delay pruning of cache for finalized epoch (fixes #2049)

beacon_chain/block_pools/chain_dag.nim

@@ -902,7 +902,8 @@ proc updateHead*(
       while tmp != dag.finalizedHead.blck:
         # leave the epoch cache in the last block of the epoch..
         tmp = tmp.parent
-        tmp.epochRefs = @[]
+        if tmp.parent != nil:
+          tmp.parent.epochRefs = @[]
 
     dag.finalizedHead = finalizedHead
 

beacon_chain/validator_duties.nim

@@ -405,11 +405,9 @@ proc handleAttestations(node: BeaconNode, head: BlockRef, slot: Slot) =
 proc handleProposal(node: BeaconNode, head: BlockRef, slot: Slot):
     Future[BlockRef] {.async.} =
   ## Perform the proposal for the given slot, iff we have a validator attached
-  ## that is supposed to do so, given the shuffling in head
-
-  # TODO here we advance the state to the new slot, but later we'll be
-  #      proposing for it - basically, we're selecting proposer based on an
-  #      empty slot
+  ## that is supposed to do so, given the shuffling at that slot for the given
+  ## head - to compute the proposer, we need to advance a state to the given
+  ## slot
 
   let proposer = node.chainDag.getProposer(head, slot)
   if proposer.isNone():

tests/test_block_pool.nim

@@ -373,7 +373,9 @@ suiteReport "chain DAG finalization tests" & preset():
     check:
       dag.heads.len() == 1
 
-    let headER = dag.heads[0].findEpochRef(dag.heads[0].slot.epoch)
+    let
+      headER = dag.heads[0].findEpochRef(dag.heads[0].slot.epoch)
+      finalER = dag.finalizedHead.blck.findEpochRef(dag.finalizedHead.slot.epoch)
     check:
 
       # Epochrefs should share validator key set when the validator set is
@@ -385,10 +387,14 @@ suiteReport "chain DAG finalization tests" & preset():
       headER.validator_key_store[1] ==
         dag.heads[0].findEpochRef(dag.heads[0].slot.epoch - 1).validator_key_store[1]
 
+      # The EpochRef for the finalized block is needed for eth1 voting, so we
+      # should never drop it!
+      not finalER.isNil
+
     block:
       var cur = dag.heads[0]
       while cur != nil:
-        if cur.slot < dag.finalizedHead.blck.slot:
+        if cur.slot < dag.finalizedHead.blck.parent.slot:
           # Cache should be cleaned on finalization
           check: cur.epochRefs.len == 0
         else: