .map_err(|e| format!("Invalid checkpoint sync URL: {:?}", e))?;
+
+            ClientGenesis::CheckpointSyncUrl {
+                genesis_state_bytes,
+                url,
+            }
+        } else {
+            // Note: re-serializing the genesis state is not so efficient, however it avoids adding
+            // trait bounds to the `ClientGenesis` enum. This would have significant flow-on
+            // effects.
+            ClientGenesis::SszBytes {
+                genesis_state_bytes,
+            }
+        }
     } else {
-        client_config.genesis = ClientGenesis::DepositContract;
+        if cli_args.is_present("checkpoint-state") || cli_args.is_present("checkpoint-sync-url") {
+            return Err(
+                "Checkpoint sync is not available for this network as no genesis state is known"
+                    .to_string(),
+            );
+        }
+        ClientGenesis::DepositContract
+    };
+
+    if cli_args.is_present("reconstruct-historic-states") {
+        client_config.chain.reconstruct_historic_states = true;
     }
 
     let raw_graffiti = if let Some(graffiti) = cli_args.value_of("graffiti") {

beacon_node/store/src/chunk_writer.rs

@@ -0,0 +1,75 @@
+use crate::chunked_vector::{chunk_key, Chunk, ChunkError, Field};
+use crate::{Error, KeyValueStore, KeyValueStoreOp};
+use types::EthSpec;
+
+/// Buffered writer for chunked vectors (block roots mainly).
+pub struct ChunkWriter<'a, F, E, S>
+where
+    F: Field<E>,
+    E: EthSpec,
+    S: KeyValueStore<E>,
+{
+    /// Buffered chunk awaiting writing to disk (always dirty).
+    chunk: Chunk<F::Value>,
+    /// Chunk index of `chunk`.
+    index: usize,
+    store: &'a S,
+}
+
+impl<'a, F, E, S> ChunkWriter<'a, F, E, S>
+where
+    F: Field<E>,
+    E: EthSpec,
+    S: KeyValueStore<E>,
+{
+    pub fn new(store: &'a S, vindex: usize) -> Result<Self, Error> {
+        let chunk_index = F::chunk_index(vindex);
+        let chunk = Chunk::load(store, F::column(), &chunk_key(chunk_index))?
+            .unwrap_or_else(|| Chunk::new(vec![F::Value::default(); F::chunk_size()]));
+
+        Ok(Self {
+            chunk,
+            index: chunk_index,
+            store,
+        })
+    }
+
+    /// Set the value at a given vector index, writing the current chunk and moving on if necessary.
+    pub fn set(
+        &mut self,
+        vindex: usize,
+        value: F::Value,
+        batch: &mut Vec<KeyValueStoreOp>,
+    ) -> Result<(), Error> {
+        let chunk_index = F::chunk_index(vindex);
+
+        // Advance to the next chunk.
+        if chunk_index != self.index {
+            self.write(batch)?;
+            *self = Self::new(self.store, vindex)?;
+        }
+
+        let i = vindex % F::chunk_size();
+        let existing_value = &self.chunk.values[i];
+
+        if existing_value == &value || existing_value == &F::Value::default() {
+            self.chunk.values[i] = value;
+            Ok(())
+        } else {
+            Err(ChunkError::Inconsistent {
+                field: F::column(),
+                chunk_index,
+                existing_value: format!("{:?}", existing_value),
+                new_value: format!("{:?}", value),
+            }
+            .into())
+        }
+    }
+
+    /// Write the current chunk to disk.
+    ///
+    /// Should be called before the writer is dropped, in order to write the final chunk to disk.
+    pub fn write(&self, batch: &mut Vec<KeyValueStoreOp>) -> Result<(), Error> {
+        self.chunk.store(F::column(), &chunk_key(self.index), batch)
+    }
+}

beacon_node/store/src/chunked_iter.rs

@@ -97,7 +97,7 @@ where
             self.current_chunk = Chunk::load(
                 &self.store.cold_db,
                 F::column(),
-                &chunk_key(self.next_cindex as u64),
+                &chunk_key(self.next_cindex),
             )
             .map_err(|e| {
                 error!(

beacon_node/store/src/chunked_vector.rs

@@ -34,8 +34,8 @@ pub enum UpdatePattern {
 /// Map a chunk index to bytes that can be used to key the NoSQL database.
 ///
 /// We shift chunks up by 1 to make room for a genesis chunk that is handled separately.
-pub fn chunk_key(cindex: u64) -> [u8; 8] {
-    (cindex + 1).to_be_bytes()
+pub fn chunk_key(cindex: usize) -> [u8; 8] {
+    (cindex as u64 + 1).to_be_bytes()
 }
 
 /// Return the database key for the genesis value.
@@ -73,6 +73,11 @@ pub trait Field<E: EthSpec>: Copy {
         128
     }
 
+    /// Convert a v-index (vector index) to a chunk index.
+    fn chunk_index(vindex: usize) -> usize {
+        vindex / Self::chunk_size()
+    }
+
     /// Get the value of this field at the given vector index, from the state.
     fn get_value(
         state: &BeaconState<E>,
@@ -399,7 +404,7 @@ where
     I: Iterator<Item = usize>,
 {
     for chunk_index in range {
-        let chunk_key = &chunk_key(chunk_index as u64)[..];
+        let chunk_key = &chunk_key(chunk_index)[..];
 
         let existing_chunk =
             Chunk::<F::Value>::load(store, F::column(), chunk_key)?.unwrap_or_else(Chunk::default);
@@ -440,7 +445,7 @@ fn range_query<S: KeyValueStore<E>, E: EthSpec, T: Decode + Encode>(
     let mut result = Vec::with_capacity(len);
 
     for chunk_index in range {
-        let key = &chunk_key(chunk_index as u64)[..];
+        let key = &chunk_key(chunk_index)[..];
         let chunk = Chunk::load(store, column, key)?.ok_or(ChunkError::Missing { chunk_index })?;
         result.push(chunk);
     }

beacon_node/store/src/config.rs

@@ -24,8 +24,6 @@ pub struct StoreConfig {
 #[derive(Debug, Clone, PartialEq, Eq, Encode, Decode)]
 pub struct OnDiskStoreConfig {
     pub slots_per_restore_point: u64,
-    // NOTE: redundant, see https://github.com/sigp/lighthouse/issues/1784
-    pub _block_cache_size: usize,
 }
 
 #[derive(Debug, Clone)]
@@ -49,7 +47,6 @@ impl StoreConfig {
     pub fn as_disk_config(&self) -> OnDiskStoreConfig {
         OnDiskStoreConfig {
             slots_per_restore_point: self.slots_per_restore_point,
-            _block_cache_size: DEFAULT_BLOCK_CACHE_SIZE,
         }
     }
 

beacon_node/store/src/errors.rs

@@ -13,13 +13,46 @@ pub enum Error {
     BeaconStateError(BeaconStateError),
     PartialBeaconStateError,
     HotColdDBError(HotColdDBError),
-    DBError { message: String },
+    DBError {
+        message: String,
+    },
     RlpError(String),
     BlockNotFound(Hash256),
     NoContinuationData,
     SplitPointModified(Slot, Slot),
     ConfigError(StoreConfigError),
     SchemaMigrationError(String),
+    /// The store's `anchor_info` was mutated concurrently, the latest modification wasn't applied.
+    AnchorInfoConcurrentMutation,
+    /// The block or state is unavailable due to weak subjectivity sync.
+    HistoryUnavailable,
+    /// State reconstruction cannot commence because not all historic blocks are known.
+    MissingHistoricBlocks {
+        oldest_block_slot: Slot,
+    },
+    /// State reconstruction failed because it didn't reach the upper limit slot.
+    ///
+    /// This should never happen (it's a logic error).
+    StateReconstructionDidNotComplete,
+    StateReconstructionRootMismatch {
+        slot: Slot,
+        expected: Hash256,
+        computed: Hash256,
+    },
+}
+
+pub trait HandleUnavailable<T> {
+    fn handle_unavailable(self) -> std::result::Result<Option<T>, Error>;
+}
+
+impl<T> HandleUnavailable<T> for Result<T> {
+    fn handle_unavailable(self) -> std::result::Result<Option<T>, Error> {
+        match self {
+            Ok(x) => Ok(Some(x)),
+            Err(Error::HistoryUnavailable) => Ok(None),
+            Err(e) => Err(e),
+        }
+    }
 }
 
 impl From<DecodeError> for Error {

beacon_node/store/src/hot_cold_store.rs

@@ -3,17 +3,15 @@ use crate::chunked_vector::{
 };
 use crate::config::{OnDiskStoreConfig, StoreConfig};
 use crate::forwards_iter::{HybridForwardsBlockRootsIterator, HybridForwardsStateRootsIterator};
-use crate::impls::{
-    beacon_block_as_kv_store_op,
-    beacon_state::{get_full_state, store_full_state},
-};
+use crate::impls::beacon_state::{get_full_state, store_full_state};
 use crate::iter::{ParentRootBlockIterator, StateRootsIterator};
 use crate::leveldb_store::BytesKey;
 use crate::leveldb_store::LevelDB;
 use crate::memory_store::MemoryStore;
 use crate::metadata::{
-    CompactionTimestamp, PruningCheckpoint, SchemaVersion, COMPACTION_TIMESTAMP_KEY, CONFIG_KEY,
-    CURRENT_SCHEMA_VERSION, PRUNING_CHECKPOINT_KEY, SCHEMA_VERSION_KEY, SPLIT_KEY,
+    AnchorInfo, CompactionTimestamp, PruningCheckpoint, SchemaVersion, ANCHOR_INFO_KEY,
+    COMPACTION_TIMESTAMP_KEY, CONFIG_KEY, CURRENT_SCHEMA_VERSION, PRUNING_CHECKPOINT_KEY,
+    SCHEMA_VERSION_KEY, SPLIT_KEY,
 };
 use crate::metrics;
 use crate::{
@@ -23,13 +21,15 @@ use crate::{
 use leveldb::iterator::LevelDBIterator;
 use lru::LruCache;
 use parking_lot::{Mutex, RwLock};
-use slog::{debug, error, info, trace, warn, Logger};
+use serde_derive::{Deserialize, Serialize};
+use slog::{debug, error, info, trace, Logger};
 use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
 use state_processing::{
     per_block_processing, per_slot_processing, BlockProcessingError, BlockSignatureStrategy,
     SlotProcessingError,
 };
+use std::cmp::min;
 use std::convert::TryInto;
 use std::marker::PhantomData;
 use std::path::Path;
@@ -57,8 +57,10 @@ pub struct HotColdDB<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> {
     ///
     /// States with slots less than `split.slot` are in the cold DB, while states with slots
     /// greater than or equal are in the hot DB.
-    split: RwLock<Split>,
-    config: StoreConfig,
+    pub(crate) split: RwLock<Split>,
+    /// The starting slots for the range of blocks & states stored in the database.
+    anchor_info: RwLock<Option<AnchorInfo>>,
+    pub(crate) config: StoreConfig,
     /// Cold database containing compact historical data.
     pub cold_db: Cold,
     /// Hot database containing duplicated but quick-to-access recent data.
@@ -68,7 +70,7 @@ pub struct HotColdDB<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> {
     /// LRU cache of deserialized blocks. Updated whenever a block is loaded.
     block_cache: Mutex<LruCache<Hash256, SignedBeaconBlock<E>>>,
     /// Chain spec.
-    spec: ChainSpec,
+    pub(crate) spec: ChainSpec,
     /// Logger.
     pub(crate) log: Logger,
     /// Mere vessel for E.
@@ -95,11 +97,13 @@ pub enum HotColdDBError {
     MissingHotStateSummary(Hash256),
     MissingEpochBoundaryState(Hash256),
     MissingSplitState(Hash256, Slot),
+    MissingAnchorInfo,
     HotStateSummaryError(BeaconStateError),
     RestorePointDecodeError(ssz::DecodeError),
     BlockReplayBeaconError(BeaconStateError),
     BlockReplaySlotError(SlotProcessingError),
     BlockReplayBlockError(BlockProcessingError),
+    MissingLowerLimitState(Slot),
     InvalidSlotsPerRestorePoint {
         slots_per_restore_point: u64,
         slots_per_historical_root: u64,
@@ -126,6 +130,7 @@ impl<E: EthSpec> HotColdDB<E, MemoryStore<E>, MemoryStore<E>> {
 
