inhibit LC sync while DAG is synced (#6505)

Normally, running LC and DAG sync at same time is fine, but on tiny
devnet where some peer may not support the LC data, we can end up in
situation where peer gets disconnected when DAG is in sync, because
DAG sync never uses any req/resp on local devnet (perfect nw conditions)
so the LC sync over minutes removes the peer as sync is stuck.

We don't need to actively sync LC from network if DAG is already synced,
preventing this specific low peer devnet issue (there are others still).
LC is still locally updated when DAG finalized checkpoint advances.

beacon_chain/beacon_node_light_client.nim

@@ -52,9 +52,15 @@ proc initLightClient*(
     optimisticProcessor = initOptimisticProcessor(
       getBeaconTime, optimisticHandler)
 
+    shouldInhibitSync = func(): bool =
+      if node.syncManager != nil:
+        not node.syncManager.inProgress  # No LC sync needed if DAG is in sync
+      else:
+        false
     lightClient = createLightClient(
       node.network, rng, config, cfg, forkDigests, getBeaconTime,
-      genesis_validators_root, LightClientFinalizationMode.Strict)
+      genesis_validators_root, LightClientFinalizationMode.Strict,
+      shouldInhibitSync = shouldInhibitSync)
 
   if config.syncLightClient:
     proc onOptimisticHeader(

beacon_chain/light_client.nim

@@ -86,7 +86,8 @@ proc createLightClient(
     getBeaconTime: GetBeaconTimeFn,
     genesis_validators_root: Eth2Digest,
     finalizationMode: LightClientFinalizationMode,
-    strictVerification = false
+    strictVerification = false,
+    shouldInhibitSync: light_client_manager.GetBoolCallback = nil
 ): LightClient =
   let lightClient = LightClient(
     network: network,
@@ -177,7 +178,8 @@ proc createLightClient(
     lightClient.network, rng, getTrustedBlockRoot,
     bootstrapVerifier, updateVerifier, finalityVerifier, optimisticVerifier,
     isLightClientStoreInitialized, isNextSyncCommitteeKnown,
-    getFinalizedPeriod, getOptimisticPeriod, getBeaconTime)
+    getFinalizedPeriod, getOptimisticPeriod, getBeaconTime,
+    shouldInhibitSync = shouldInhibitSync)
 
   lightClient.gossipState = {}
 
@@ -191,13 +193,15 @@ proc createLightClient*(
     forkDigests: ref ForkDigests,
     getBeaconTime: GetBeaconTimeFn,
     genesis_validators_root: Eth2Digest,
-    finalizationMode: LightClientFinalizationMode
+    finalizationMode: LightClientFinalizationMode,
+    shouldInhibitSync: light_client_manager.GetBoolCallback = nil
 ): LightClient =
   createLightClient(
     network, rng,
     config.dumpEnabled, config.dumpDirInvalid, config.dumpDirIncoming,
     cfg, forkDigests, getBeaconTime, genesis_validators_root, finalizationMode,
-    strictVerification = config.strictVerification)
+    strictVerification = config.strictVerification,
+    shouldInhibitSync = shouldInhibitSync)
 
 proc createLightClient*(
     network: Eth2Node,
@@ -207,12 +211,14 @@ proc createLightClient*(
     forkDigests: ref ForkDigests,
     getBeaconTime: GetBeaconTimeFn,
     genesis_validators_root: Eth2Digest,
-    finalizationMode: LightClientFinalizationMode
+    finalizationMode: LightClientFinalizationMode,
+    shouldInhibitSync: light_client_manager.GetBoolCallback = nil
 ): LightClient =
   createLightClient(
     network, rng,
     dumpEnabled = false, dumpDirInvalid = ".", dumpDirIncoming = ".",
-    cfg, forkDigests, getBeaconTime, genesis_validators_root, finalizationMode)
+    cfg, forkDigests, getBeaconTime, genesis_validators_root, finalizationMode,
+    shouldInhibitSync = shouldInhibitSync)
 
 proc start*(lightClient: LightClient) =
   notice "Starting light client",

beacon_chain/sync/light_client_manager.nim

@@ -65,6 +65,7 @@ type
     getFinalizedPeriod: GetSyncCommitteePeriodCallback
     getOptimisticPeriod: GetSyncCommitteePeriodCallback
     getBeaconTime: GetBeaconTimeFn
+    shouldInhibitSync: GetBoolCallback
     loopFuture: Future[void].Raising([CancelledError])
 
 func init*(
@@ -80,7 +81,8 @@ func init*(
     isNextSyncCommitteeKnown: GetBoolCallback,
     getFinalizedPeriod: GetSyncCommitteePeriodCallback,
     getOptimisticPeriod: GetSyncCommitteePeriodCallback,
-    getBeaconTime: GetBeaconTimeFn
+    getBeaconTime: GetBeaconTimeFn,
+    shouldInhibitSync: GetBoolCallback = nil
 ): LightClientManager =
   ## Initialize light client manager.
   LightClientManager(
@@ -95,8 +97,8 @@ func init*(
     isNextSyncCommitteeKnown: isNextSyncCommitteeKnown,
     getFinalizedPeriod: getFinalizedPeriod,
     getOptimisticPeriod: getOptimisticPeriod,
-    getBeaconTime: getBeaconTime
-  )
+    getBeaconTime: getBeaconTime,
+    shouldInhibitSync: shouldInhibitSync)
 
 proc isGossipSupported*(
     self: LightClientManager,
@@ -335,6 +337,7 @@ proc loop(self: LightClientManager) {.async: (raises: [CancelledError]).} =
     # Periodically wake and check for changes
     let wallTime = self.getBeaconTime()
     if wallTime < nextSyncTaskTime or
+        (self.shouldInhibitSync != nil and self.shouldInhibitSync()) or
         self.network.peerPool.lenAvailable < 1:
       await sleepAsync(chronos.seconds(2))
       continue