add hooks for observing LC progress (#4401)

For Fluffy injection, add observer callbacks that get called whenever
new light client data is sucecssfully processed.

```
  proc onLightClientObject(
      lightClient: LightClient, obj: SomeLightClientObject) =
    info "New LC object", obj

  lightClient.bootstrapObserver =
    proc(lightClient: LightClient, obj: altair.LightClientBootstrap) =
      lightClient.onLightClientObject(obj)
  lightClient.updateObserver =
    proc(lightClient: LightClient, obj: altair.LightClientUpdate) =
      lightClient.onLightClientObject(obj)
  lightClient.finalityUpdateObserver =
    proc(lightClient: LightClient, obj: altair.LightClientFinalityUpdate) =
      lightClient.onLightClientObject(obj)
  lightClient.optimisticUpdateObserver =
    proc(lightClient: LightClient, obj: altair.LightClientOptimisticUpdate) =
      lightClient.onLightClientObject(obj)
```

beacon_chain/gossip_processing/light_client_processor.nim

@@ -31,11 +31,24 @@ declareHistogram light_client_store_object_duration_seconds,
   "storeObject() duration", buckets = [0.25, 0.5, 1, 2, 4, 8, Inf]
 
 type
+  Nothing = object
+
   GetTrustedBlockRootCallback* =
     proc(): Option[Eth2Digest] {.gcsafe, raises: [Defect].}
   VoidCallback* =
     proc() {.gcsafe, raises: [Defect].}
 
+  ValueObserver[V] =
+    proc(v: V) {.gcsafe, raises: [Defect].}
+  BootstrapObserver* =
+    ValueObserver[altair.LightClientBootstrap]
+  UpdateObserver* =
+    ValueObserver[altair.LightClientUpdate]
+  FinalityUpdateObserver* =
+    ValueObserver[altair.LightClientFinalityUpdate]
+  OptimisticUpdateObserver* =
+    ValueObserver[altair.LightClientOptimisticUpdate]
+
   LightClientFinalizationMode* {.pure.} = enum
     Strict
       ## Only finalize light client data that:
@@ -91,6 +104,10 @@ type
     getBeaconTime: GetBeaconTimeFn
     getTrustedBlockRoot: GetTrustedBlockRootCallback
     onStoreInitialized, onFinalizedHeader, onOptimisticHeader: VoidCallback
+    bootstrapObserver: BootstrapObserver
+    updateObserver: UpdateObserver
+    finalityUpdateObserver: FinalityUpdateObserver
+    optimisticUpdateObserver: OptimisticUpdateObserver
 
     cfg: RuntimeConfig
     genesis_validators_root: Eth2Digest
@@ -127,7 +144,11 @@ proc new*(
     getTrustedBlockRoot: GetTrustedBlockRootCallback,
     onStoreInitialized: VoidCallback = nil,
     onFinalizedHeader: VoidCallback = nil,
-    onOptimisticHeader: VoidCallback = nil
+    onOptimisticHeader: VoidCallback = nil,
+    bootstrapObserver: BootstrapObserver = nil,
+    updateObserver: UpdateObserver = nil,
+    finalityUpdateObserver: FinalityUpdateObserver = nil,
+    optimisticUpdateObserver: OptimisticUpdateObserver = nil
 ): ref LightClientProcessor =
   (ref LightClientProcessor)(
     dumpEnabled: dumpEnabled,
@@ -139,6 +160,10 @@ proc new*(
     onStoreInitialized: onStoreInitialized,
     onFinalizedHeader: onFinalizedHeader,
     onOptimisticHeader: onOptimisticHeader,
+    bootstrapObserver: bootstrapObserver,
+    updateObserver: updateObserver,
+    finalityUpdateObserver: finalityUpdateObserver,
+    optimisticUpdateObserver: optimisticUpdateObserver,
     cfg: cfg,
     genesis_validators_root: genesis_validators_root,
     finalizationMode: finalizationMode)
@@ -252,10 +277,16 @@ proc processObject(
 
   res
 
-template withReportedProgress(expectFinalityUpdate: bool, body: untyped): bool =
+template withReportedProgress(
+    obj: SomeLightClientObject | Nothing, body: untyped): bool =
   block:
     let
       previousWasInitialized = store[].isSome
+      previousNextCommitteeKnown =
+        if store[].isSome:
+          store[].get.is_next_sync_committee_known
+        else:
+          false
       previousFinalized =
         if store[].isSome:
           store[].get.finalized_header
@@ -269,29 +300,55 @@ template withReportedProgress(expectFinalityUpdate: bool, body: untyped): bool =
 
     body
 
-    var didProgress = false
+    var
+      didProgress = false
+      didSignificantProgress = false
 
     if store[].isSome != previousWasInitialized:
       didProgress = true
+      didSignificantProgress = true
       if self.onStoreInitialized != nil:
         self.onStoreInitialized()
         self.onStoreInitialized = nil
 
-    if store[].get.optimistic_header != previousOptimistic:
-      when not expectFinalityUpdate:
+    if store[].isSome:
+      if store[].get.optimistic_header != previousOptimistic:
         didProgress = true
-      if self.onOptimisticHeader != nil:
-        self.onOptimisticHeader()
+        when obj isnot SomeLightClientUpdateWithFinality:
+          didSignificantProgress = true
+        if self.onOptimisticHeader != nil:
+          self.onOptimisticHeader()
 
