attest to known valid block when possible (#5313)

* attest to known valid block when possible

* cleaner approach; slot is always == attestation slot itself

* copyright year linting

beacon_chain/beacon_node.nim

@@ -100,6 +100,7 @@ type
     dutyValidatorCount*: int
       ## Number of validators that we've checked for activation
     processingDelay*: Opt[Duration]
+    lastValidAttestedBlock*: Opt[BlockSlot]
 
 const
   MaxEmptySlotCount* = uint64(10*60) div SECONDS_PER_SLOT

beacon_chain/rpc/rest_validator_api.nim

@@ -736,9 +736,17 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
               return RestApiResponse.jsonError(Http503, BeaconNodeInSyncError,
                                                $res.error())
             let tres = res.get()
-            if not tres.executionValid:
-              return RestApiResponse.jsonError(Http503, BeaconNodeInSyncError)
-            tres
+            if tres.executionValid:
+              tres
+            else:
+              let qbs = node.lastValidAttestedBlock.valueOr:
+                return RestApiResponse.jsonError(
+                  Http503, BeaconNodeInSyncError)
+              if qbs.blck.slot > qslot:
+                return RestApiResponse.jsonError(
+                  Http503, BeaconNodeInSyncError)
+              qbs.blck
+
         let epochRef = node.dag.getEpochRef(qhead, qslot.epoch, true).valueOr:
           return RestApiResponse.jsonError(Http400, PrunedStateError, $error)
         makeAttestationData(epochRef, qhead.atSlot(qslot), qindex)

beacon_chain/validators/beacon_validators.nim

@@ -1785,6 +1785,23 @@ proc updateValidators(
             index: index, validator: validators[int index]
           )))
 
+proc handleFallbackAttestations(node: BeaconNode, lastSlot, slot: Slot) =
+  # Neither block proposal nor sync committee duties can be done in this
+  # situation.
+  let attestationHead = node.lastValidAttestedBlock.valueOr:
+    return
+
+  if attestationHead.slot + SLOTS_PER_EPOCH < slot:
+    return
+
+  # Using `slot` and `curSlot` here ensure that the attestation data created
+  # is for the wall slot, regardless of attestationHead's block and slot.
+  for curSlot in (lastSlot + 1) ..< slot:
+    notice "Catching up on fallback attestation duties", curSlot, slot
+    handleAttestations(node, attestationHead.blck, curSlot)
+
+  handleAttestations(node, attestationHead.blck, slot)
+
 proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
   ## Perform validator duties - create blocks, vote and aggregate existing votes
   if node.attachedValidators[].count == 0:
@@ -1807,6 +1824,8 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
     info "Execution client not in sync; skipping validator duties for now",
       slot, headSlot = head.slot
 
+    handleFallbackAttestations(node, lastSlot, slot)
+
     # Rewards will be growing though, as we sync..
     updateValidatorMetrics(node)
 
@@ -1814,6 +1833,8 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
   else:
     discard # keep going
 
+  node.lastValidAttestedBlock = Opt.some head.atSlot()
+
   withState(node.dag.headState):
     node.updateValidators(forkyState.data.validators.asSeq())