[Split fork choice PR] Derisk-ed attestation checks changes (#1154)

* Derisked attestation pool improvements

* tune down frequent logs

* VoteTracker logging

beacon_chain/attestation_aggregation.nim

@@ -8,10 +8,10 @@
 {.push raises: [Defect].}
 
 import
-  options,
+  options, chronicles,
   ./spec/[beaconstate, datatypes, crypto, digest, helpers, validator,
     state_transition_block],
-  ./attestation_pool, ./beacon_node_types
+  ./block_pool, ./attestation_pool, ./beacon_node_types, ./ssz
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#aggregation-selection
 func is_aggregator(state: BeaconState, slot: Slot, index: CommitteeIndex,
@@ -69,3 +69,81 @@ proc aggregate_attestations*(
         selection_proof: slot_signature))
 
   none(AggregateAndProof)
+
+
+# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#attestation-subnets
+proc isValidAttestation*(
+    pool: AttestationPool, attestation: Attestation, current_slot: Slot,
+    topicCommitteeIndex: uint64): bool =
+  # The attestation's committee index (attestation.data.index) is for the
+  # correct subnet.
+  if attestation.data.index != topicCommitteeIndex:
+    debug "isValidAttestation: attestation's committee index not for the correct subnet",
+      topicCommitteeIndex = topicCommitteeIndex,
+      attestation_data_index = attestation.data.index
+    return false
+
+  if not (attestation.data.slot + ATTESTATION_PROPAGATION_SLOT_RANGE >=
+      current_slot and current_slot >= attestation.data.slot):
+    debug "isValidAttestation: attestation.data.slot not within ATTESTATION_PROPAGATION_SLOT_RANGE"
+    return false
+
+  # The attestation is unaggregated -- that is, it has exactly one
+  # participating validator (len([bit for bit in attestation.aggregation_bits
+  # if bit == 0b1]) == 1).
+  # TODO a cleverer algorithm, along the lines of countOnes() in nim-stew
+  # But that belongs in nim-stew, since it'd break abstraction layers, to
+  # use details of its representation from nim-beacon-chain.
+  var onesCount = 0
+  for aggregation_bit in attestation.aggregation_bits:
+    if not aggregation_bit:
+      continue
+    onesCount += 1
+    if onesCount > 1:
+      debug "isValidAttestation: attestation has too many aggregation bits",
+        aggregation_bits = attestation.aggregation_bits
+      return false
+  if onesCount != 1:
+    debug "isValidAttestation: attestation has too few aggregation bits"
+    return false
+
+  # The attestation is the first valid attestation received for the
+  # participating validator for the slot, attestation.data.slot.
+  let maybeAttestationsSeen = getAttestationsForSlot(pool, attestation.data.slot)
+  if maybeAttestationsSeen.isSome:
+    for attestationEntry in maybeAttestationsSeen.get.attestations:
+      if attestation.data != attestationEntry.data:
+        continue
+      # Attestations might be aggregated eagerly or lazily; allow for both.
+      for validation in attestationEntry.validations:
+        if attestation.aggregation_bits.isSubsetOf(validation.aggregation_bits):
+          debug "isValidAttestation: attestation already exists at slot",
+            attestation_data_slot = attestation.data.slot,
+            attestation_aggregation_bits = attestation.aggregation_bits,
+            attestation_pool_validation = validation.aggregation_bits
+          return false
+
+  # The block being voted for (attestation.data.beacon_block_root) passes
+  # validation.
+  # We rely on the block pool to have been validated, so check for the
+  # existence of the block in the pool.
+  # TODO: consider a "slush pool" of attestations whose blocks have not yet
+  # propagated - i.e. imagine that attestations are smaller than blocks and
+  # therefore propagate faster, thus reordering their arrival in some nodes
+  if pool.blockPool.get(attestation.data.beacon_block_root).isNone():
+    debug "isValidAttestation: block doesn't exist in block pool",
+      attestation_data_beacon_block_root = attestation.data.beacon_block_root
+    return false
+
+  # The signature of attestation is valid.
+  # TODO need to know above which validator anyway, and this is too general
+  # as it supports aggregated attestations (which this can't be)
+  var cache = get_empty_per_epoch_cache()
+  if not is_valid_indexed_attestation(
+      pool.blockPool.headState.data.data,
+      get_indexed_attestation(
+        pool.blockPool.headState.data.data, attestation, cache), {}):
+    debug "isValidAttestation: signature verification failed"
+    return false
+
+  true