broadcast optimistic light client updates (#3499)

After proposing a new block, broadcasts a `OptimisticLightClientUpdate`.
Works for both locally proposed blocks as well as VC submitted ones.

beacon_chain/networking/eth2_network.nim

@@ -2355,3 +2355,17 @@ 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/spec/beacon_time.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/[hashes, typetraits],
   chronicles,
@@ -146,6 +150,9 @@ const
   # https://github.com/ethereum/consensus-specs/blob/v1.1.10/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/vFuture/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 +174,8 @@ 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_time*(s: Slot): BeaconTime =
+  s.start_beacon_time + optimisticLightClientUpdateSlotOffset
 
 func slotOrZero*(time: BeaconTime): Slot =
   let exSlot = time.toSlot

beacon_chain/validators/validator_duties.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
   # Standard library
   std/[os, osproc, sequtils, streams, tables],
@@ -738,6 +742,19 @@ proc handleSyncCommitteeMessages(node: BeaconNode, head: BlockRef, slot: Slot) =
       asyncSpawn createAndSendSyncCommitteeMessage(node, slot, validator,
                                                    subcommitteeIdx, head)
 
+proc handleOptimisticLightClientUpdates(
+    node: BeaconNode, head: BlockRef, slot: Slot) =
+  if slot < node.dag.cfg.ALTAIR_FORK_EPOCH.start_slot():
+    return
+  doAssert head.parent != nil, "Newly proposed block lacks parent reference"
+  let msg = node.dag.lightClientCache.optimisticUpdate
+  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,
@@ -1070,7 +1087,10 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
 
     curSlot += 1
 
-  head = await handleProposal(node, head, slot)
+  let
+    newHead = await handleProposal(node, head, slot)
+    didSubmitBlock = (newHead != head)
+  head = newHead
 
   let
     # The latest point in time when we'll be sending out attestations
@@ -1120,6 +1140,16 @@ proc handleValidatorDuties*(node: BeaconNode, lastSlot, slot: Slot) {.async.} =
   handleAttestations(node, head, slot)
   handleSyncCommitteeMessages(node, head, slot)
 
+  if node.config.serveLightClientData and didSubmitBlock:
+    let cutoff = node.beaconClock.fromNow(
+      slot.optimistic_light_client_update_time())
+    if cutoff.inFuture:
+      debug "Waiting to send optimistic light client update",
+        head = shortLog(head),
+        optimisticLightClientUpdateCutoff = shortLog(cutoff.offset)
+      await sleepAsync(cutoff.offset)
+    handleOptimisticLightClientUpdates(node, head, slot)
+
   updateValidatorMetrics(node) # the important stuff is done, update the vanity numbers
 
   # https://github.com/ethereum/consensus-specs/blob/v1.1.10/specs/phase0/validator.md#broadcast-aggregate
@@ -1277,6 +1307,25 @@ proc sendBeaconBlock*(node: BeaconNode, forked: ForkedSignedBeaconBlock
         notice "Block published",
           blockRoot = shortLog(blck.root), blck = shortLog(blck.message),
           signature = shortLog(blck.signature)
+
+        if node.config.serveLightClientData:
+          # The optimistic light client update is sent with a delay because it
+          # only validates once the new block has been processed by the peers.
+          # https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#block-proposal
+          proc publishOptimisticLightClientUpdate() {.async.} =
+            let cutoff = node.beaconClock.fromNow(
+              wallTime.slotOrZero.optimistic_light_client_update_time())
+            if cutoff.inFuture:
+              debug "Waiting to publish optimistic light client update",
+                blockRoot = shortLog(blck.root), blck = shortLog(blck.message),
+                signature = shortLog(blck.signature),
+                optimisticLightClientUpdateCutoff = shortLog(cutoff.offset)
+              await sleepAsync(cutoff.offset)
+            handleOptimisticLightClientUpdates(
+              node, newBlockRef.get, wallTime.slotOrZero)
+
+          asyncSpawn publishOptimisticLightClientUpdate()
+
         true
       else:
         warn "Unable to add proposed block to block pool",