Fix block dag pruning frequency (#2469)

Should always prune after finality change but not more than once

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -874,7 +874,7 @@ proc delState(dag: ChainDAGRef, bs: BlockSlot) =
 proc pruneBlocksDAG(dag: ChainDAGRef) =
   ## This prunes the block DAG
   ## This does NOT prune the cached state checkpoints and EpochRef
-  ## This should be done after a new finalization point is reached
+  ## This must be done after a new finalization point is reached
   ## to invalidate pending blocks or attestations referring
   ## to a now invalid fork.
   ##
@@ -882,7 +882,6 @@ proc pruneBlocksDAG(dag: ChainDAGRef) =
   ## as the caches and fork choice can be pruned at a later time.
 
   # Clean up block refs, walking block by block
-  if dag.lastPrunePoint != dag.finalizedHead:
   let start = Moment.now()
 
   # Finalization means that we choose a single chain as the canonical one -
@@ -897,10 +896,6 @@ proc pruneBlocksDAG(dag: ChainDAGRef) =
 
     var cur = head.atSlot(head.slot)
     while not cur.blck.isAncestorOf(dag.finalizedHead.blck):
-        # TODO there may be more empty states here: those that have a slot
-        #      higher than head.slot and those near the branch point - one
-        #      needs to be careful though because those close to the branch
-        #      point should not necessarily be cleaned up
       dag.delState(cur) # TODO: should we move that disk I/O to `onSlotEnd`
 
       if cur.blck.slot == cur.slot:
@@ -918,6 +913,7 @@ proc pruneBlocksDAG(dag: ChainDAGRef) =
 
   debug "Pruned the blockchain DAG",
     currentCandidateHeads = dag.heads.len,
+    prunedHeads = hlen - dag.heads.len,
     dagPruningDuration = dur
 
 func needStateCachesAndForkChoicePruning*(dag: ChainDAGRef): bool =
@@ -1069,6 +1065,9 @@ proc updateHead*(
     beacon_finalized_root.set(
       dag.headState.data.data.finalized_checkpoint.root.toGaugeValue)
 
+    # Pruning the block dag is required every time the finalized head changes
+    # in order to clear out blocks that are no longer viable and should
+    # therefore no longer be considered as part of the chain we're following
     dag.pruneBlocksDAG()
 
 proc isInitialized*(T: type ChainDAGRef, db: BeaconChainDB): bool =