initial dynamic subscribe/unsubscribe for attestations to/from subnets (#1462)

* initial dynamic subscribe/unsubscribe for attestations to/from subnets

* implement random stability subnet and clean up

* switch from HashSet[uint64] to set[uint8]

* refactor subnet logic out from beacon_node and actual (un)subscribing

* only try to subscribe to marginally different subnets

* add assertions

* maintain ENR subnets

* assert that beacon_node and eth2_network have consistent view of subscribed subnets

* disable actual cycling

beacon_chain/beacon_node.nim

@@ -8,6 +8,7 @@
 import
   # Standard library
   std/[algorithm, os, tables, strutils, sequtils, times, math, terminal],
+  std/random,
 
   # Nimble packages
   stew/[objects, byteutils, endians2], stew/shims/macros,
@@ -392,6 +393,82 @@ func verifyFinalization(node: BeaconNode, slot: Slot) =
     # finalization occurs every slot, to 4 slots vs scheduledSlot.
     doAssert finalizedEpoch + 4 >= epoch
 
+proc installAttestationSubnetHandlers(node: BeaconNode, subnets: set[uint8]) =
+  proc attestationHandler(attestation: Attestation) =
+    # Avoid double-counting attestation-topic attestations on shared codepath
+    # when they're reflected through beacon blocks
+    beacon_attestations_received.inc()
+    beacon_attestation_received_seconds_from_slot_start.observe(
+      node.beaconClock.now.int64 - attestation.data.slot.toBeaconTime.int64)
+
+    node.onAttestation(attestation)
+
+  var attestationSubscriptions: seq[Future[void]] = @[]
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/p2p-interface.md#attestations-and-aggregation
+  for subnet in subnets:
+    attestationSubscriptions.add(node.network.subscribe(
+      getAttestationTopic(node.forkDigest, subnet),
+      attestationHandler,
+    ))
+
+  waitFor allFutures(attestationSubscriptions)
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/p2p-interface.md#metadata
+  node.network.metadata.seq_number += 1
+  for subnet in subnets:
+    node.network.metadata.attnets[subnet] = true
+
+proc cycleAttestationSubnets(node: BeaconNode, slot: Slot) =
+  let epochParity = slot.epoch mod 2
+  var attachedValidators: seq[ValidatorIndex]
+  for validatorIndex in 0 ..< node.chainDag.headState.data.data.validators.len:
+    if node.getAttachedValidator(
+        node.chainDag.headState.data.data, validatorIndex.ValidatorIndex) != nil:
+      attachedValidators.add validatorIndex.ValidatorIndex
+
+  let (newAttestationSubnets, expiringSubnets, newSubnets) =
+    get_attestation_subnet_changes(
+      node.chainDag.headState.data.data, attachedValidators,
+      node.attestationSubnets, slot.epoch)
+
+  node.attestationSubnets = newAttestationSubnets
+  debug "Attestation subnets",
+    expiring_subnets = expiringSubnets,
+    current_epoch_subnets =
+      node.attestationSubnets.subscribedSubnets[1 - epochParity],
+    upcoming_subnets = node.attestationSubnets.subscribedSubnets[epochParity],
+    new_subnets = newSubnets,
+    stability_subnet = node.attestationSubnets.stabilitySubnet,
+    stability_subnet_expiration_epoch =
+      node.attestationSubnets.stabilitySubnetExpirationEpoch
+
+  block:
+    var unsubscriptions: seq[Future[void]] = @[]
+    for expiringSubnet in expiringSubnets:
+      unsubscriptions.add(node.network.unsubscribe(
+        getAttestationTopic(node.forkDigest, expiringSubnet)))
+
+    waitFor allFutures(unsubscriptions)
+
+    # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/p2p-interface.md#metadata
+    # The race condition window is smaller by placing the fast, local, and
+    # synchronous operation after a variable-latency, asynchronous action.
+    node.network.metadata.seq_number += 1
+    for expiringSubnet in expiringSubnets:
+      node.network.metadata.attnets[expiringSubnet] = false
+
+  node.installAttestationSubnetHandlers(newSubnets)
+
+  block:
+    let subscribed_subnets =
+      node.attestationSubnets.subscribedSubnets[0] +
+      node.attestationSubnets.subscribedSubnets[1] +
+      {node.attestationSubnets.stabilitySubnet.uint8}
+    for subnet in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
+      doAssert node.network.metadata.attnets[subnet] ==
+        (subnet in subscribed_subnets)
+
 proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, async.} =
   ## Called at the beginning of a slot - usually every slot, but sometimes might
   ## skip a few in case we're running late.
@@ -461,6 +538,10 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
   if node.config.verifyFinalization:
     verifyFinalization(node, scheduledSlot)
 
+  when false:
+    if slot.isEpoch:
+      node.cycleAttestationSubnets(slot)
+
   if slot > lastSlot + SLOTS_PER_EPOCH:
     # We've fallen behind more than an epoch - there's nothing clever we can
     # do here really, except skip all the work and try again later.
@@ -776,29 +857,39 @@ proc installAttestationHandlers(node: BeaconNode) =
       return false
     node.attestationPool.isValidAggregatedAttestation(signedAggregateAndProof, slot)
 
