serve libp2p protocol for light client sync

This extends the `--serve-light-client-data` launch option to serve
locally collected light client data via libp2p.
See ethereum/consensus-specs#2802

Backfill of historic best `LightClientUpdate` data is not implemented.

To test, in `conf.nim` change `serveLightClientData`'s `defaultValue` to
`true`, then run this command:
```
scripts/launch_local_testnet.sh --kill-old-processes --preset minimal \
    --nodes 4 --disable-htop --stop-at-epoch 7
```
The log files of the beacon nodes will be in the `local_testnet_data`
directory. They are named `log0.txt` through `log3.txt`. The logs can be
browsed for light client related messages.

Author: etan-status

beacon_chain/gossip_processing/eth2_processor.nim

@@ -58,6 +58,10 @@ 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]
 
@@ -528,3 +532,23 @@ proc contributionValidator*(
     beacon_sync_committee_contributions_dropped.inc(1, [$v.error[0]])
 
     err(v.error())
+
+proc optimisticLightClientUpdateValidator*(
+    self: var Eth2Processor, src: MsgSource,
+    optimistic_update: altair.OptimisticLightClientUpdate
+): Result[void, ValidationError] =
+  logScope:
+    optimisticUpdate = shortLog(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

@@ -984,3 +984,20 @@ proc validateContribution*(
     sig.get()
 
   return ok((sig, participants))
+
+# https://github.com/ethereum/consensus-specs/blob/v1.1.8/specs/altair/sync-protocol.md#optimistic_light_client_update
+proc validateOptimisticLightClientUpdate*(
+    dag: ChainDAGRef, optimistic_update: altair.OptimisticLightClientUpdate):
+    Result[void, ValidationError] =
+  template latest_local_update(): auto = dag.optimisticLightClientUpdate
+
+  if optimistic_update != latest_local_update:
+    # [IGNORE] The optimistic update is not attesting to the latest block's
+    # parent block.
+    if optimistic_update.attested_header != latest_local_update.attested_header:
+      return errIgnore("OptimisticLightClientUpdate: different attested block")
+
+    # [REJECT] The optimistic update does not match the expected value.
+    return errReject("OptimisticLightClientUpdate: update does not match block")
+
+  ok()

beacon_chain/networking/eth2_network.nim

@@ -2225,3 +2225,17 @@ proc broadcastSignedContributionAndProof*(
     node: Eth2Node, msg: SignedContributionAndProof) =
   let topic = getSyncCommitteeContributionAndProofTopic(node.forkDigests.altair)
   node.broadcast(topic, msg)
+
+proc broadcastOptimisticLightClientUpdate*(
+    node: Eth2Node, msg: altair.OptimisticLightClientUpdate) =
+  let
+    forkDigest =
+      if msg.fork_version == node.cfg.SHARDING_FORK_VERSION:
+        node.forkDigests.sharding
+      elif msg.fork_version == node.cfg.BELLATRIX_FORK_VERSION:
+        node.forkDigests.bellatrix
+      else:
+        doAssert msg.fork_version == node.cfg.ALTAIR_FORK_VERSION
+        node.forkDigests.altair
+    topic = getOptimisticLightClientUpdateTopic(forkDigest)
+  node.broadcast(topic, msg)

beacon_chain/nimbus_beacon_node.nim

@@ -730,6 +730,10 @@ proc addAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest, slot: Sl
     getSyncCommitteeContributionAndProofTopic(forkDigest), basicParams)
   node.network.updateSyncnetsMetadata(syncnets)
 
+  if node.config.serveLightClientData:
+    node.network.subscribe(
+      getOptimisticLightClientUpdateTopic(forkDigest), basicParams)
+
 proc removeAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest) =
   node.removePhase0MessageHandlers(forkDigest)
 
@@ -742,6 +746,9 @@ proc removeAltairMessageHandlers(node: BeaconNode, forkDigest: ForkDigest) =
   node.network.unsubscribe(
     getSyncCommitteeContributionAndProofTopic(forkDigest))
 
