Backfiller

Backfilling is the process of downloading historical blocks via P2P that
are required to fulfill `GetBlocksByRange` duties - this happens during
both trusted node and finalized checkpoint syncs.

In particular, backfilling happens after syncing to head, such that
attestation work can start as soon as possible.

beacon_chain/beacon_node.nim

@@ -60,6 +60,7 @@ type
     vcProcess*: Process
     requestManager*: RequestManager
     syncManager*: SyncManager[Peer, PeerID]
+    backfiller*: SyncManager[Peer, PeerID]
     genesisSnapshotContent*: string
     actionTracker*: ActionTracker
     processor*: ref Eth2Processor

beacon_chain/conf.nim

@@ -232,12 +232,10 @@ type
         name: "weak-subjectivity-checkpoint" }: Option[Checkpoint]
 
       finalizedCheckpointState* {.
-        hidden # TODO unhide when backfilling is done
         desc: "SSZ file specifying a recent finalized state"
         name: "finalized-checkpoint-state" }: Option[InputFile]
 
       finalizedCheckpointBlock* {.
-        hidden # TODO unhide when backfilling is done
         desc: "SSZ file specifying a recent finalized block"
         name: "finalized-checkpoint-block" }: Option[InputFile]
 

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -1664,3 +1664,6 @@ proc getBlockSSZ*(dag: ChainDAGRef, id: BlockId, bytes: var seq[byte]): bool =
     dag.db.getAltairBlockSSZ(id.root, bytes)
   of BeaconBlockFork.Bellatrix:
     dag.db.getMergeBlockSSZ(id.root, bytes)
+
+func needsBackfill*(dag: ChainDAGRef): bool =
+  dag.backfill.slot > dag.genesis.slot

beacon_chain/nimbus_beacon_node.nim

@@ -469,6 +469,10 @@ proc init*(T: type BeaconNode,
     syncManager = newSyncManager[Peer, PeerID](
       network.peerPool, SyncQueueKind.Forward, getLocalHeadSlot, getLocalWallSlot,
       getFirstSlotAtFinalizedEpoch, getBackfillSlot, blockVerifier)
+    backfiller = newSyncManager[Peer, PeerID](
+      network.peerPool, SyncQueueKind.Backward, getLocalHeadSlot, getLocalWallSlot,
+      getFirstSlotAtFinalizedEpoch, getBackfillSlot, blockVerifier,
+      maxHeadAge = 0)
 
   let stateTtlCache = if config.restCacheSize > 0:
     StateTtlCache.init(
@@ -499,6 +503,7 @@ proc init*(T: type BeaconNode,
     eventBus: eventBus,
     requestManager: RequestManager.init(network, blockVerifier),
     syncManager: syncManager,
+    backfiller: backfiller,
     actionTracker: ActionTracker.init(rng, config.subscribeAllSubnets),
     processor: processor,
     blockProcessor: blockProcessor,
@@ -916,6 +921,11 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
   # above, this will be done just before the next slot starts
   await node.updateGossipStatus(slot + 1)
 
+proc syncStatus(node: BeaconNode): string =
+  if node.syncManager.inProgress: node.syncManager.syncStatus
+  elif node.backfiller.inProgress: "backfill: " & node.backfiller.syncStatus
+  else: "synced"
+
 proc onSlotStart(
     node: BeaconNode, wallTime: BeaconTime, lastSlot: Slot) {.async.} =
   ## Called at the beginning of a slot - usually every slot, but sometimes might
@@ -936,9 +946,7 @@ proc onSlotStart(
   info "Slot start",
     slot = shortLog(wallSlot),
     epoch = shortLog(wallSlot.epoch),
-    sync =
-      if node.syncManager.inProgress: node.syncManager.syncStatus
-      else: "synced",
+    sync = node.syncStatus(),
     peers = len(node.network.peerPool),
     head = shortLog(node.dag.head),
     finalized = shortLog(getStateField(
@@ -1127,6 +1135,18 @@ proc stop(node: BeaconNode) =
   node.db.close()
   notice "Databases closed"
 
+proc startBackfillTask(node: BeaconNode) {.async.} =
+  while node.dag.needsBackfill:
+    if not node.syncManager.inProgress:
+      # Only start the backfiller if it's needed _and_ head sync has completed -
+      # if we lose sync after having synced head, we could stop the backfilller,
+      # but this should be a fringe case - might as well keep the logic simple for
+      # now
+      node.backfiller.start()
+      return
+
+    await sleepAsync(chronos.seconds(2))
+
 proc run(node: BeaconNode) {.raises: [Defect, CatchableError].} =
   bnStatus = BeaconNodeStatus.Running
 
@@ -1150,6 +1170,8 @@ proc run(node: BeaconNode) {.raises: [Defect, CatchableError].} =
   node.requestManager.start()
   node.syncManager.start()
 
+  if node.dag.needsBackfill(): asyncSpawn node.startBackfillTask()
+
   waitFor node.updateGossipStatus(wallSlot)
 
   asyncSpawn runSlotLoop(node, wallTime, onSlotStart)
@@ -1327,13 +1349,7 @@ proc initStatusBar(node: BeaconNode) {.raises: [Defect, ValueError].} =
       formatGwei(node.attachedValidatorBalanceTotal)
 
     of "sync_status":
-      if isNil(node.syncManager):
-        "pending"
-      else:
-        if node.syncManager.inProgress:
-          node.syncManager.syncStatus
-        else:
-          "synced"
+      node.syncStatus()
     else:
       # We ignore typos for now and just render the expression
       # as it was written. TODO: come up with a good way to show