adjust checkpoint tracking for devnets (#4039)

Track checkpoints more defensively on devnets with low participation.

beacon_chain/consensus_object_pools/attestation_pool.nim

@@ -97,7 +97,9 @@ proc init*(T: type AttestationPool, dag: ChainDAGRef,
   ## holding a zero_root.
   let finalizedEpochRef = dag.getFinalizedEpochRef()
 
-  var forkChoice = ForkChoice.init(finalizedEpochRef, dag.finalizedHead.blck)
+  var forkChoice = ForkChoice.init(
+    finalizedEpochRef, dag.finalizedHead.blck,
+    lowParticipation in dag.updateFlags)
 
   # Feed fork choice with unfinalized history - during startup, block pool only
   # keeps track of a single history so we just need to follow it

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -775,8 +775,9 @@ proc init*(T: type ChainDAGRef, cfg: RuntimeConfig, db: BeaconChainDB,
            lcDataConfig = default(LightClientDataConfig)): ChainDAGRef =
   cfg.checkForkConsistency()
 
-  doAssert updateFlags - {strictVerification, enableTestFeatures} == {},
-    "Other flags not supported in ChainDAG"
+  doAssert updateFlags - {
+      strictVerification, enableTestFeatures, lowParticipation
+    } == {}, "Other flags not supported in ChainDAG"
 
   # TODO we require that the db contains both a head and a tail block -
   #      asserting here doesn't seem like the right way to go about it however..
@@ -803,7 +804,9 @@ proc init*(T: type ChainDAGRef, cfg: RuntimeConfig, db: BeaconChainDB,
 
       # The only allowed flag right now is strictVerification, as the others all
       # allow skipping some validation.
-      updateFlags: {strictVerification, enableTestFeatures} * updateFlags,
+      updateFlags: updateFlags * {
+        strictVerification, enableTestFeatures, lowParticipation
+      },
       cfg: cfg,
 
       vanityLogs: vanityLogs,

beacon_chain/extras.nim

@@ -41,5 +41,7 @@ type
     ## should skip calculating that last state root.
     enableTestFeatures ##\
     ## Whether to enable extra features for testing.
+    lowParticipation ##\
+    ## Whether the network is prone to low participation.
 
   UpdateFlags* = set[UpdateFlag]

beacon_chain/fork_choice/fork_choice.nim

@@ -51,10 +51,14 @@ func compute_deltas(
 
 logScope: topics = "fork_choice"
 
-func init*(T: type ForkChoiceBackend, checkpoints: FinalityCheckpoints): T =
-  T(proto_array: ProtoArray.init(checkpoints))
+func init*(
+    T: type ForkChoiceBackend, checkpoints: FinalityCheckpoints,
+    hasLowParticipation = false): T =
+  T(proto_array: ProtoArray.init(checkpoints, hasLowParticipation))
 
-proc init*(T: type ForkChoice, epochRef: EpochRef, blck: BlockRef): T =
+proc init*(
+    T: type ForkChoice, epochRef: EpochRef, blck: BlockRef,
+    hasLowParticipation = false): T =
   ## Initialize a fork choice context for a finalized state - in the finalized
   ## state, the justified and finalized checkpoints are the same, so only one
   ## is used here
@@ -66,7 +70,8 @@ proc init*(T: type ForkChoice, epochRef: EpochRef, blck: BlockRef): T =
     backend: ForkChoiceBackend.init(
       FinalityCheckpoints(
         justified: checkpoint,
-        finalized: checkpoint)),
+        finalized: checkpoint),
+      hasLowParticipation),
     checkpoints: Checkpoints(
       justified: BalanceCheckpoint(
         checkpoint: checkpoint,
@@ -369,6 +374,7 @@ proc process_block*(self: var ForkChoice,
 
 func find_head*(
        self: var ForkChoiceBackend,
+       current_epoch: Epoch,
        checkpoints: FinalityCheckpoints,
        justified_state_balances: seq[Gwei],
        proposer_boost_root: Eth2Digest
@@ -387,7 +393,8 @@ func find_head*(
 
   # Apply score changes
   ? self.proto_array.applyScoreChanges(
-    deltas, checkpoints, justified_state_balances, proposer_boost_root)
+    deltas, current_epoch, checkpoints,
+    justified_state_balances, proposer_boost_root)
 
   self.balances = justified_state_balances
 
@@ -407,6 +414,7 @@ proc get_head*(self: var ForkChoice,
   ? self.update_time(dag, wallTime)
 
   self.backend.find_head(
+    self.checkpoints.time.slotOrZero.epoch,
     FinalityCheckpoints(
       justified: self.checkpoints.justified.checkpoint,
       finalized: self.checkpoints.finalized),

beacon_chain/fork_choice/fork_choice_types.nim

@@ -92,6 +92,8 @@ type
     ## to get the physical index
 
   ProtoArray* = object
+    hasLowParticipation*: bool
+    currentEpoch*: Epoch
     checkpoints*: FinalityCheckpoints
     nodes*: ProtoNodes
     indices*: Table[Eth2Digest, Index]

beacon_chain/fork_choice/proto_array.nim

@@ -74,6 +74,9 @@ func len*(nodes: ProtoNodes): int =
 func add(nodes: var ProtoNodes, node: ProtoNode) =
   nodes.buf.add node
 
+func isPreviousEpochJustified(self: ProtoArray): bool =
+  self.checkpoints.justified.epoch + 1 == self.currentEpoch
+
 # Forward declarations
 # ----------------------------------------------------------------------
 