+  if node.config.serveLightClientData:
+    node.network.unsubscribe(getOptimisticLightClientUpdateTopic(forkDigest))
+
 proc trackSyncCommitteeTopics*(node: BeaconNode) =
   # TODO
   discard
@@ -1149,6 +1156,18 @@ proc installMessageValidators(node: BeaconNode) =
   installSyncCommitteeeValidators(node.dag.forkDigests.altair)
   installSyncCommitteeeValidators(node.dag.forkDigests.bellatrix)
 
+  if node.config.serveLightClientData:
+    template installOptimisticLightClientUpdateValidator(digest: auto) =
+      node.network.addValidator(
+        getOptimisticLightClientUpdateTopic(digest),
+        proc(msg: altair.OptimisticLightClientUpdate): ValidationResult =
+          toValidationResult(
+            node.processor[].optimisticLightClientUpdateValidator(
+              MsgSource.gossip, msg)))
+
+    installOptimisticLightClientUpdateValidator(node.dag.forkDigests.altair)
+    installOptimisticLightClientUpdateValidator(node.dag.forkDigests.bellatrix)
+
 proc stop(node: BeaconNode) =
   bnStatus = BeaconNodeStatus.Stopping
   notice "Graceful shutdown"

beacon_chain/spec/beacon_time.nim

@@ -1,5 +1,5 @@
 # beacon_chain
-# Copyright (c) 2018-2021 Status Research & Development GmbH
+# Copyright (c) 2018-2022 Status Research & Development GmbH
 # Licensed and distributed under either of
 #   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
 #   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
@@ -146,6 +146,9 @@ const
   # https://github.com/ethereum/consensus-specs/blob/v1.1.8/specs/altair/validator.md#broadcast-sync-committee-contribution
   syncContributionSlotOffset* = TimeDiff(nanoseconds:
     NANOSECONDS_PER_SLOT.int64  * 2 div INTERVALS_PER_SLOT)
+  # https://github.com/ethereum/consensus-specs/blob/v1.1.8/specs/altair/sync-protocol.md#block-proposal
+  optimisticLightClientUpdateSlotOffset* = TimeDiff(nanoseconds:
+    NANOSECONDS_PER_SLOT.int64 div INTERVALS_PER_SLOT)
 
 func toFloatSeconds*(t: TimeDiff): float =
   float(t.nanoseconds) / 1_000_000_000.0
@@ -167,6 +170,8 @@ func sync_committee_message_deadline*(s: Slot): BeaconTime =
   s.start_beacon_time + syncCommitteeMessageSlotOffset
 func sync_contribution_deadline*(s: Slot): BeaconTime =
   s.start_beacon_time + syncContributionSlotOffset
+func optimistic_light_client_update_deadline*(s: Slot): BeaconTime =
+  s.start_beacon_time + optimisticLightClientUpdateSlotOffset
 
 func slotOrZero*(time: BeaconTime): Slot =
   let exSlot = time.toSlot

beacon_chain/spec/datatypes/altair.nim

@@ -604,6 +604,12 @@ chronicles.formatIt SignedContributionAndProof: shortLog(it)
 template hash*(x: LightClientUpdate): Hash =
   hash(x.header)
 
+func shortLog*(v: OptimisticLightClientUpdate): auto =
+  (
+    attested_header: shortLog(v.attested_header),
+    sync_aggregate: shortLog(v.sync_aggregate)
+  )
+
 func clear*(info: var EpochInfo) =
   info.validators.setLen(0)
   info.balances = UnslashedParticipatingBalances()

beacon_chain/spec/network.nim

@@ -94,6 +94,11 @@ 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/v1.1.8/specs/altair/sync-protocol.md#optimistic_light_client_update
+func getOptimisticLightClientUpdateTopic*(forkDigest: ForkDigest): string =
+  ## For broadcasting the latest `OptimisticLightClientUpdate` to light clients.
+  eth2Prefix(forkDigest) & "optimistic_light_client_update/ssz_snappy"
+
 func getENRForkID*(cfg: RuntimeConfig,
                    epoch: Epoch,
                    genesis_validators_root: Eth2Digest): ENRForkID =

beacon_chain/validators/validator_duties.nim

@@ -739,6 +739,18 @@ proc handleSyncCommitteeMessages(node: BeaconNode, head: BlockRef, slot: Slot) =
       asyncSpawn createAndSendSyncCommitteeMessage(node, slot, validator,
                                                    subcommitteeIdx, head)
 