         let db = HotColdDB {
             split: RwLock::new(Split::default()),
+            anchor_info: RwLock::new(None),
             cold_db: MemoryStore::open(),
             hot_db: MemoryStore::open(),
             block_cache: Mutex::new(LruCache::new(config.block_cache_size)),
@@ -158,6 +163,7 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
 
         let db = Arc::new(HotColdDB {
             split: RwLock::new(Split::default()),
+            anchor_info: RwLock::new(None),
             cold_db: LevelDB::open(cold_path)?,
             hot_db: LevelDB::open(hot_path)?,
             block_cache: Mutex::new(LruCache::new(config.block_cache_size)),
@@ -190,6 +196,9 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
         // Load the previous split slot from the database (if any). This ensures we can
         // stop and restart correctly.
         if let Some(split) = db.load_split()? {
+            *db.split.write() = split;
+            *db.anchor_info.write() = db.load_anchor_info()?;
+
             info!(
                 db.log,
                 "Hot-Cold DB initialized";
@@ -197,7 +206,6 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
                 "split_slot" => split.slot,
                 "split_state" => format!("{:?}", split.state_root)
             );
-            *db.split.write() = split;
         }
 
         // Run a garbage collection pass.
@@ -243,8 +251,8 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         block: SignedBeaconBlock<E>,
     ) -> Result<(), Error> {
         // Store on disk.
-        self.hot_db
-            .do_atomically(vec![beacon_block_as_kv_store_op(block_root, &block)])?;
+        let op = self.block_as_kv_store_op(block_root, &block);
+        self.hot_db.do_atomically(vec![op])?;
 
         // Update cache.
         self.block_cache.lock().put(*block_root, block);
@@ -252,6 +260,18 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         Ok(())
     }
 
+    /// Prepare a signed beacon block for storage in the database.
+    #[must_use]
+    pub fn block_as_kv_store_op(
+        &self,
+        key: &Hash256,
+        block: &SignedBeaconBlock<E>,
+    ) -> KeyValueStoreOp {
+        // FIXME(altair): re-add block write/overhead metrics, or remove them
+        let db_key = get_key_for_col(DBColumn::BeaconBlock.into(), key.as_bytes());
+        KeyValueStoreOp::PutKeyValue(db_key, block.as_ssz_bytes())
+    }
+
     /// Fetch a block from the store.
     pub fn get_block(&self, block_root: &Hash256) -> Result<Option<SignedBeaconBlock<E>>, Error> {
         metrics::inc_counter(&metrics::BEACON_BLOCK_GET_COUNT);
@@ -467,7 +487,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
                 Some(state_slot) => {
                     let epoch_boundary_slot =
                         state_slot / E::slots_per_epoch() * E::slots_per_epoch();
-                    self.load_cold_state_by_slot(epoch_boundary_slot).map(Some)
+                    self.load_cold_state_by_slot(epoch_boundary_slot)
                 }
                 None => Ok(None),
             }
@@ -492,7 +512,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         for op in batch {
             match op {
                 StoreOp::PutBlock(block_root, block) => {
-                    key_value_batch.push(beacon_block_as_kv_store_op(block_root, block));
+                    key_value_batch.push(self.block_as_kv_store_op(block_root, block));
                 }
 
                 StoreOp::PutState(state_root, state) => {
@@ -563,6 +583,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         }
         Ok(())
     }
+
     /// Store a post-finalization state efficiently in the hot database.
     ///
     /// On an epoch boundary, store a full state. On an intermediate slot, store
@@ -639,21 +660,16 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
 
     /// Store a pre-finalization state in the freezer database.
     ///
-    /// Will log a warning and not store anything if the state does not lie on a restore point
-    /// boundary.
+    /// If the state doesn't lie on a restore point boundary then just its summary will be stored.
     pub fn store_cold_state(
         &self,
         state_root: &Hash256,
         state: &BeaconState<E>,
         ops: &mut Vec<KeyValueStoreOp>,
     ) -> Result<(), Error> {
+        ops.push(ColdStateSummary { slot: state.slot() }.as_kv_store_op(*state_root));
+
         if state.slot() % self.config.slots_per_restore_point != 0 {
-            warn!(
-                self.log,
-                "Not storing non-restore point state in freezer";
-                "slot" => state.slot().as_u64(),
-                "state_root" => format!("{:?}", state_root)
-            );
             return Ok(());
         }
 