-    if store[].get.finalized_header != previousFinalized:
-      didProgress = true
-      if self.onFinalizedHeader != nil:
-        self.onFinalizedHeader()
+      if store[].get.finalized_header != previousFinalized:
+        didProgress = true
+        didSignificantProgress = true
+        if self.onFinalizedHeader != nil:
+          self.onFinalizedHeader()
 
-    didProgress
+      if store[].get.is_next_sync_committee_known != previousNextCommitteeKnown:
+        didProgress = true
+
+    if didProgress:
+      when obj is Nothing:
+        discard
+      elif obj is altair.LightClientBootstrap:
+        if self.bootstrapObserver != nil:
+          self.bootstrapObserver(obj)
+      elif obj is altair.LightClientUpdate:
+        if self.updateObserver != nil:
+          self.updateObserver(obj)
+      elif obj is altair.LightClientFinalityUpdate:
+        if self.finalityUpdateObserver != nil:
+          self.finalityUpdateObserver(obj)
+      elif obj is altair.LightClientOptimisticUpdate:
+        if self.optimisticUpdateObserver != nil:
+          self.optimisticUpdateObserver(obj)
+      else: raiseAssert "Unreachable"
+
+    didSignificantProgress
 
 template withReportedProgress(body: untyped): bool =
-  withReportedProgress(false, body)
+  withReportedProgress(Nothing(), body)
 
 proc storeObject*(
     self: var LightClientProcessor,
@@ -304,8 +361,8 @@ proc storeObject*(
     startTick = Moment.now()
     store = self.store
 
-    didProgress =
-      withReportedProgress(obj is SomeLightClientUpdateWithFinality):
+    didSignificantProgress =
+      withReportedProgress(obj):
         ? self.processObject(obj, wallTime)
 
         let
@@ -328,7 +385,7 @@ proc storeObject*(
           kind = typeof(obj).name,
           objectSlot = objSlot,
           storeObjectDur
-  ok didProgress
+  ok didSignificantProgress
 
 proc resetToFinalizedHeader*(
     self: var LightClientProcessor,
@@ -393,8 +450,8 @@ func toValidationError(
     wallTime: BeaconTime,
     obj: SomeLightClientObject): Result[void, ValidationError] =
   if r.isOk:
-    let didProgress = r.get
-    if didProgress:
+    let didSignificantProgress = r.get
+    if didSignificantProgress:
       let
         signature_slot = obj.signature_slot
         currentTime = wallTime + MAXIMUM_GOSSIP_CLOCK_DISPARITY

beacon_chain/light_client.nim

@@ -29,6 +29,17 @@ type
     proc(lightClient: LightClient, header: BeaconBlockHeader) {.
       gcsafe, raises: [Defect].}
 
+  LightClientValueObserver[V] =
+    proc(lightClient: LightClient, v: V) {.gcsafe, raises: [Defect].}
+  LightClientBootstrapObserver* =
+    LightClientValueObserver[altair.LightClientBootstrap]
+  LightClientUpdateObserver* =
+    LightClientValueObserver[altair.LightClientUpdate]
+  LightClientFinalityUpdateObserver* =
+    LightClientValueObserver[altair.LightClientFinalityUpdate]
+  LightClientOptimisticUpdateObserver* =
+    LightClientValueObserver[altair.LightClientOptimisticUpdate]
+
   LightClient* = ref object
     network: Eth2Node
     cfg: RuntimeConfig
@@ -39,6 +50,10 @@ type
     manager: LightClientManager
     gossipState: GossipState
     onFinalizedHeader*, onOptimisticHeader*: LightClientHeaderCallback
+    bootstrapObserver*: LightClientBootstrapObserver
+    updateObserver*: LightClientUpdateObserver
+    finalityUpdateObserver*: LightClientFinalityUpdateObserver
+    optimisticUpdateObserver*: LightClientOptimisticUpdateObserver
     trustedBlockRoot*: Option[Eth2Digest]
 
 func finalizedHeader*(lightClient: LightClient): Opt[BeaconBlockHeader] =
@@ -94,11 +109,28 @@ proc createLightClient(
       lightClient.onOptimisticHeader(
         lightClient, lightClient.optimisticHeader.get)
 
+  proc bootstrapObserver(obj: altair.LightClientBootstrap) =
+    if lightClient.bootstrapObserver != nil:
+      lightClient.bootstrapObserver(lightClient, obj)
+
+  proc updateObserver(obj: altair.LightClientUpdate) =
+    if lightClient.updateObserver != nil:
+      lightClient.updateObserver(lightClient, obj)
+
+  proc finalityObserver(obj: altair.LightClientFinalityUpdate) =
+    if lightClient.finalityUpdateObserver != nil:
+      lightClient.finalityUpdateObserver(lightClient, obj)
+
+  proc optimisticObserver(obj: altair.LightClientOptimisticUpdate) =
+    if lightClient.optimisticUpdateObserver != nil:
+      lightClient.optimisticUpdateObserver(lightClient, obj)
+
   lightClient.processor = LightClientProcessor.new(
     dumpEnabled, dumpDirInvalid, dumpDirIncoming,
     cfg, genesis_validators_root, finalizationMode,
     lightClient.store, getBeaconTime, getTrustedBlockRoot,
-    onStoreInitialized, onFinalizedHeader, onOptimisticHeader)
+    onStoreInitialized, onFinalizedHeader, onOptimisticHeader,
+    bootstrapObserver, updateObserver, finalityObserver, optimisticObserver)
 
   proc lightClientVerifier(obj: SomeLightClientObject):
       Future[Result[void, VerifierError]] =