Deposit Cache Finalization & Fast WS Sync (#2915)

## Summary

The deposit cache now has the ability to finalize deposits. This will cause it to drop unneeded deposit logs and hashes in the deposit Merkle tree that are no longer required to construct deposit proofs. The cache is finalized whenever the latest finalized checkpoint has a new `Eth1Data` with all deposits imported.

This has three benefits:

1. Improves the speed of constructing Merkle proofs for deposits as we can just replay deposits since the last finalized checkpoint instead of all historical deposits when re-constructing the Merkle tree.
2. Significantly faster weak subjectivity sync as the deposit cache can be transferred to the newly syncing node in compressed form. The Merkle tree that stores `N` finalized deposits requires a maximum of `log2(N)` hashes. The newly syncing node then only needs to download deposits since the last finalized checkpoint to have a full tree.
3. Future proofing in preparation for [EIP-4444](https://eips.ethereum.org/EIPS/eip-4444) as execution nodes will no longer be required to store logs permanently so we won't always have all historical logs available to us.

## More Details

Image to illustrate how the deposit contract merkle tree evolves and finalizes along with the resulting `DepositTreeSnapshot`
![image](https://user-images.githubusercontent.com/37123614/151465302-5fc56284-8a69-4998-b20e-45db3934ac70.png)

## Other Considerations

I've changed the structure of the `SszDepositCache` so once you load & save your database from this version of lighthouse, you will no longer be able to load it from older versions.

Co-authored-by: ethDreamer <37123614+ethDreamer@users.noreply.github.com>

beacon_node/eth1/src/service.rs

@@ -20,7 +20,7 @@ use std::path::PathBuf;
 use std::sync::Arc;
 use std::time::{SystemTime, UNIX_EPOCH};
 use tokio::time::{interval_at, Duration, Instant};
-use types::{ChainSpec, EthSpec, Unsigned};
+use types::{ChainSpec, DepositTreeSnapshot, Eth1Data, EthSpec, Unsigned};
 
 /// Indicates the default eth1 chain id we use for the deposit contract.
 pub const DEFAULT_CHAIN_ID: Eth1Id = Eth1Id::Goerli;
@@ -63,7 +63,13 @@ async fn endpoint_state(
     config_chain_id: &Eth1Id,
     log: &Logger,
 ) -> EndpointState {
-    let error_connecting = |e| {
+    let error_connecting = |e: String| {
+        debug!(
+            log,
+            "eth1 endpoint error";
+            "endpoint" => %endpoint,
+            "error" => &e,
+        );
         warn!(
             log,
             "Error connecting to eth1 node endpoint";
@@ -213,6 +219,10 @@ pub enum Error {
     GetDepositLogsFailed(String),
     /// There was an unexpected internal error.
     Internal(String),
+    /// Error finalizing deposit
+    FailedToFinalizeDeposit(String),
+    /// There was a problem Initializing from deposit snapshot
+    FailedToInitializeFromSnapshot(String),
 }
 
 /// The success message for an Eth1Data cache update.
@@ -395,6 +405,7 @@ impl Service {
                     config.deposit_contract_deploy_block,
                 )),
                 endpoint: endpoint_from_config(&config)?,
+                to_finalize: RwLock::new(None),
                 remote_head_block: RwLock::new(None),
                 config: RwLock::new(config),
                 spec,
@@ -407,6 +418,36 @@ impl Service {
         &self.inner.endpoint
     }
 
+    /// Creates a new service, initializing the deposit tree from a snapshot.
+    pub fn from_deposit_snapshot(
+        config: Config,
+        log: Logger,
+        spec: ChainSpec,
+        deposit_snapshot: &DepositTreeSnapshot,
+    ) -> Result<Self, Error> {
+        let deposit_cache =
+            DepositUpdater::from_snapshot(config.deposit_contract_deploy_block, deposit_snapshot)
+                .map_err(Error::FailedToInitializeFromSnapshot)?;
+
+        Ok(Self {
+            inner: Arc::new(Inner {
+                block_cache: <_>::default(),
+                deposit_cache: RwLock::new(deposit_cache),
+                endpoint: endpoint_from_config(&config)
+                    .map_err(Error::FailedToInitializeFromSnapshot)?,
+                to_finalize: RwLock::new(None),
+                remote_head_block: RwLock::new(None),
+                config: RwLock::new(config),
+                spec,
+            }),
+            log,
+        })
+    }
+
+    pub fn set_to_finalize(&self, eth1_data: Option<Eth1Data>) {
+        *(self.inner.to_finalize.write()) = eth1_data;
+    }
+
     /// Returns the follow distance that has been shortened to accommodate for differences in the
     /// spacing between blocks.
     ///
@@ -521,7 +562,7 @@ impl Service {
         let deposits = self.deposits().read();
         deposits
             .cache
-            .get_valid_signature_count(deposits.cache.latest_block_number()?)
+            .get_valid_signature_count(deposits.cache.latest_block_number())
     }
 
     /// Returns the number of deposits with valid signatures that have been observed up to and
@@ -619,7 +660,8 @@ impl Service {
                     "old_block_number" => deposit_cache.last_processed_block,
                     "new_block_number" => deposit_cache.cache.latest_block_number(),
                 );
-                deposit_cache.last_processed_block = deposit_cache.cache.latest_block_number();
+                deposit_cache.last_processed_block =
+                    Some(deposit_cache.cache.latest_block_number());
             }
 
             let outcome =
@@ -698,6 +740,37 @@ impl Service {
                 "deposits" => format!("{:?}", deposit),
             ),
         };
+        let optional_eth1data = self.inner.to_finalize.write().take();
+        if let Some(eth1data_to_finalize) = optional_eth1data {
+            let already_finalized = self
+                .inner
+                .deposit_cache
+                .read()
+                .cache
+                .finalized_deposit_count();
+            let deposit_count_to_finalize = eth1data_to_finalize.deposit_count;
+            if deposit_count_to_finalize > already_finalized {
+                match self.finalize_deposits(eth1data_to_finalize) {
+                    Err(e) => error!(
+                        self.log,
+                        "Failed to finalize deposit cache";
+                        "error" => ?e,
+                    ),
+                    Ok(()) => info!(
+                        self.log,
+                        "Successfully finalized deposit tree";
+                        "finalized deposit count" => deposit_count_to_finalize,
+                    ),
+                }
+            } else {
+                debug!(
+                    self.log,
+                    "Deposits tree already finalized";
+                    "already_finalized" => already_finalized,
+                    "deposit_count_to_finalize" => deposit_count_to_finalize,
+                );
+            }
+        }
         Ok(())
     }
 
@@ -733,6 +806,30 @@ impl Service {
         )
     }
 
+    pub fn finalize_deposits(&self, eth1_data: Eth1Data) -> Result<(), Error> {
+        let eth1_block = self
+            .inner
+            .block_cache
+            .read()
+            .block_by_hash(&eth1_data.block_hash)
+            .cloned()
+            .ok_or_else(|| {
+                Error::FailedToFinalizeDeposit(
+                    "Finalized block not found in block cache".to_string(),
+                )
+            })?;
+        self.inner
+            .deposit_cache
+            .write()
+            .cache
+            .finalize(eth1_block)
+            .map_err(|e| Error::FailedToFinalizeDeposit(format!("{:?}", e)))
+    }
+
+    pub fn get_deposit_snapshot(&self) -> Option<DepositTreeSnapshot> {
+        self.inner.deposit_cache.read().cache.get_deposit_snapshot()
+    }
+
     /// Contacts the remote eth1 node and attempts to import deposit logs up to the configured
     /// follow-distance block.
     ///