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

Not yet implemented:
- Backfill of historic best `LightClientUpdate`.
- Client side of the protocol.

Known issues:
- Heavy forking for more than a sync committee periods may result in
  incorrect best `LightClientUpdate` computation.

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.

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: 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: 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

@@ -156,6 +156,7 @@ type
     Success
     InvalidRequest
     ServerError
+    ResourceUnavailable
 
   PeerStateInitializer* = proc(peer: Peer): RootRef {.gcsafe, raises: [Defect].}
   NetworkStateInitializer* = proc(network: EthereumNode): RootRef {.gcsafe, raises: [Defect].}
@@ -204,6 +205,8 @@ type
 
   InvalidInputsError* = object of CatchableError
 
+  ResourceUnavailableError* = object of CatchableError
+
   NetRes*[T] = Result[T, Eth2NetworkingError]
     ## This is type returned from all network requests
 
@@ -707,6 +710,13 @@ proc handleIncomingStream(network: Eth2Node,
     template returnInvalidRequest(msg: string) =
       returnInvalidRequest(ErrorMsg msg.toBytes)
 
+    template returnResourceUnavailable(msg: ErrorMsg) =
+      await sendErrorResponse(peer, conn, ResourceUnavailable, msg)
+      return
+
+    template returnResourceUnavailable(msg: string) =
+      returnResourceUnavailable(ErrorMsg msg.toBytes)
+
     let s = when useNativeSnappy:
       let fs = libp2pInput(conn)
 
@@ -771,8 +781,8 @@ proc handleIncomingStream(network: Eth2Node,
       await callUserHandler(MsgType, peer, conn, msg.get)
     except InvalidInputsError as err:
       returnInvalidRequest err.msg
-      await sendErrorResponse(peer, conn, ServerError,
-                              ErrorMsg err.msg.toBytes)
+    except ResourceUnavailableError as err:
+      returnResourceUnavailable err.msg
     except CatchableError as err:
       await sendErrorResponse(peer, conn, ServerError,
                               ErrorMsg err.msg.toBytes)
@@ -2225,3 +2235,17 @@ proc broadcastSignedContributionAndProof*(
     node: Eth2Node, msg: SignedContributionAndProof) =
   let topic = getSyncCommitteeContributionAndProofTopic(node.forkDigests.altair)
   node.broadcast(topic, msg)
+
+proc broadcastOptimisticLightClientUpdate*(
+    node: Eth2Node, msg: 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/networking/faststreams_backend.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).
@@ -112,7 +112,7 @@ proc readResponseChunk(s: AsyncInputStream,
 
   let responseCode = ResponseCode responseCodeByte
   case responseCode:
-  of InvalidRequest, ServerError:
+  of InvalidRequest, ServerError, ResourceUnavailable:
     let errorMsgChunk = await readChunkPayload(s, noSnappy, string)
     let errorMsg = if errorMsgChunk.isOk: errorMsgChunk.value
                    else: return err(errorMsgChunk.error)

beacon_chain/networking/libp2p_streams_backend.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).
@@ -148,7 +148,7 @@ proc readResponseChunk(conn: Connection, peer: Peer,
 
     let responseCode = ResponseCode responseCodeByte
     case responseCode:
-    of InvalidRequest, ServerError:
+    of InvalidRequest, ServerError, ResourceUnavailable:
       let errorMsgChunk = await readChunkPayload(conn, peer, ErrorMsg)
       let errorMsg = if errorMsgChunk.isOk: errorMsgChunk.value
                      else: return err(errorMsgChunk.error)

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: 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

@@ -27,7 +27,7 @@
 import
   std/[typetraits, sets, hashes],
   chronicles,
-  stew/[assign2, bitops2],
+  stew/[assign2, bitops2, objects],
   "."/[base, phase0]
 
 export base, sets
@@ -601,9 +601,24 @@ chronicles.formatIt SyncCommitteeContribution: shortLog(it)
 chronicles.formatIt ContributionAndProof: shortLog(it)
 chronicles.formatIt SignedContributionAndProof: shortLog(it)
 
+func shortLog*(v: LightClientUpdate): auto =
+  (
+    attested_header: shortLog(v.attested_header),
+    finalized_header: shortLog(v.finalized_header),
+    num_active_participants: countOnes(v.sync_aggregate.sync_committee_bits),
+    is_signed_by_next: v.next_sync_committee.isZeroMemory
+  )
+
 template hash*(x: LightClientUpdate): Hash =
   hash(x.header)
 
+func shortLog*(v: OptimisticLightClientUpdate): auto =
+  (
+    attested_header: shortLog(v.attested_header),
+    num_active_participants: countOnes(v.sync_aggregate.sync_committee_bits),
+    is_signed_by_next: v.is_signed_by_next_sync_committee
+  )
+
 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/spec/ssz_codec.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).
@@ -17,7 +17,7 @@ export codec, base, typetraits
 
 # Coding and decoding of SSZ to spec-specific types
 
-template toSszType*(v: Slot|Epoch): auto = uint64(v)
+template toSszType*(v: Slot|Epoch|SyncCommitteePeriod): auto = uint64(v)
 template toSszType*(v: BlsCurveType): auto = toRaw(v)
 template toSszType*(v: ForkDigest|GraffitiBytes): auto = distinctBase(v)
 template toSszType*(v: Version): auto = distinctBase(v)
@@ -34,6 +34,9 @@ template fromSszBytes*(T: type Slot, bytes: openArray[byte]): T =
 template fromSszBytes*(T: type Epoch, bytes: openArray[byte]): T =
   T fromSszBytes(uint64, bytes)
 
+template fromSszBytes*(T: type SyncCommitteePeriod, bytes: openArray[byte]): T =
+  T fromSszBytes(uint64, bytes)
+
 func fromSszBytes*(T: type ForkDigest, bytes: openArray[byte]): T {.raisesssz.} =
   if bytes.len != sizeof(result):
     raiseIncorrectSize T