libp2p light client gossip validation (#3486)

When `--serve-light-client-data` is specified, provides stability on the
`optimistic_light_client_update` GossipSub topic.

beacon_chain/gossip_processing/eth2_processor.nim

@@ -7,6 +7,10 @@
 
 {.push raises: [Defect].}
 
+# References to `vFuture` refer to the pre-release proposal of the libp2p based
+# light client sync protocol. Conflicting release versions are not in use.
+# https://github.com/ethereum/consensus-specs/pull/2802
+
 import
   std/tables,
   stew/results, bearssl,
@@ -58,6 +62,10 @@ declareCounter beacon_sync_committee_contributions_received,
   "Number of valid sync committee contributions processed by this node"
 declareCounter beacon_sync_committee_contributions_dropped,
   "Number of invalid sync committee contributions dropped by this node", labels = ["reason"]
+declareCounter beacon_optimistic_light_client_updates_received,
+  "Number of valid optimistic light client updates processed by this node"
+declareCounter beacon_optimistic_light_client_updates_dropped,
+  "Number of invalid optimistic light client updates dropped by this node", labels = ["reason"]
 
 const delayBuckets = [2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, Inf]
 
@@ -529,3 +537,24 @@ proc contributionValidator*(
     beacon_sync_committee_contributions_dropped.inc(1, [$v.error[0]])
 
     err(v.error())
+
+# https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#optimistic_light_client_update
+proc optimisticLightClientUpdateValidator*(
+    self: var Eth2Processor, src: MsgSource,
+    optimistic_update: OptimisticLightClientUpdate
+): Result[void, ValidationError] =
+  logScope:
+    optimistic_update
+
+  debug "Optimistic light client update received"
+
+  let v = self.dag.validateOptimisticLightClientUpdate(optimistic_update)
+  if v.isOk():
+    trace "Optimistic light client update validated"
+
+    beacon_optimistic_light_client_updates_received.inc()
+  else:
+    debug "Dropping optimistic light client update", error = v.error
+    beacon_optimistic_light_client_updates_dropped.inc(1, [$v.error[0]])
+
+  v

beacon_chain/gossip_processing/gossip_validation.nim

@@ -7,6 +7,10 @@
 
 {.push raises: [Defect].}
 
+# References to `vFuture` refer to the pre-release proposal of the libp2p based
+# light client sync protocol. Conflicting release versions are not in use.
+# https://github.com/ethereum/consensus-specs/pull/2802
+
 import
   # Status
   chronicles, chronos, metrics,
@@ -1028,3 +1032,20 @@ proc validateContribution*(
     sig.get()
 
   return ok((sig, participants))
+
+# https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#optimistic_light_client_update
+proc validateOptimisticLightClientUpdate*(
+    dag: ChainDAGRef, optimistic_update: OptimisticLightClientUpdate):
+    Result[void, ValidationError] =
+  template local_update(): auto = dag.lightClientCache.optimisticUpdate
+
+  if optimistic_update != local_update:
+    # [IGNORE] The optimistic update is not attesting to the latest block's
+    # parent block.
+    if optimistic_update.attested_header != local_update.attested_header:
+      return errIgnore("OptimisticLightClientUpdate: not attesting to latest")
+
+    # [REJECT] The optimistic update does not match the expected value.
+    return errReject("OptimisticLightClientUpdate: not matching expected value")
+
+  ok()

beacon_chain/nimbus_beacon_node.nim

@@ -814,6 +814,10 @@ proc addAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest, slot: Sl
 
   node.network.updateSyncnetsMetadata(currentSyncCommitteeSubnets)
 
+  if node.config.serveLightClientData:
+    node.network.subscribe(
+      getOptimisticLightClientUpdateTopic(forkDigest), basicParams)
+
 proc removeAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest) =
   node.removePhase0MessageHandlers(forkDigest)
 
@@ -825,6 +829,9 @@ proc removeAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest) =
   node.network.unsubscribe(
     getSyncCommitteeContributionAndProofTopic(forkDigest))
 
+  if node.config.serveLightClientData:
+    node.network.unsubscribe(getOptimisticLightClientUpdateTopic(forkDigest))
+
 proc trackCurrentSyncCommitteeTopics(node: BeaconNode, slot: Slot) =
   # Unlike trackNextSyncCommitteeTopics, just snap to the currently correct
   # set of subscriptions, and use current_sync_committee. Furthermore, this
@@ -1323,6 +1330,21 @@ proc installMessageValidators(node: BeaconNode) =
   installSyncCommitteeeValidators(node.dag.forkDigests.altair)
   installSyncCommitteeeValidators(node.dag.forkDigests.bellatrix)
 
+  template installOptimisticLightClientUpdateValidator(digest: auto) =
+    node.network.addValidator(
+      getOptimisticLightClientUpdateTopic(digest),
+      proc(msg: OptimisticLightClientUpdate): ValidationResult =
+        if node.config.serveLightClientData:
+          toValidationResult(
+            node.processor[].optimisticLightClientUpdateValidator(
+              MsgSource.gossip, msg))
+        else:
+          debug "Ignoring optimistic light client update: Feature disabled"
+          ValidationResult.Ignore)
+
+  installOptimisticLightClientUpdateValidator(node.dag.forkDigests.altair)
+  installOptimisticLightClientUpdateValidator(node.dag.forkDigests.bellatrix)
+
 proc stop(node: BeaconNode) =
   bnStatus = BeaconNodeStatus.Stopping
   notice "Graceful shutdown"

beacon_chain/spec/network.nim

@@ -7,6 +7,10 @@
 
 {.push raises: [Defect].}
 
+# References to `vFuture` refer to the pre-release proposal of the libp2p based
+# light client sync protocol. Conflicting release versions are not in use.
+# https://github.com/ethereum/consensus-specs/pull/2802
+
 import
   "."/[helpers, forks],
   "."/datatypes/base
@@ -94,6 +98,11 @@ func getSyncCommitteeContributionAndProofTopic*(forkDigest: ForkDigest): string
   ## For subscribing and unsubscribing to/from a subnet.
   eth2Prefix(forkDigest) & "sync_committee_contribution_and_proof/ssz_snappy"
 
+# https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#optimistic_light_client_update
+func getOptimisticLightClientUpdateTopic*(forkDigest: ForkDigest): string =
+  ## For broadcasting or obtaining the latest `OptimisticLightClientUpdate`.
+  eth2Prefix(forkDigest) & "optimistic_light_client_update_v0/ssz_snappy"
+
 func getENRForkID*(cfg: RuntimeConfig,
                    epoch: Epoch,
                    genesis_validators_root: Eth2Digest): ENRForkID =