Remove Already Exited Validators From Queue (#4695)

* added regression test

* fixed the regression test units

Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

beacon-chain/rpc/beacon/validators.go

@@ -540,7 +540,9 @@ func (bs *Server) GetValidatorQueue(
 	minEpoch := exitQueueEpoch + params.BeaconConfig().MinValidatorWithdrawabilityDelay
 	exitQueueIndices := make([]uint64, 0)
 	for _, valIdx := range awaitingExit {
-		if headState.Validators[valIdx].WithdrawableEpoch < minEpoch {
+		val := headState.Validators[valIdx]
+		// Ensure the validator has not yet exited before adding its index to the exit queue.
+		if val.WithdrawableEpoch < minEpoch && !validatorHasExited(val, helpers.CurrentEpoch(headState)) {
 			exitQueueIndices = append(exitQueueIndices, valIdx)
 		}
 	}
@@ -611,3 +613,24 @@ func (bs *Server) GetValidatorPerformance(
 		TotalActiveValidators:         activeCount,
 	}, nil
 }
+
+// Determines whether a validator has already exited.
+func validatorHasExited(validator *ethpb.Validator, currentEpoch uint64) bool {
+	farFutureEpoch := params.BeaconConfig().FarFutureEpoch
+	if currentEpoch < validator.ActivationEligibilityEpoch {
+		return false
+	}
+	if currentEpoch < validator.ActivationEpoch {
+		return false
+	}
+	if validator.ExitEpoch == farFutureEpoch {
+		return false
+	}
+	if currentEpoch < validator.ExitEpoch {
+		if validator.Slashed {
+			return false
+		}
+		return false
+	}
+	return true
+}

beacon-chain/rpc/beacon/validators_test.go

@@ -1222,6 +1222,67 @@ func TestServer_GetValidatorQueue_PendingActivation(t *testing.T) {
 	}
 }
 
+func TestServer_GetValidatorQueue_ExitedValidatorLeavesQueue(t *testing.T) {
+	headState := &pbp2p.BeaconState{
+		Validators: []*ethpb.Validator{
+			{
+				ActivationEpoch:   0,
+				ExitEpoch:         params.BeaconConfig().FarFutureEpoch,
+				WithdrawableEpoch: params.BeaconConfig().FarFutureEpoch,
+				PublicKey:         []byte("1"),
+			},
+			{
+				ActivationEpoch:   0,
+				ExitEpoch:         4,
+				WithdrawableEpoch: 6,
+				PublicKey:         []byte("2"),
+			},
+		},
+		FinalizedCheckpoint: &ethpb.Checkpoint{
+			Epoch: 0,
+		},
+	}
+	bs := &Server{
+		HeadFetcher: &mock.ChainService{
+			State: headState,
+		},
+	}
+
+	// First we check if validator with index 1 is in the exit queue.
+	res, err := bs.GetValidatorQueue(context.Background(), &ptypes.Empty{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	wanted := [][]byte{
+		[]byte("2"),
+	}
+	activeValidatorCount, err := helpers.ActiveValidatorCount(headState, helpers.CurrentEpoch(headState))
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantChurn, err := helpers.ValidatorChurnLimit(activeValidatorCount)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.ChurnLimit != wantChurn {
+		t.Errorf("Wanted churn %d, received %d", wantChurn, res.ChurnLimit)
+	}
+	if !reflect.DeepEqual(res.ExitPublicKeys, wanted) {
+		t.Errorf("Wanted %v, received %v", wanted, res.ExitPublicKeys)
+	}
+
+	// Now, we move the state.slot past the exit epoch of the validator, and now
+	// the validator should no longer exist in the queue.
+	headState.Slot = helpers.StartSlot(headState.Validators[1].ExitEpoch + 1)
+	res, err = bs.GetValidatorQueue(context.Background(), &ptypes.Empty{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.ExitPublicKeys) != 0 {
+		t.Errorf("Wanted empty exit queue, received %v", res.ExitPublicKeys)
+	}
+}
+
 func TestServer_GetValidatorQueue_PendingExit(t *testing.T) {
 	headState := &pbp2p.BeaconState{
 		Validators: []*ethpb.Validator{