@@ -87,7 +90,9 @@ func nodeLeadsToViableHead(self: ProtoArray, node: ProtoNode): FcResult[bool]
 # ProtoArray routines
 # ----------------------------------------------------------------------
 
-func init*(T: type ProtoArray, checkpoints: FinalityCheckpoints): T =
+func init*(
+    T: type ProtoArray, checkpoints: FinalityCheckpoints,
+    hasLowParticipation: bool): T =
   let node = ProtoNode(
     root: checkpoints.finalized.root,
     parent: none(int),
@@ -96,10 +101,29 @@ func init*(T: type ProtoArray, checkpoints: FinalityCheckpoints): T =
     bestChild: none(int),
     bestDescendant: none(int))
 
-  T(checkpoints: checkpoints,
+  T(hasLowParticipation: hasLowParticipation,
+    checkpoints: checkpoints,
     nodes: ProtoNodes(buf: @[node], offset: 0),
     indices: {node.root: 0}.toTable())
 
+iterator realizePendingCheckpoints*(
+    self: var ProtoArray, resetTipTracking = true): FinalityCheckpoints =
+  # Pull-up chain tips from previous epoch
+  for idx, unrealized in self.currentEpochTips.pairs():
+    let physicalIdx = idx - self.nodes.offset
+    if unrealized != self.nodes.buf[physicalIdx].checkpoints:
+      trace "Pulling up chain tip",
+        blck = self.nodes.buf[physicalIdx].root,
+        checkpoints = self.nodes.buf[physicalIdx].checkpoints,
+        unrealized
+      self.nodes.buf[physicalIdx].checkpoints = unrealized
+
+    yield unrealized
+
+  # Reset tip tracking for new epoch
+  if resetTipTracking:
+    self.currentEpochTips.clear()
+
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.3/specs/phase0/fork-choice.md#configuration
 # https://github.com/ethereum/consensus-specs/blob/v1.1.10/specs/phase0/fork-choice.md#get_latest_attesting_balance
 const PROPOSER_SCORE_BOOST* = 40
@@ -124,6 +148,7 @@ func calculateProposerBoost(validatorBalances: openArray[Gwei]): int64 =
 
 func applyScoreChanges*(self: var ProtoArray,
                         deltas: var openArray[Delta],
+                        currentEpoch: Epoch,
                         checkpoints: FinalityCheckpoints,
                         newBalances: openArray[Gwei],
                         proposerBoostRoot: Eth2Digest): FcResult[void] =
@@ -148,8 +173,14 @@ func applyScoreChanges*(self: var ProtoArray,
       deltasLen: deltas.len,
       indicesLen: self.indices.len)
 
+  self.currentEpoch = currentEpoch
   self.checkpoints = checkpoints
 
+  # If previous epoch is justified, pull up all current tips to previous epoch
+  if self.hasLowParticipation and self.isPreviousEpochJustified:
+    for realized in self.realizePendingCheckpoints(resetTipTracking = false):
+      discard
+
   ## Alias
   # This cannot raise the IndexError exception, how to tell compiler?
   template node: untyped {.dirty.} =
@@ -300,21 +331,6 @@ func onBlock*(self: var ProtoArray,
 
   ok()
 
-iterator realizePendingCheckpoints*(self: var ProtoArray): FinalityCheckpoints =
-  # Pull-up chain tips from previous epoch
-  for idx, unrealized in self.currentEpochTips.pairs():
-    let physicalIdx = idx - self.nodes.offset
-    trace "Pulling up chain tip",
-      blck = self.nodes.buf[physicalIdx].root,
-      checkpoints = self.nodes.buf[physicalIdx].checkpoints,
-      unrealized
-    self.nodes.buf[physicalIdx].checkpoints = unrealized
-
-    yield unrealized
-
-  # Reset tip tracking for new epoch
-  self.currentEpochTips.clear()
-
 func findHead*(self: var ProtoArray,
                head: var Eth2Digest,
                justifiedRoot: Eth2Digest): FcResult[void] =
@@ -518,7 +534,14 @@ func nodeLeadsToViableHead(self: ProtoArray, node: ProtoNode): FcResult[bool] =
 func nodeIsViableForHead(self: ProtoArray, node: ProtoNode): bool =
   ## This is the equivalent of `filter_block_tree` function in eth2 spec
   ## https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.3/specs/phase0/fork-choice.md#filter_block_tree
-  ##
+  if self.hasLowParticipation:
+    if node.checkpoints.justified.epoch < self.checkpoints.justified.epoch:
+      return false
+    if self.isPreviousEpochJustified:
+      return true
+    return node.checkpoints.finalized == self.checkpoints.finalized or
+      self.checkpoints.finalized.epoch == GENESIS_EPOCH
+
   ## Any node that has a different finalized or justified epoch
   ## should not be viable for the head.
   (

beacon_chain/nimbus_beacon_node.nim

@@ -154,7 +154,7 @@ proc loadChainDag(
 
   let
     extraFlags =
-      if shouldEnableTestFeatures: {enableTestFeatures}
+      if shouldEnableTestFeatures: {enableTestFeatures, lowParticipation}
       else: {enableTestFeatures}
     chainDagFlags =
       if config.strictVerification: {strictVerification}

tests/fork_choice/interpreter.nim

@@ -65,6 +65,7 @@ func apply(ctx: var ForkChoiceBackend, id: int, op: Operation) =
   case op.kind
   of FindHead, InvalidFindHead:
     let r = ctx.find_head(
+      GENESIS_EPOCH,
       op.checkpoints,
       op.justified_state_balances,
       # Don't use proposer boosting