Builder Specs v0.2.0 (#3134)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/3091

Extends https://github.com/sigp/lighthouse/pull/3062, adding pre-bellatrix block support on blinded endpoints and allowing the normal proposal flow (local payload construction) on blinded endpoints. This resulted in better fallback logic because the VC will not have to switch endpoints on failure in the BN <> Builder API, the BN can just fallback immediately and without repeating block processing that it shouldn't need to. We can also keep VC fallback from the VC<>BN API's blinded endpoint to full endpoint.

## Proposed Changes

- Pre-bellatrix blocks on blinded endpoints
- Add a new `PayloadCache` to the execution layer
- Better fallback-from-builder logic

## Todos

- [x] Remove VC transition logic
- [x] Add logic to only enable builder flow after Merge transition finalization
- [x] Tests
- [x] Fix metrics
- [x] Rustdocs


Co-authored-by: Mac L <mjladson@pm.me>
Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -53,7 +53,9 @@ use crate::BeaconForkChoiceStore;
 use crate::BeaconSnapshot;
 use crate::{metrics, BeaconChainError};
 use eth2::types::{EventKind, SseBlock, SyncDuty};
-use execution_layer::{ExecutionLayer, PayloadAttributes, PayloadStatus};
+use execution_layer::{
+    BuilderParams, ChainHealth, ExecutionLayer, FailedCondition, PayloadAttributes, PayloadStatus,
+};
 use fork_choice::{
     AttestationFromBlock, ExecutionStatus, ForkChoice, ForkchoiceUpdateParameters,
     InvalidationOperation, PayloadVerificationStatus,
@@ -3315,10 +3317,21 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
         let proposer_index = state.get_beacon_proposer_index(state.slot(), &self.spec)? as u64;
 
-        let pubkey_opt = state
+        let pubkey = state
             .validators()
             .get(proposer_index as usize)
-            .map(|v| v.pubkey);
+            .map(|v| v.pubkey)
+            .ok_or(BlockProductionError::BeaconChain(
+                BeaconChainError::ValidatorIndexUnknown(proposer_index as usize),
+            ))?;
+
+        let builder_params = BuilderParams {
+            pubkey,
+            slot: state.slot(),
+            chain_health: self
+                .is_healthy()
+                .map_err(BlockProductionError::BeaconChain)?,
+        };
 
         // If required, start the process of loading an execution payload from the EL early. This
         // allows it to run concurrently with things like attestation packing.
@@ -3326,7 +3339,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             BeaconState::Base(_) | BeaconState::Altair(_) => None,
             BeaconState::Merge(_) => {
                 let prepare_payload_handle =
-                    get_execution_payload(self.clone(), &state, proposer_index, pubkey_opt)?;
+                    get_execution_payload(self.clone(), &state, proposer_index, builder_params)?;
                 Some(prepare_payload_handle)
             }
         };
@@ -4539,6 +4552,74 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .map(|duration| (fork_name, duration))
     }
 
+    /// This method serves to get a sense of the current chain health. It is used in block proposal
+    /// to determine whether we should outsource payload production duties.
+    ///
+    /// Since we are likely calling this during the slot we are going to propose in, don't take into
+    /// account the current slot when accounting for skips.
+    pub fn is_healthy(&self) -> Result<ChainHealth, Error> {
+        // Check if the merge has been finalized.
+        if let Some(finalized_hash) = self
+            .canonical_head
+            .cached_head()
+            .forkchoice_update_parameters()
+            .finalized_hash
+        {
+            if ExecutionBlockHash::zero() == finalized_hash {
+                return Ok(ChainHealth::PreMerge);
+            }
+        } else {
+            return Ok(ChainHealth::PreMerge);
+        };
+
+        if self.config.builder_fallback_disable_checks {
+            return Ok(ChainHealth::Healthy);
+        }
+
+        let current_slot = self.slot()?;
+
+        // Check slots at the head of the chain.
+        let prev_slot = current_slot.saturating_sub(Slot::new(1));
+        let head_skips = prev_slot.saturating_sub(self.canonical_head.cached_head().head_slot());
+        let head_skips_check = head_skips.as_usize() <= self.config.builder_fallback_skips;
+
+        // Check if finalization is advancing.
+        let current_epoch = current_slot.epoch(T::EthSpec::slots_per_epoch());
+        let epochs_since_finalization = current_epoch.saturating_sub(
+            self.canonical_head
+                .cached_head()
+                .finalized_checkpoint()
+                .epoch,
+        );
+        let finalization_check = epochs_since_finalization.as_usize()
+            <= self.config.builder_fallback_epochs_since_finalization;
+
+        // Check skip slots in the last `SLOTS_PER_EPOCH`.
+        let start_slot = current_slot.saturating_sub(T::EthSpec::slots_per_epoch());
+        let mut epoch_skips = 0;
+        for slot in start_slot.as_u64()..current_slot.as_u64() {
+            if self
+                .block_root_at_slot_skips_none(Slot::new(slot))?
+                .is_none()
+            {
+                epoch_skips += 1;
+            }
+        }
+        let epoch_skips_check = epoch_skips <= self.config.builder_fallback_skips_per_epoch;
+
+        if !head_skips_check {
+            Ok(ChainHealth::Unhealthy(FailedCondition::Skips))
+        } else if !finalization_check {
+            Ok(ChainHealth::Unhealthy(
+                FailedCondition::EpochsSinceFinalization,
+            ))
+        } else if !epoch_skips_check {
+            Ok(ChainHealth::Unhealthy(FailedCondition::SkipsPerEpoch))
+        } else {
+            Ok(ChainHealth::Healthy)
+        }
+    }
+
     pub fn dump_as_dot<W: Write>(&self, output: &mut W) {
         let canonical_head_hash = self.canonical_head.cached_head().head_block_root();
         let mut visited: HashSet<Hash256> = HashSet::new();