harden sync sub committee selection (#2965)

* harden sync sub committee selection

also turn it into an iterator

* fix test and warning

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -1027,19 +1027,16 @@ proc pruneBlocksDAG(dag: ChainDAGRef) =
     prunedHeads = hlen - dag.heads.len,
     dagPruneDur = Moment.now() - startTick
 
-func syncSubcommittee*(syncCommittee: openarray[ValidatorPubKey],
-                       committeeIdx: SyncCommitteeIndex): seq[ValidatorPubKey] =
-  ## TODO Return a view type
-  ## Unfortunately, this doesn't work as a template right now.
-  if syncCommittee.len == 0:
-    return @[]
+iterator syncSubcommittee*(
+    syncCommittee: openarray[ValidatorPubKey],
+    committeeIdx: SyncCommitteeIndex): ValidatorPubKey =
+  var
+    i = committeeIdx.asInt * SYNC_SUBCOMMITTEE_SIZE
+    onePastEndIdx = min(syncCommittee.len, i + SYNC_SUBCOMMITTEE_SIZE)
 
-  let
-    startIdx = committeeIdx.asInt * SYNC_SUBCOMMITTEE_SIZE
-    onePastEndIdx = startIdx + SYNC_SUBCOMMITTEE_SIZE
-  doAssert startIdx < syncCommittee.len
-
-  @(toOpenArray(syncCommittee, startIdx, onePastEndIdx - 1))
+  while i < onePastEndIdx:
+    yield syncCommittee[i]
+    inc i
 
 func syncCommitteeParticipants*(dagParam: ChainDAGRef,
                                 slotParam: Slot): seq[ValidatorPubKey] =
@@ -1081,7 +1078,7 @@ func getSubcommitteePositionsAux(
     return @[]
   let validatorPubKey = validatorKey.get().toPubKey
 
-  for pos, key in syncCommittee.syncSubcommittee(committeeIdx):
+  for pos, key in toSeq(syncCommittee.syncSubcommittee(committeeIdx)):
     if validatorPubKey == key:
       result.add uint64(pos)
 

beacon_chain/spec/forks.nim

@@ -68,7 +68,7 @@ type
     of BeaconBlockFork.Merge:
       mergeBlock*:  merge.TrustedSignedBeaconBlock
 
-  ForkDigests* {.requiresInit.} = object
+  ForkDigests* = object
     phase0*: ForkDigest
     altair*: ForkDigest
     merge*:  ForkDigest

tests/test_gossip_validation.nim

@@ -178,13 +178,13 @@ suite "Gossip validation " & preset():
 suite "Gossip validation - Extra": # Not based on preset config
   test "validateSyncCommitteeMessage":
     const num_validators = SLOTS_PER_EPOCH
-    let 
+    let
       cfg = block:
         var cfg = defaultRuntimeConfig
         cfg.ALTAIR_FORK_EPOCH = (GENESIS_EPOCH + 1).Epoch
         cfg
       dag = block:
-        let 
+        let
           dag = ChainDAGRef.init(cfg, makeTestDB(num_validators), {})
           taskpool = Taskpool.new()
           quarantine = QuarantineRef.init(keys.newRng(), taskpool)
@@ -192,7 +192,7 @@ suite "Gossip validation - Extra": # Not based on preset config
         for blck in makeTestBlocks(
             dag.headState.data, dag.head.root, cache,
             int(SLOTS_PER_EPOCH), false, cfg = cfg):
-          let added = 
+          let added =
             case blck.kind
             of BeaconBlockFork.Phase0:
               const nilCallback = OnPhase0BlockAdded(nil)
@@ -210,7 +210,7 @@ suite "Gossip validation - Extra": # Not based on preset config
 
       syncCommitteeIdx = 0.SyncCommitteeIndex
       syncCommittee = @(dag.syncCommitteeParticipants(state[].data.slot))
-      subcommittee = syncCommittee.syncSubcommittee(syncCommitteeIdx)
+      subcommittee = toSeq(syncCommittee.syncSubcommittee(syncCommitteeIdx))
 
       pubkey = subcommittee[0]
       expectedCount = subcommittee.count(pubkey)
@@ -225,7 +225,7 @@ suite "Gossip validation - Extra": # Not based on preset config
 
       syncCommitteeMsgPool = newClone(SyncCommitteeMsgPool.init())
       res = validateSyncCommitteeMessage(
-        dag, syncCommitteeMsgPool, msg, syncCommitteeIdx, 
+        dag, syncCommitteeMsgPool, msg, syncCommitteeIdx,
         state[].data.slot.toBeaconTime(), true)
       contribution = block:
         var contribution: SyncCommitteeContribution