fix attestation selection when calculating rewards and penalties

Previously, the we'd use the first source attestation regardless if it
matches the filter or not leading to the wrong minimum being used

beacon_chain/spec/state_transition_epoch.nim

@@ -176,6 +176,7 @@ func get_winning_crosslink_and_attesting_indices(
 
   let winning_attestations =
     filterIt(attestations, it.data.crosslink == winning_crosslink)
+
   (winning_crosslink,
    get_unslashed_attesting_indices(state, winning_attestations, stateCache))
 
@@ -364,6 +365,7 @@ func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
       get_matching_target_attestations(state, previous_epoch)
     matching_head_attestations =
       get_matching_head_attestations(state, previous_epoch)
+
   for attestations in
       [matching_source_attestations, matching_target_attestations,
        matching_head_attestations]:
@@ -392,23 +394,33 @@ func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
       state, matching_source_attestations, stateCache):
     # Translation of attestation = min([...])
     doAssert matching_source_attestations.len > 0
-    var attestation = matching_source_attestations[0]
+
+    # Start by filtering the right attestations
+    var filtered_matching_source_attestations: seq[PendingAttestation]
+
     for source_attestation_index, a in matching_source_attestations:
       if index notin
           source_attestation_attesting_indices[source_attestation_index]:
         continue
+      filtered_matching_source_attestations.add a
 
+    # The first filtered attestation serves as min until we find something
+    # better
+    var attestation = filtered_matching_source_attestations[0]
+    for source_attestation_index, a in filtered_matching_source_attestations:
       if a.inclusion_delay < attestation.inclusion_delay:
         attestation = a
 
     let proposer_reward =
       (get_base_reward(state, index) div PROPOSER_REWARD_QUOTIENT).Gwei
+
     rewards[attestation.proposer_index.int] += proposer_reward
     let max_attester_reward = get_base_reward(state, index) - proposer_reward
+
     rewards[index] +=
-      (max_attester_reward *
+      ((max_attester_reward *
        ((SLOTS_PER_EPOCH + MIN_ATTESTATION_INCLUSION_DELAY).uint64 -
-       attestation.inclusion_delay) div SLOTS_PER_EPOCH).Gwei
+       attestation.inclusion_delay)) div SLOTS_PER_EPOCH).Gwei
 
   # Inactivity penalty
   let finality_delay = previous_epoch - state.finalized_checkpoint.epoch
@@ -444,6 +456,7 @@ func get_crosslink_deltas*(state: BeaconState, cache: var StateCache):
           state, epoch, shard, cache)
       attesting_balance = get_total_balance(state, attesting_indices)
       committee_balance = get_total_balance(state, crosslink_committee)
+
     for index in crosslink_committee:
       let base_reward = get_base_reward(state, index)
       if index in attesting_indices:
@@ -463,6 +476,7 @@ func process_rewards_and_penalties(
   let
     (rewards1, penalties1) = get_attestation_deltas(state, cache)
     (rewards2, penalties2) = get_crosslink_deltas(state, cache)
+
   for i in 0 ..< len(state.validators):
     increase_balance(state, i.ValidatorIndex, rewards1[i] + rewards2[i])
     decrease_balance(state, i.ValidatorIndex, penalties1[i] + penalties2[i])