@@ -688,7 +704,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
     /// Return `None` if no state with `state_root` lies in the freezer.
     pub fn load_cold_state(&self, state_root: &Hash256) -> Result<Option<BeaconState<E>>, Error> {
         match self.load_cold_state_slot(state_root)? {
-            Some(slot) => self.load_cold_state_by_slot(slot).map(Some),
+            Some(slot) => self.load_cold_state_by_slot(slot),
             None => Ok(None),
         }
     }
@@ -696,12 +712,22 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
     /// Load a pre-finalization state from the freezer database.
     ///
     /// Will reconstruct the state if it lies between restore points.
-    pub fn load_cold_state_by_slot(&self, slot: Slot) -> Result<BeaconState<E>, Error> {
-        if slot % self.config.slots_per_restore_point == 0 {
-            let restore_point_idx = slot.as_u64() / self.config.slots_per_restore_point;
-            self.load_restore_point_by_index(restore_point_idx)
+    pub fn load_cold_state_by_slot(&self, slot: Slot) -> Result<Option<BeaconState<E>>, Error> {
+        // Guard against fetching states that do not exist due to gaps in the historic state
+        // database, which can occur due to checkpoint sync or re-indexing.
+        // See the comments in `get_historic_state_limits` for more information.
+        let (lower_limit, upper_limit) = self.get_historic_state_limits();
+
+        if slot <= lower_limit || slot >= upper_limit {
+            if slot % self.config.slots_per_restore_point == 0 {
+                let restore_point_idx = slot.as_u64() / self.config.slots_per_restore_point;
+                self.load_restore_point_by_index(restore_point_idx)
+            } else {
+                self.load_cold_intermediate_state(slot)
+            }
+            .map(Some)
         } else {
-            self.load_cold_intermediate_state(slot)
+            Ok(None)
         }
     }
 
@@ -742,17 +768,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         let split = self.split.read_recursive();
 
         let low_restore_point = self.load_restore_point_by_index(low_restore_point_idx)?;
-        // If the slot of the high point lies outside the freezer, use the split state
-        // as the upper restore point.
-        let high_restore_point = if high_restore_point_idx * self.config.slots_per_restore_point
-            >= split.slot.as_u64()
-        {
-            self.get_state(&split.state_root, Some(split.slot))?.ok_or(
-                HotColdDBError::MissingSplitState(split.state_root, split.slot),
-            )?
-        } else {
-            self.load_restore_point_by_index(high_restore_point_idx)?
-        };
+        let high_restore_point = self.get_restore_point(high_restore_point_idx, &split)?;
 
         // 2. Load the blocks from the high restore point back to the low restore point.
         let blocks = self.load_blocks_to_replay(
@@ -765,6 +781,24 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         self.replay_blocks(low_restore_point, blocks, slot, BlockReplay::Accurate)
     }
 
+    /// Get the restore point with the given index, or if it is out of bounds, the split state.
+    pub(crate) fn get_restore_point(
+        &self,
+        restore_point_idx: u64,
+        split: &Split,
+    ) -> Result<BeaconState<E>, Error> {
+        if restore_point_idx * self.config.slots_per_restore_point >= split.slot.as_u64() {
+            self.get_state(&split.state_root, Some(split.slot))?
+                .ok_or(HotColdDBError::MissingSplitState(
+                    split.state_root,
+                    split.slot,
+                ))
+                .map_err(Into::into)
+        } else {
+            self.load_restore_point_by_index(restore_point_idx)
+        }
+    }
+
     /// Get a suitable block root for backtracking from `high_restore_point` to the state at `slot`.
     ///
     /// Defaults to the block root for `slot`, which *should* be in range.
@@ -800,12 +834,21 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
                         .as_ref()
                         .map_or(true, |block| block.slot() <= end_slot)
                 })
-                // Include the block at the start slot (if any). Whilst it doesn't need to be applied
-                // to the state, it contains a potentially useful state root.
-                .take_while(|result| {
-                    result
-                        .as_ref()
-                        .map_or(true, |block| block.slot() >= start_slot)
+                // Include the block at the start slot (if any). Whilst it doesn't need to be
+                // applied to the state, it contains a potentially useful state root.
+                //
+                // Return `true` on an `Err` so that the `collect` fails, unless the error is a
+                // `BlockNotFound` error and some blocks are intentionally missing from the DB.
+                // This complexity is unfortunately necessary to avoid loading the parent of the
+                // oldest known block -- we can't know that we have all the required blocks until we
+                // load a block with slot less than the start slot, which is impossible if there are
+                // no blocks with slot less than the start slot.
+                .take_while(|result| match result {
+                    Ok(block) => block.slot() >= start_slot,
+                    Err(Error::BlockNotFound(_)) => {
+                        self.get_oldest_block_slot() == self.spec.genesis_slot
+                    }
+                    Err(_) => true,
                 })
                 .collect::<Result<_, _>>()?;
         blocks.reverse();
@@ -904,6 +947,15 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         self.split.read_recursive().slot
     }
 
+    /// Fetch a copy of the current split slot from memory.
+    pub fn get_split_info(&self) -> Split {
+        *self.split.read_recursive()
+    }
+
+    pub fn set_split(&self, slot: Slot, state_root: Hash256) {
+        *self.split.write() = Split { slot, state_root };
+    }
+
     /// Fetch the slot of the most recently stored restore point.
     pub fn get_latest_restore_point_slot(&self) -> Slot {
         (self.get_split_slot() - 1) / self.config.slots_per_restore_point
@@ -920,6 +972,122 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         self.hot_db.put(&SCHEMA_VERSION_KEY, &schema_version)
     }
 
+    /// Initialise the anchor info for checkpoint sync starting from `block`.
+    pub fn init_anchor_info(&self, block: BeaconBlockRef<'_, E>) -> Result<(), Error> {
+        let anchor_slot = block.slot();
+        let slots_per_restore_point = self.config.slots_per_restore_point;
+
+        // Set the `state_upper_limit` to the slot of the *next* restore point.
+        // See `get_state_upper_limit` for rationale.
+        let next_restore_point_slot = if anchor_slot % slots_per_restore_point == 0 {
+            anchor_slot
+        } else {
+            (anchor_slot / slots_per_restore_point + 1) * slots_per_restore_point
+        };
+        let anchor_info = AnchorInfo {
+            anchor_slot,
+            oldest_block_slot: anchor_slot,
+            oldest_block_parent: block.parent_root(),
+            state_upper_limit: next_restore_point_slot,
+            state_lower_limit: self.spec.genesis_slot,
+        };
+        self.compare_and_set_anchor_info(None, Some(anchor_info))
+    }
+
+    /// Get a clone of the store's anchor info.
+    ///
+    /// To do mutations, use `compare_and_set_anchor_info`.
+    pub fn get_anchor_info(&self) -> Option<AnchorInfo> {
+        self.anchor_info.read_recursive().clone()
+    }
+
+    /// Atomically update the anchor info from `prev_value` to `new_value`.
+    ///
+    /// Return an `AnchorInfoConcurrentMutation` error if the `prev_value` provided
+    /// is not correct.
+    pub fn compare_and_set_anchor_info(
+        &self,
+        prev_value: Option<AnchorInfo>,
+        new_value: Option<AnchorInfo>,
+    ) -> Result<(), Error> {
+        let mut anchor_info = self.anchor_info.write();
+        if *anchor_info == prev_value {
+            self.store_anchor_info(&new_value)?;
+            *anchor_info = new_value;
+            Ok(())
+        } else {
+            Err(Error::AnchorInfoConcurrentMutation)
+        }
+    }
+
+    /// Load the anchor info from disk, but do not set `self.anchor_info`.
+    fn load_anchor_info(&self) -> Result<Option<AnchorInfo>, Error> {
+        self.hot_db.get(&ANCHOR_INFO_KEY)
+    }
+
+    /// Store the given `anchor_info` to disk.
+    ///
+    /// The argument is intended to be `self.anchor_info`, but is passed manually to avoid issues
+    /// with recursive locking.
+    fn store_anchor_info(&self, anchor_info: &Option<AnchorInfo>) -> Result<(), Error> {
+        if let Some(ref anchor_info) = anchor_info {
+            self.hot_db.put(&ANCHOR_INFO_KEY, anchor_info)?;
+        } else {
+            self.hot_db.delete::<AnchorInfo>(&ANCHOR_INFO_KEY)?;
+        }
+        Ok(())
+    }
+
+    /// If an anchor exists, return its `anchor_slot` field.
+    pub fn get_anchor_slot(&self) -> Option<Slot> {
+        self.anchor_info
+            .read_recursive()
+            .as_ref()
+            .map(|a| a.anchor_slot)
+    }
+
+    /// Return the slot-window describing the available historic states.
+    ///
+    /// Returns `(lower_limit, upper_limit)`.
+    ///
+    /// The lower limit is the maximum slot such that frozen states are available for all
+    /// previous slots (<=).
+    ///
+    /// The upper limit is the minimum slot such that frozen states are available for all
+    /// subsequent slots (>=).
+    ///
+    /// If `lower_limit >= upper_limit` then all states are available. This will be true
+    /// if the database is completely filled in, as we'll return `(split_slot, 0)` in this
+    /// instance.
+    pub fn get_historic_state_limits(&self) -> (Slot, Slot) {
+        // If checkpoint sync is used then states in the hot DB will always be available, but may
+        // become unavailable as finalisation advances due to the lack of a restore point in the
+        // database. For this reason we take the minimum of the split slot and the
+        // restore-point-aligned `state_upper_limit`, which should be set _ahead_ of the checkpoint
+        // slot during initialisation.
+        //
+        // E.g. if we start from a checkpoint at slot 2048+1024=3072 with SPRP=2048, then states
+        // with slots 3072-4095 will be available only while they are in the hot database, and this
+        // function will return the current split slot as the upper limit. Once slot 4096 is reached
+        // a new restore point will be created at that slot, making all states from 4096 onwards
+        // permanently available.
+        let split_slot = self.get_split_slot();
+        self.anchor_info
+            .read_recursive()
+            .as_ref()
+            .map_or((split_slot, self.spec.genesis_slot), |a| {
+                (a.state_lower_limit, min(a.state_upper_limit, split_slot))
+            })
+    }
+
+    /// Return the minimum slot such that blocks are available for all subsequent slots.
+    pub fn get_oldest_block_slot(&self) -> Slot {
+        self.anchor_info
+            .read_recursive()
+            .as_ref()
+            .map_or(self.spec.genesis_slot, |anchor| anchor.oldest_block_slot)
+    }
+
     /// Load previously-stored config from disk.
     fn load_config(&self) -> Result<Option<OnDiskStoreConfig>, Error> {
         self.hot_db.get(&CONFIG_KEY)
@@ -935,6 +1103,12 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         self.hot_db.get(&SPLIT_KEY)
     }
 
