seed sync committee state cache (#4592)

By pre-seeding the sync committee cache when applying blocks, we avoid a
significantly expensive validator set traversal / sync committee index
construction during sync / block application - 20-30% sync speedup
post-altair.

* also cache/reload total active balance for another cool 10%

beacon_chain/consensus_object_pools/block_pools_types.nim

@@ -271,6 +271,8 @@ type
 
     shufflingRef*: ShufflingRef
 
+    total_active_balance*: Gwei
+
     # balances, as used in fork choice
     effective_balances_bytes*: seq[byte]
 

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -534,6 +534,8 @@ func init*(
 
     attester_dependent_root = withState(state):
       forkyState.attester_dependent_root
+    total_active_balance = withState(state):
+      get_total_active_balance(forkyState.data, cache)
     epochRef = EpochRef(
       key: dag.epochKey(state.latest_block_id, epoch).expect(
         "Valid epoch ancestor when processing state"),
@@ -551,7 +553,8 @@ func init*(
       # beacon_proposers: Separately filled below
       proposer_dependent_root: proposer_dependent_root,
 
-      shufflingRef: shufflingRef
+      shufflingRef: shufflingRef,
+      total_active_balance: total_active_balance
     )
     epochStart = epoch.start_slot()
 
@@ -597,12 +600,28 @@ func loadStateCache(
           let start_slot = epoch.start_slot()
           for i, idx in epochRef[][].beacon_proposers:
             cache.beacon_proposer_indices[start_slot + i] = idx
+          cache.total_active_balance[epoch] = epochRef[][].total_active_balance
 
   load(epoch)
 
   if epoch > 0:
     load(epoch - 1)
 
+  if dag.head != nil: # nil during init.. sigh
+    let period = dag.head.slot.sync_committee_period
+    if period == epoch.sync_committee_period and
+        period notin cache.sync_committees and
+        period > dag.cfg.ALTAIR_FORK_EPOCH.sync_committee_period():
+      # If the block we're aiming for shares ancestry with head, we can reuse
+      # the cached head committee - this accounts for most "live" cases like
+      # syncing and checking blocks since the committees rarely change
+      let periodBsi = dag.atSlot(bid, period.start_slot)
+      if periodBsi.isSome and periodBsi ==
+          dag.atSlot(dag.head.bid, period.start_slot):
+        # We often end up sharing sync committees with head during sync / gossip
+        # validation / head updates
+        cache.sync_committees[period] = dag.headSyncCommittees
+
 func containsForkBlock*(dag: ChainDAGRef, root: Eth2Digest): bool =
   ## Checks for blocks at the finalized checkpoint or newer
   KeyedBlockRef.asLookupKey(root) in dag.forkBlocks
@@ -1014,6 +1033,10 @@ proc init*(T: type ChainDAGRef, cfg: RuntimeConfig, db: BeaconChainDB,
         head = shortLog(head), tail = shortLog(dag.tail)
       quit 1
 
+  withState(dag.headState):
+    when stateFork >= ConsensusFork.Altair:
+      dag.headSyncCommittees = forkyState.data.get_sync_committee_cache(cache)
+
   block:
     # EpochRef needs an epoch boundary state
     assign(dag.epochRefState, dag.headState)
@@ -1170,10 +1193,6 @@ proc init*(T: type ChainDAGRef, cfg: RuntimeConfig, db: BeaconChainDB,
   # have a cache
   dag.updateValidatorKeys(getStateField(dag.headState, validators).asSeq())
 
-  withState(dag.headState):
-    when stateFork >= ConsensusFork.Altair:
-      dag.headSyncCommittees = forkyState.data.get_sync_committee_cache(cache)
-
   info "Block DAG initialized",
     head = shortLog(dag.head),
     finalizedHead = shortLog(dag.finalizedHead),