cache new LC data after creating new LC updates (#5607)

When creating new LC updates, information about the parent block's post
state must be available (cached), but information about current block's
post state is not yet required. Caching information about the current
block's post state can be delayed, simplifying the LC data collection
logic a bit and allowing more future flexibility with the cache design.

beacon_chain/consensus_object_pools/blockchain_dag_light_client.nim

@@ -676,23 +676,26 @@ proc initLightClientDataCache*(dag: ChainDAGRef) =
   let targetTailSlot = max(dag.targetLightClientTailSlot, dag.tail.slot)
   dag.lcDataStore.cache.tailSlot = max(dag.head.slot, targetTailSlot)
 
-  # Import head state
+  # In `OnlyNew` mode, only head state needs to be cached
   if dag.head.slot < dag.lcDataStore.cache.tailSlot:
     return
-  withState(dag.headState):
-    when consensusFork >= ConsensusFork.Altair:
-      dag.cacheLightClientData(forkyState, dag.head.bid)
-    else: raiseAssert "Unreachable" # `tailSlot` cannot be before Altair
   if dag.lcDataStore.importMode == LightClientDataImportMode.OnlyNew:
+    withState(dag.headState):
+      when consensusFork >= ConsensusFork.Altair:
+        dag.cacheLightClientData(forkyState, dag.head.bid)
+      else: raiseAssert "Unreachable" # `tailSlot` cannot be before Altair
     return
 
   # Import light client data for finalized period through finalized head
-  let finalizedSlot = max(dag.finalizedHead.blck.slot, targetTailSlot)
-  if finalizedSlot >= dag.lcDataStore.cache.tailSlot:
-    return
-  dag.lcDataStore.cache.tailSlot = finalizedSlot
-  let finalizedPeriod = finalizedSlot.sync_committee_period
-  var res = dag.initLightClientDataForPeriod(finalizedPeriod)
+  let
+    finalizedSlot = max(dag.finalizedHead.blck.slot, targetTailSlot)
+    finalizedPeriod = finalizedSlot.sync_committee_period
+  var res =
+    if finalizedSlot < dag.lcDataStore.cache.tailSlot:
+      dag.lcDataStore.cache.tailSlot = finalizedSlot
+      dag.initLightClientDataForPeriod(finalizedPeriod)
+    else:
+      Opt[void].ok()
 
   let lightClientStartTick = Moment.now()
   logScope: lightClientDataMaxPeriods = dag.lcDataStore.maxPeriods
@@ -728,14 +731,13 @@ proc initLightClientDataCache*(dag: ChainDAGRef) =
       continue
     withStateAndBlck(dag.headState, bdata):
       when consensusFork >= ConsensusFork.Altair:
-        # Cache light client data (non-finalized blocks may refer to this)
-        if i != blocks.low:
-          dag.cacheLightClientData(forkyState, bid)  # `dag.head` already cached
-
         # Create `LightClientUpdate` instances
         if i < blocks.high:
           dag.createLightClientUpdates(
             forkyState, forkyBlck, parentBid = blocks[i + 1])
+
+        # Cache light client data (non-finalized blocks may refer to this)
+        dag.cacheLightClientData(forkyState, bid)
       else: raiseAssert "Unreachable"
 
   let lightClientEndTick = Moment.now()
@@ -775,8 +777,8 @@ proc processNewBlockForLightClient*(
   const consensusFork = typeof(signedBlock).kind
   when consensusFork >= ConsensusFork.Altair:
     template forkyState: untyped = state.forky(consensusFork)
-    dag.cacheLightClientData(forkyState, signedBlock.toBlockId())
     dag.createLightClientUpdates(forkyState, signedBlock, parentBid)
+    dag.cacheLightClientData(forkyState, signedBlock.toBlockId())
   else:
     raiseAssert "Unreachable"  # `tailSlot` cannot be before Altair