+    /// Store the split point to disk.
+    pub fn store_split(&self) -> Result<(), Error> {
+        self.hot_db.put_sync(&SPLIT_KEY, &*self.split.read())?;
+        Ok(())
+    }
+
     /// Load the state root of a restore point.
     fn load_restore_point_hash(&self, restore_point_index: u64) -> Result<Hash256, Error> {
         let key = Self::restore_point_key(restore_point_index);
@@ -1037,6 +1211,12 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
             .map(|pc: PruningCheckpoint| pc.checkpoint))
     }
 
+    /// Store the checkpoint to begin pruning from (the "old finalized checkpoint").
+    pub fn store_pruning_checkpoint(&self, checkpoint: Checkpoint) -> Result<(), Error> {
+        self.hot_db
+            .do_atomically(vec![self.pruning_checkpoint_store_op(checkpoint)])
+    }
+
     /// Create a staged store for the pruning checkpoint.
     pub fn pruning_checkpoint_store_op(&self, checkpoint: Checkpoint) -> KeyValueStoreOp {
         PruningCheckpoint { checkpoint }.as_kv_store_op(PRUNING_CHECKPOINT_KEY)
@@ -1075,6 +1255,11 @@ pub fn migrate_database<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
     // The new frozen head must increase the current split slot, and lie on an epoch
     // boundary (in order for the hot state summary scheme to work).
     let current_split_slot = store.split.read_recursive().slot;
+    let anchor_slot = store
+        .anchor_info
+        .read_recursive()
+        .as_ref()
+        .map(|a| a.anchor_slot);
 
     if frozen_head.slot() < current_split_slot {
         return Err(HotColdDBError::FreezeSlotError {
@@ -1094,7 +1279,10 @@ pub fn migrate_database<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
     // to the cold DB.
     let state_root_iter = StateRootsIterator::new(store.clone(), frozen_head);
     for maybe_pair in state_root_iter.take_while(|result| match result {
-        Ok((_, slot)) => slot >= &current_split_slot,
+        Ok((_, slot)) => {
+            slot >= &current_split_slot
+                && anchor_slot.map_or(true, |anchor_slot| slot >= &anchor_slot)
+        }
         Err(_) => true,
     }) {
         let (state_root, slot) = maybe_pair?;
@@ -1183,10 +1371,10 @@ pub fn migrate_database<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
 }
 
 /// Struct for storing the split slot and state root in the database.
-#[derive(Debug, Clone, Copy, Default, Encode, Decode)]
+#[derive(Debug, Clone, Copy, PartialEq, Default, Encode, Decode, Deserialize, Serialize)]
 pub struct Split {
-    slot: Slot,
-    state_root: Hash256,
+    pub(crate) slot: Slot,
+    pub(crate) state_root: Hash256,
 }
 
 impl StoreItem for Split {
@@ -1252,8 +1440,8 @@ impl HotStateSummary {
 
 /// Struct for summarising a state in the freezer database.
 #[derive(Debug, Clone, Copy, Default, Encode, Decode)]
-struct ColdStateSummary {
-    slot: Slot,
+pub(crate) struct ColdStateSummary {
+    pub slot: Slot,
 }
 
 impl StoreItem for ColdStateSummary {

beacon_node/store/src/impls.rs

@@ -1,15 +1 @@
-use crate::*;
-use ssz::Encode;
-
 pub mod beacon_state;
-
-/// Prepare a signed beacon block for storage in the database.
-#[must_use]
-pub fn beacon_block_as_kv_store_op<T: EthSpec>(
-    key: &Hash256,
-    block: &SignedBeaconBlock<T>,
-) -> KeyValueStoreOp {
-    // FIXME(altair): re-add block write/overhead metrics, or remove them
-    let db_key = get_key_for_col(DBColumn::BeaconBlock.into(), key.as_bytes());
-    KeyValueStoreOp::PutKeyValue(db_key, block.as_ssz_bytes())
-}

beacon_node/store/src/iter.rs

@@ -1,3 +1,4 @@
+use crate::errors::HandleUnavailable;
 use crate::{Error, HotColdDB, ItemStore};
 use std::borrow::Cow;
 use std::marker::PhantomData;
@@ -201,15 +202,20 @@ impl<'a, T: EthSpec, Hot: ItemStore<T>, Cold: ItemStore<T>> RootsIterator<'a, T,
             (Ok(block_root), Ok(state_root)) => Ok(Some((*block_root, *state_root, self.slot))),
             (Err(BeaconStateError::SlotOutOfBounds), Err(BeaconStateError::SlotOutOfBounds)) => {
                 // Read a `BeaconState` from the store that has access to prior historical roots.
-                let beacon_state =
-                    next_historical_root_backtrack_state(&*self.store, &self.beacon_state)?;
+                if let Some(beacon_state) =
+                    next_historical_root_backtrack_state(&*self.store, &self.beacon_state)
+                        .handle_unavailable()?
+                {
+                    self.beacon_state = Cow::Owned(beacon_state);
 
-                self.beacon_state = Cow::Owned(beacon_state);
+                    let block_root = *self.beacon_state.get_block_root(self.slot)?;
+                    let state_root = *self.beacon_state.get_state_root(self.slot)?;
 
-                let block_root = *self.beacon_state.get_block_root(self.slot)?;
-                let state_root = *self.beacon_state.get_state_root(self.slot)?;
-
-                Ok(Some((block_root, state_root, self.slot)))
+                    Ok(Some((block_root, state_root, self.slot)))
+                } else {
+                    // No more states available due to weak subjectivity sync.
+                    Ok(None)
+                }
             }
             (Err(e), _) => Err(e.into()),
             (Ok(_), Err(e)) => Err(e.into()),
@@ -329,6 +335,9 @@ impl<'a, T: EthSpec, Hot: ItemStore<T>, Cold: ItemStore<T>> Iterator
 }
 
