fix invalid_signature_no_participants test case (#2741)

FixtureAll-mainnet.md

@@ -135,6 +135,7 @@ FixtureAll-mainnet
 + [Invalid] invalid_signature_infinite_signature_with_all_participants                       OK
 + [Invalid] invalid_signature_infinite_signature_with_single_participant                     OK
 + [Invalid] invalid_signature_missing_participant                                            OK
++ [Invalid] invalid_signature_no_participants                                                OK
 + [Invalid] invalid_signature_past_block                                                     OK
 + [Invalid] invalid_slot_block_header                                                        OK
 + [Invalid] mismatched_target_and_slot                                                       OK
@@ -278,7 +279,7 @@ FixtureAll-mainnet
 + fork_random_low_balances                                                                   OK
 + fork_random_misc_balances                                                                  OK
 ```
-OK: 275/275 Fail: 0/275 Skip: 0/275
+OK: 276/276 Fail: 0/276 Skip: 0/276
 ## Official - Altair - Epoch Processing - Effective balance updates [Preset: mainnet]
 ```diff
 + Effective balance updates - effective_balance_hysteresis [Preset: mainnet]                 OK
@@ -511,4 +512,4 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 OK: 27/27 Fail: 0/27 Skip: 0/27
 
 ---TOTAL---
-OK: 421/421 Fail: 0/421 Skip: 0/421
+OK: 422/422 Fail: 0/422 Skip: 0/422

beacon_chain/spec/state_transition.nim

@@ -427,7 +427,9 @@ proc makeBeaconBlock*(
       attestations: List[Attestation, Limit MAX_ATTESTATIONS](attestations),
       deposits: List[Deposit, Limit MAX_DEPOSITS](deposits),
       voluntary_exits:
-        List[SignedVoluntaryExit, Limit MAX_VOLUNTARY_EXITS](voluntaryExits)))
+        List[SignedVoluntaryExit, Limit MAX_VOLUNTARY_EXITS](voluntaryExits),
+      sync_aggregate: SyncAggregate(sync_committee_signature:
+        default(CookedSig).toValidatorSig)))
 
   # TODO sync committees
 

beacon_chain/spec/state_transition_block.nim

@@ -425,6 +425,12 @@ proc process_sync_aggregate*(
     if aggregate.sync_committee_bits[i]:
       participant_pubkeys.add committee_pubkeys[i]
 
+  # p2p-interface message validators check for empty sync committees, so it
+  # shouldn't run except as part of test suite.
+  if  participant_pubkeys.len == 0 and
+      aggregate.sync_committee_signature != default(CookedSig).toValidatorSig():
+    return err("process_sync_aggregate: empty sync aggregates need signature of point at infinity")
+
   # Empty participants allowed
   if participant_pubkeys.len > 0 and not blsFastAggregateVerify(
       participant_pubkeys, signing_root.data, aggregate.sync_committee_signature):

tests/official/altair/test_fixture_operations_sync_aggregate.nim

@@ -61,14 +61,11 @@ proc runTest(dir, identifier: string) =
         reportDiff(preState, postState)
       else:
         let done = process_sync_aggregate(preState[], syncAggregate, cache).isOk
-        doAssert done == false, "We didn't expect this invalid proposer slashing to be processed."
+        doAssert done == false, "We didn't expect this invalid sync aggregate to be processed."
 
   `testImpl_sync_committee _ identifier`()
 
 suite "Official - Altair - Operations - Sync Aggregate" & preset():
   for kind, path in walkDir(
       OpSyncAggregateDir, relative = true, checkDir = true):
-    # TODO remove when it works
-    if path == "invalid_signature_no_participants":
-      continue
     runTest(OpSyncAggregateDir, path)