don't rely on head updates for topic subscription decision

beacon_chain/beacon_node.nim

@@ -467,7 +467,7 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
   # Syncing tends to be ~1 block/s, and allow for an epoch of time for libp2p
   # subscribing to spin up. The faster the sync, the more wallSlot - headSlot
   # lead time is required.
-  const GOSSIP_ACTIVATION_THRESHOLD_SLOTS = 64
+  const TOPIC_SUBSCRIBE_THRESHOLD_SLOTS = 96
 
   if slot > lastSlot + SLOTS_PER_EPOCH:
     # We've fallen behind more than an epoch - there's nothing clever we can
@@ -526,6 +526,7 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
     finalizedHead = shortLog(node.chainDag.finalizedHead.blck),
     finalizedEpoch = shortLog(node.chainDag.finalizedHead.blck.slot.compute_epoch_at_slot())
 
+  let syncQueueLen = node.syncManager.syncQueueLen
   if
       # Don't enable if already enabled; to avoid race conditions requires care,
       # but isn't crucial, as this condition spuriously fail, but the next time,
@@ -533,13 +534,22 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
       node.attestationSubnets.subscribedSubnets[0].len +
       node.attestationSubnets.subscribedSubnets[1].len == 0 and
       # SyncManager forward sync by default runs until maxHeadAge slots, or one
-      # epoch range is achieved.
-      slot < node.chainDag.head.slot + GOSSIP_ACTIVATION_THRESHOLD_SLOTS:
+      # epoch range is achieved. This particular condition has a couple caveats
+      # including that under certain conditions, debtsCount appears to push len
+      # (here, syncQueueLen) to underflow-like values; and even when exactly at
+      # the expected walltime slot the queue isn't necessarily empty. Therefore
+      # TOPIC_SUBSCRIBE_THRESHOLD_SLOTS is not exactly the number of slots that
+      # are left. Furthermore, even when 0 peers are being used, this won't get
+      # to 0 slots in syncQueueLen, but that's a vacuous condition given that a
+      # networking interaction cannot happen under such circumstances.
+      syncQueueLen < TOPIC_SUBSCRIBE_THRESHOLD_SLOTS:
     # When node.cycleAttestationSubnets() is enabled more properly, integrate
     # this into the node.cycleAttestationSubnets() call.
     debug "Enabling topic subscriptions",
       wallSlot = slot,
-      headSlot = node.chainDag.head.slot
+      headSlot = node.chainDag.head.slot,
+      syncQueueLen
+
     await node.addMessageHandlers()
 
   when false:

beacon_chain/sync_manager.nim

@@ -653,6 +653,9 @@ template checkPeerScore(peer, body: untyped): untyped =
           topics = "syncman"
     break
 
+func syncQueueLen*[A, B](man: SyncManager[A, B]): uint64 =
+  man.queue.len
+
 proc syncWorker*[A, B](man: SyncManager[A, B],
                        peer: A): Future[A] {.async.} =
   # Sync worker is the lowest level loop which performs syncing with single