 /// Fetch the next state to use whilst backtracking in `*RootsIterator`.
+///
+/// Return `Err(HistoryUnavailable)` in the case where no more backtrack states are available
+/// due to weak subjectivity sync.
 fn next_historical_root_backtrack_state<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
     store: &HotColdDB<E, Hot, Cold>,
     current_state: &BeaconState<E>,
@@ -338,10 +347,17 @@ fn next_historical_root_backtrack_state<E: EthSpec, Hot: ItemStore<E>, Cold: Ite
     // not frozen, this just means we might not jump back by the maximum amount on
     // our first jump (i.e. at most 1 extra state load).
     let new_state_slot = slot_of_prev_restore_point::<E>(current_state.slot());
-    let new_state_root = current_state.get_state_root(new_state_slot)?;
-    Ok(store
-        .get_state(new_state_root, Some(new_state_slot))?
-        .ok_or_else(|| BeaconStateError::MissingBeaconState((*new_state_root).into()))?)
+
+    let (_, historic_state_upper_limit) = store.get_historic_state_limits();
+
+    if new_state_slot >= historic_state_upper_limit {
+        let new_state_root = current_state.get_state_root(new_state_slot)?;
+        Ok(store
+            .get_state(new_state_root, Some(new_state_slot))?
+            .ok_or_else(|| BeaconStateError::MissingBeaconState((*new_state_root).into()))?)
+    } else {
+        Err(Error::HistoryUnavailable)
+    }
 }
 
 /// Compute the slot of the last guaranteed restore point in the freezer database.

beacon_node/store/src/lib.rs

@@ -10,6 +10,7 @@
 #[macro_use]
 extern crate lazy_static;
 
+mod chunk_writer;
 pub mod chunked_iter;
 pub mod chunked_vector;
 pub mod config;
@@ -23,9 +24,11 @@ mod memory_store;
 pub mod metadata;
 pub mod metrics;
 mod partial_beacon_state;
+pub mod reconstruct;
 
 pub mod iter;
 
+pub use self::chunk_writer::ChunkWriter;
 pub use self::config::StoreConfig;
 pub use self::hot_cold_store::{BlockReplay, HotColdDB, HotStateSummary, Split};
 pub use self::leveldb_store::LevelDB;
@@ -33,6 +36,7 @@ pub use self::memory_store::MemoryStore;
 pub use self::partial_beacon_state::PartialBeaconState;
 pub use errors::Error;
 pub use impls::beacon_state::StorageContainer as BeaconStateStorageContainer;
+pub use metadata::AnchorInfo;
 pub use metrics::scrape_for_metrics;
 use parking_lot::MutexGuard;
 pub use types::*;

beacon_node/store/src/metadata.rs

@@ -1,8 +1,10 @@
 use crate::{DBColumn, Error, StoreItem};
+use serde_derive::{Deserialize, Serialize};
 use ssz::{Decode, Encode};
-use types::{Checkpoint, Hash256};
+use ssz_derive::{Decode, Encode};
+use types::{Checkpoint, Hash256, Slot};
 
-pub const CURRENT_SCHEMA_VERSION: SchemaVersion = SchemaVersion(4);
+pub const CURRENT_SCHEMA_VERSION: SchemaVersion = SchemaVersion(5);
 
 // All the keys that get stored under the `BeaconMeta` column.
 //
@@ -12,6 +14,7 @@ pub const CONFIG_KEY: Hash256 = Hash256::repeat_byte(1);
 pub const SPLIT_KEY: Hash256 = Hash256::repeat_byte(2);
 pub const PRUNING_CHECKPOINT_KEY: Hash256 = Hash256::repeat_byte(3);
 pub const COMPACTION_TIMESTAMP_KEY: Hash256 = Hash256::repeat_byte(4);
+pub const ANCHOR_INFO_KEY: Hash256 = Hash256::repeat_byte(5);
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct SchemaVersion(pub u64);
@@ -76,3 +79,41 @@ impl StoreItem for CompactionTimestamp {
         Ok(CompactionTimestamp(u64::from_ssz_bytes(bytes)?))
     }
 }
+
+/// Database parameters relevant to weak subjectivity sync.
+#[derive(Debug, PartialEq, Eq, Clone, Encode, Decode, Serialize, Deserialize)]
+pub struct AnchorInfo {
+    /// The slot at which the anchor state is present and which we cannot revert.
+    pub anchor_slot: Slot,
+    /// The slot from which historical blocks are available (>=).
+    pub oldest_block_slot: Slot,
+    /// The block root of the next block that needs to be added to fill in the history.
+    ///
+    /// Zero if we know all blocks back to genesis.
+    pub oldest_block_parent: Hash256,
+    /// The slot from which historical states are available (>=).
+    pub state_upper_limit: Slot,
+    /// The slot before which historical states are available (<=).
+    pub state_lower_limit: Slot,
+}
+
+impl AnchorInfo {
+    /// Returns true if the block backfill has completed.
+    pub fn block_backfill_complete(&self) -> bool {
+        self.oldest_block_slot == 0
+    }
+}
+
+impl StoreItem for AnchorInfo {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconMeta
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
+        Ok(Self::from_ssz_bytes(bytes)?)
+    }
+}