-  var attestationSubscriptions: seq[Future[void]] = @[]
-
   # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/p2p-interface.md#attestations-and-aggregation
+  # These validators stay around the whole time, regardless of which specific
+  # subnets are subscribed to during any given epoch.
   for it in 0'u64 ..< ATTESTATION_SUBNET_COUNT.uint64:
     closureScope:
       let ci = it
-      attestationSubscriptions.add(node.network.subscribe(
+      node.network.addValidator(
         getAttestationTopic(node.forkDigest, ci),
-        attestationHandler,
         # This proc needs to be within closureScope; don't lift out of loop.
         proc(attestation: Attestation): bool =
           attestationValidator(attestation, ci)
-      ))
+      )
 
-  attestationSubscriptions.add(node.network.subscribe(
+  var initialSubnets: set[uint8]
+  for i in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
+    initialSubnets.incl i
+  node.installAttestationSubnetHandlers(initialSubnets)
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/validator.md#phase-0-attestation-subnet-stability
+  node.attestationSubnets.stabilitySubnet = rand(ATTESTATION_SUBNET_COUNT - 1).uint64
+  node.attestationSubnets.stabilitySubnetExpirationEpoch =
+    GENESIS_EPOCH + getStabilitySubnetLength()
+
+  # Relative to epoch 0, this sets the "current" attestation subnets.
+  node.attestationSubnets.subscribedSubnets[1 - (GENESIS_EPOCH mod 2)] =
+    initialSubnets
+
+  waitFor node.network.subscribe(
     getAggregateAndProofsTopic(node.forkDigest),
     proc(signedAggregateAndProof: SignedAggregateAndProof) =
       attestationHandler(signedAggregateAndProof.message.aggregate),
     proc(signedAggregateAndProof: SignedAggregateAndProof): bool =
-      aggregatedAttestationValidator(signedAggregateAndProof)
+      aggregatedAttestationValidator(signedAggregateAndProof))
-  ))
-
-  waitFor allFutures(attestationSubscriptions)
 
 proc stop*(node: BeaconNode) =
   status = BeaconNodeStatus.Stopping

beacon_chain/beacon_node_common.nim

@@ -54,6 +54,7 @@ type
     blockProcessingLoop*: Future[void]
     onSecondLoop*: Future[void]
     genesisSnapshotContent*: string
+    attestationSubnets*: AttestationSubnets
 
 const
   MaxEmptySlotCount* = uint64(10*60) div SECONDS_PER_SLOT

beacon_chain/spec/datatypes.nim

@@ -442,6 +442,11 @@ type
       Table[Epoch, seq[ValidatorIndex]]
     beacon_proposer_indices*: Table[Slot, Option[ValidatorIndex]]
 
+  AttestationSubnets* = object
+    subscribedSubnets*: array[2, set[uint8]]
+    stabilitySubnet*: uint64
+    stabilitySubnetExpirationEpoch*: Epoch
+
 func shortValidatorKey*(state: BeaconState, validatorIdx: int): string =
     ($state.validators[validatorIdx].pubkey)[0..7]
 

beacon_chain/spec/network.nim

@@ -8,7 +8,7 @@
 {.push raises: [Defect].}
 
 import
-  std/[strformat, sets],
+  std/[strformat, sets, random],
   ./datatypes, ./helpers, ./validator
 
 const
@@ -110,3 +110,51 @@ func get_committee_assignments(
         result.add(
           (compute_subnet_for_attestation(committees_per_slot, slot, idx),
             slot))
+
+# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/validator.md#phase-0-attestation-subnet-stability
+proc getStabilitySubnetLength*(): uint64 =
+  EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION +
+    rand(EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION.int).uint64
+
+proc get_attestation_subnet_changes*(
+    state: BeaconState, attachedValidators: openarray[ValidatorIndex],
+    prevAttestationSubnets: AttestationSubnets, epoch: Epoch):
+    tuple[a: AttestationSubnets, b: set[uint8], c: set[uint8]] =
+  static:
+    doAssert ATTESTATION_SUBNET_COUNT == 64
+
+  var attestationSubnets = prevAttestationSubnets
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/validator.md#phase-0-attestation-subnet-stability
+  let prevStabilitySubnet = {attestationSubnets.stabilitySubnet.uint8}
+  if epoch >= attestationSubnets.stabilitySubnetExpirationEpoch:
+    attestationSubnets.stabilitySubnet =
+      rand(ATTESTATION_SUBNET_COUNT - 1).uint64
+    attestationSubnets.stabilitySubnetExpirationEpoch =
+      epoch + getStabilitySubnetLength()
+
+  var nextEpochSubnets: set[uint8]
+  for it in get_committee_assignments(
+      state, state.slot.epoch + 1, attachedValidators.toHashSet):
+    nextEpochSubnets.incl it.subnetIndex.uint8
+
+  doAssert nextEpochSubnets.len > 0 and
+    nextEpochSubnets.len <= attachedValidators.len
+
+  let
+    epochParity = epoch mod 2
+    stabilitySet = {attestationSubnets.stabilitySubnet.uint8}
+    currentEpochSubnets = attestationSubnets.subscribedSubnets[1 - epochParity]
+
+    expiringSubnets =
+      (prevStabilitySubnet +
+        attestationSubnets.subscribedSubnets[epochParity]) -
+        nextEpochSubnets - currentEpochSubnets - stabilitySet
+    newSubnets =
+      (nextEpochSubnets + stabilitySet) -
+        (currentEpochSubnets + prevStabilitySubnet)
+
+  doAssert newSubnets.len <= attachedValidators.len + 1
+
+  attestationSubnets.subscribedSubnets[epochParity] = newSubnets
+  (attestationSubnets, expiringSubnets, newSubnets)