+proc handleOptimisticLightClientUpdates(
+    node: BeaconNode, head: BlockRef, slot: Slot, didPropose: bool) =
+  if not didPropose:
+    return
+  let msg = node.dag.optimisticLightClientUpdate
+  if msg.attested_header.slot != head.parent.bid.slot:
+    notice "No optimistic light client update for proposed block",
+      slot = slot, block_root = shortLog(head.root)
+    return
+  node.network.broadcastOptimisticLightClientUpdate(msg)
+  notice "Sent optimistic light client update", message = shortLog(msg)
+
 proc signAndSendContribution(node: BeaconNode,
                              validator: AttachedValidator,
                              contribution: SyncCommitteeContribution,
@@ -841,14 +853,14 @@ proc handleSyncCommitteeContributions(node: BeaconNode,
               slot, head, subnet_id = candidateAggregators[i].subcommitteeIdx
 
 proc handleProposal(node: BeaconNode, head: BlockRef, slot: Slot):
-    Future[BlockRef] {.async.} =
+    Future[tuple[head: BlockRef, didPropose: bool]] {.async.} =
   ## Perform the proposal for the given slot, iff we have a validator attached
   ## that is supposed to do so, given the shuffling at that slot for the given
   ## head - to compute the proposer, we need to advance a state to the given
   ## slot
   let proposer = node.dag.getProposer(head, slot)
   if proposer.isNone():
-    return head
+    return (head: head, didPropose: false)
 
   let
     proposerKey = node.dag.validatorKey(proposer.get).get().toPubKey
@@ -862,9 +874,12 @@ proc handleProposal(node: BeaconNode, head: BlockRef, slot: Slot):
         proposer_index = proposer.get(),
         proposer = shortLog(proposerKey)
 
-      head
+      (head: head, didPropose: false)
     else:
-      await proposeBlock(node, validator, proposer.get(), head, slot)
+      (
+        head: await proposeBlock(node, validator, proposer.get(), head, slot),
+        didPropose: true
+      )
 
 proc makeAggregateAndProof*(
     pool: var AttestationPool, epochRef: EpochRef, slot: Slot, index: CommitteeIndex,
@@ -1052,6 +1067,7 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
 
   # Start by checking if there's work we should have done in the past that we
   # can still meaningfully do
+  var didPropose = false
   while curSlot < slot:
     notice "Catching up on validator duties",
       curSlot = shortLog(curSlot),
@@ -1061,7 +1077,7 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
     # For every slot we're catching up, we'll propose then send
     # attestations - head should normally be advancing along the same branch
     # in this case
-    head = await handleProposal(node, head, curSlot)
+    (head, didPropose) = await handleProposal(node, head, curSlot)
 
     # For each slot we missed, we need to send out attestations - if we were
     # proposing during this time, we'll use the newly proposed head, else just
@@ -1071,7 +1087,7 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
 
     curSlot += 1
 
-  head = await handleProposal(node, head, slot)
+  (head, didPropose) = await handleProposal(node, head, slot)
 
   let
     # The latest point in time when we'll be sending out attestations
@@ -1116,11 +1132,16 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
     node.consensusManager[].updateHead(slot)
     head = node.dag.head
 
-  static: doAssert attestationSlotOffset == syncCommitteeMessageSlotOffset
-
+  static:
+    doAssert attestationSlotOffset == syncCommitteeMessageSlotOffset
   handleAttestations(node, head, slot)
   handleSyncCommitteeMessages(node, head, slot)
 
+  if node.config.serveLightClientData:
+    static:
+      doAssert attestationSlotOffset == optimisticLightClientUpdateSlotOffset
+    handleOptimisticLightClientUpdates(node, head, slot, didPropose)
+
   updateValidatorMetrics(node) # the important stuff is done, update the vanity numbers
 
   # https://github.com/ethereum/consensus-specs/blob/v1.1.8/specs/phase0/validator.md#broadcast-aggregate