beacon_node/store/src/reconstruct.rs

@@ -0,0 +1,160 @@
+//! Implementation of historic state reconstruction (given complete block history).
+use crate::hot_cold_store::{HotColdDB, HotColdDBError};
+use crate::{Error, ItemStore, KeyValueStore};
+use itertools::{process_results, Itertools};
+use slog::info;
+use state_processing::{per_block_processing, per_slot_processing, BlockSignatureStrategy};
+use std::sync::Arc;
+use types::{EthSpec, Hash256};
+
+impl<E, Hot, Cold> HotColdDB<E, Hot, Cold>
+where
+    E: EthSpec,
+    Hot: KeyValueStore<E> + ItemStore<E>,
+    Cold: KeyValueStore<E> + ItemStore<E>,
+{
+    pub fn reconstruct_historic_states(self: &Arc<Self>) -> Result<(), Error> {
+        let mut anchor = if let Some(anchor) = self.get_anchor_info() {
+            anchor
+        } else {
+            // Nothing to do, history is complete.
+            return Ok(());
+        };
+
+        // Check that all historic blocks are known.
+        if anchor.oldest_block_slot != 0 {
+            return Err(Error::MissingHistoricBlocks {
+                oldest_block_slot: anchor.oldest_block_slot,
+            });
+        }
+
+        info!(
+            self.log,
+            "Beginning historic state reconstruction";
+            "start_slot" => anchor.state_lower_limit,
+        );
+
+        let slots_per_restore_point = self.config.slots_per_restore_point;
+
+        // Iterate blocks from the state lower limit to the upper limit.
+        let lower_limit_slot = anchor.state_lower_limit;
+        let split = self.get_split_info();
+        let upper_limit_state = self.get_restore_point(
+            anchor.state_upper_limit.as_u64() / slots_per_restore_point,
+            &split,
+        )?;
+        let upper_limit_slot = upper_limit_state.slot();
+
+        // Use a dummy root, as we never read the block for the upper limit state.
+        let upper_limit_block_root = Hash256::repeat_byte(0xff);
+
+        let block_root_iter = Self::forwards_block_roots_iterator(
+            self.clone(),
+            lower_limit_slot,
+            upper_limit_state,
+            upper_limit_block_root,
+            &self.spec,
+        )?;
+
+        // The state to be advanced.
+        let mut state = self
+            .load_cold_state_by_slot(lower_limit_slot)?
+            .ok_or(HotColdDBError::MissingLowerLimitState(lower_limit_slot))?;
+
+        state.build_all_caches(&self.spec)?;
+
+        process_results(block_root_iter, |iter| -> Result<(), Error> {
+            let mut io_batch = vec![];
+
+            let mut prev_state_root = None;
+
+            for ((prev_block_root, _), (block_root, slot)) in iter.tuple_windows() {
+                let is_skipped_slot = prev_block_root == block_root;
+
+                let block = if is_skipped_slot {
+                    None
+                } else {
+                    Some(
+                        self.get_block(&block_root)?
+                            .ok_or(Error::BlockNotFound(block_root))?,
+                    )
+                };
+
+                // Advance state to slot.
+                per_slot_processing(&mut state, prev_state_root.take(), &self.spec)
+                    .map_err(HotColdDBError::BlockReplaySlotError)?;
+
+                // Apply block.
+                if let Some(block) = block {
+                    per_block_processing(
+                        &mut state,
+                        &block,
+                        Some(block_root),
+                        BlockSignatureStrategy::NoVerification,
+                        &self.spec,
+                    )
+                    .map_err(HotColdDBError::BlockReplayBlockError)?;
+
+                    prev_state_root = Some(block.state_root());
+                }
+
+                let state_root = prev_state_root
+                    .ok_or(())
+                    .or_else(|_| state.update_tree_hash_cache())?;
+
+                // Stage state for storage in freezer DB.
+                self.store_cold_state(&state_root, &state, &mut io_batch)?;
+
+                // If the slot lies on an epoch boundary, commit the batch and update the anchor.
+                if slot % slots_per_restore_point == 0 || slot + 1 == upper_limit_slot {
+                    info!(
+                        self.log,
+                        "State reconstruction in progress";
+                        "slot" => slot,
+                        "remaining" => upper_limit_slot - 1 - slot
+                    );
+
+                    self.cold_db.do_atomically(std::mem::take(&mut io_batch))?;
+
+                    // Update anchor.
+                    let old_anchor = Some(anchor.clone());
+
+                    if slot + 1 == upper_limit_slot {
+                        // The two limits have met in the middle! We're done!
+                        // Perform one last integrity check on the state reached.
+                        let computed_state_root = state.update_tree_hash_cache()?;
+                        if computed_state_root != state_root {
+                            return Err(Error::StateReconstructionRootMismatch {
+                                slot,
+                                expected: state_root,
+                                computed: computed_state_root,
+                            });
+                        }
+
+                        self.compare_and_set_anchor_info(old_anchor, None)?;
+
+                        return Ok(());
+                    } else {
+                        // The lower limit has been raised, store it.
+                        anchor.state_lower_limit = slot;
+
+                        self.compare_and_set_anchor_info(old_anchor, Some(anchor.clone()))?;
+                    }
+                }
+            }
+
+            // Should always reach the `upper_limit_slot` and return early above.
+            Err(Error::StateReconstructionDidNotComplete)
+        })??;
+
+        // Check that the split point wasn't mutated during the state reconstruction process.
+        // It shouldn't have been, due to the serialization of requests through the store migrator,
+        // so this is just a paranoid check.
+        let latest_split = self.get_split_info();
+        if split != latest_split {
+            return Err(Error::SplitPointModified(latest_split.slot, split.slot));
+        }
+
+        Ok(())
+    }
+}