Handle idx panic (#1492)

* fix panic on empty first committee

* fix issue in validator server too

beacon-chain/core/validators/validator.go

@@ -64,6 +64,10 @@ func BeaconProposerIdx(state *pb.BeaconState, slot uint64) (uint64, error) {
 	}
 	firstCommittee := committeeArray[0].Committee
 
+	if len(firstCommittee) == 0 {
+		return 0, fmt.Errorf("empty first committee at slot %d", slot)
+	}
+
 	return firstCommittee[slot%uint64(len(firstCommittee))], nil
 }
 

beacon-chain/core/validators/validator_test.go

@@ -274,6 +274,14 @@ func TestBeaconProposerIdx(t *testing.T) {
 	}
 }
 
+func TestBeaconProposerIdx_returnsErrorWithEmptyCommittee(t *testing.T) {
+	_, err := BeaconProposerIdx(&pb.BeaconState{}, 0)
+	expected := "empty first committee at slot 0"
+	if err.Error() != expected {
+		t.Errorf("Unexpected error. got=%v want=%s", err, expected)
+	}
+}
+
 func TestAttestingValidatorIndices_Ok(t *testing.T) {
 	if params.BeaconConfig().EpochLength != 64 {
 		t.Errorf("EpochLength should be 64 for these tests to pass")

beacon-chain/rpc/validator_server.go

@@ -58,20 +58,22 @@ func (vs *ValidatorServer) ValidatorEpochAssignments(
 	var attesterSlot uint64
 	var proposerSlot uint64
 
-	for i := req.EpochStart; i < req.EpochStart+params.BeaconConfig().EpochLength; i++ {
-		crossLinkCommittees, err := v.CrosslinkCommitteesAtSlot(beaconState, i)
+	for slot := req.EpochStart; slot < req.EpochStart+params.BeaconConfig().EpochLength; slot++ {
+		crossLinkCommittees, err := v.CrosslinkCommitteesAtSlot(beaconState, slot)
 		if err != nil {
-			return nil, fmt.Errorf("could not get crosslink committees at slot %d: %v", i, err)
+			return nil, err
+		}
+		proposerIndex, err := v.BeaconProposerIdx(beaconState, slot)
+		if err != nil {
+			return nil, err
 		}
-		firstCommittee := crossLinkCommittees[0].Committee
-		proposerIndex := firstCommittee[i%uint64(len(firstCommittee))]
 		if proposerIndex == validatorIndex {
-			proposerSlot = i
+			proposerSlot = slot
 		}
 		for _, committee := range crossLinkCommittees {
 			for _, idx := range committee.Committee {
 				if idx == validatorIndex {
-					attesterSlot = i
+					attesterSlot = slot
 					shard = committee.Shard
 				}
 			}