[release-22.0] Fix bug where query consolidator returns empty result without error when the waiter cap exceeded (#18782)

go/sync2/fake_consolidator.go

@@ -51,8 +51,12 @@ type FakePendingResult struct {
 	BroadcastCalls int
 	// WaitCalls can be used to inspect Wait calls.
 	WaitCalls int
-	err       error
-	result    *sqltypes.Result
+	// AddWaiterCounterCalls can be used to inspect AddWaiterCounter calls.
+	AddWaiterCounterCalls []int64
+	// WaiterCount simulates the current waiter count
+	WaiterCount int64
+	err         error
+	result      *sqltypes.Result
 }
 
 var (
@@ -113,7 +117,9 @@ func (fr *FakePendingResult) Wait() {
 	fr.WaitCalls++
 }
 
-// AddWaiterCounter is currently a no-op.
-func (fr *FakePendingResult) AddWaiterCounter(int64) *int64 {
-	return new(int64)
+// AddWaiterCounter records the call and simulates waiter count changes.
+func (fr *FakePendingResult) AddWaiterCounter(delta int64) *int64 {
+	fr.AddWaiterCounterCalls = append(fr.AddWaiterCounterCalls, delta)
+	fr.WaiterCount += delta
+	return &fr.WaiterCount
 }

go/vt/vttablet/tabletserver/query_executor.go

@@ -710,6 +710,7 @@ func (qre *QueryExecutor) execSelect() (*sqltypes.Result, error) {
 	// Check tablet type.
 	if qre.shouldConsolidate() {
 		q, original := qre.tsv.qe.consolidator.Create(sqlWithoutComments)
+		waiterCapExceeded := false
 		if original {
 			defer q.Broadcast()
 			conn, err := qre.getConn()
@@ -729,13 +730,21 @@ func (qre *QueryExecutor) execSelect() (*sqltypes.Result, error) {
 				startTime := time.Now()
 				q.Wait()
 				qre.tsv.stats.WaitTimings.Record("Consolidations", startTime)
+			} else {
+				// Waiter cap exceeded, fall back to independent query execution
+				waiterCapExceeded = true
 			}
 			q.AddWaiterCounter(-1)
 		}
-		if q.Err() != nil {
-			return nil, q.Err()
+
+		// Return consolidation results unless waiter cap was exceeded
+		if !waiterCapExceeded {
+			if q.Err() != nil {
+				return nil, q.Err()
+			}
+			return q.Result(), nil
 		}
-		return q.Result(), nil
+		// If waiter cap exceeded, fall through to independent execution
 	}
 	conn, err := qre.getConn()
 	if err != nil {

go/vt/vttablet/tabletserver/query_executor_test.go

@@ -1466,6 +1466,79 @@ func TestQueryExecutorShouldConsolidate(t *testing.T) {
 	}
 }
 
+func TestQueryExecutorConsolidatorWaiterCapFallback(t *testing.T) {
+	// Test that when the consolidator waiter cap is reached, queries fall back
+	// to independent execution instead of returning empty results.
+
+	db := setUpQueryExecutorTest(t)
+	defer db.Close()
+
+	ctx := context.Background()
+	tsv := newTestTabletServer(ctx, enableConsolidator, db)
+	defer tsv.StopService()
+
+	// Set a waiter cap of 1
+	tsv.config.ConsolidatorQueryWaiterCap = 1
+
+	fakeConsolidator := sync2.NewFakeConsolidator()
+	tsv.qe.consolidator = fakeConsolidator
+
+	input := "select * from t limit 10001"
+	result := &sqltypes.Result{
+		Fields: getTestTableFields(),
+		Rows: [][]sqltypes.Value{{
+			sqltypes.NewInt32(1),   // pk
+			sqltypes.NewInt32(100), // name
+			sqltypes.NewInt32(200), // addr
+		}},
+	}
+
+	// Set up consolidator to simulate an identical query already running (Created=false)
+	fakePendingResult := &sync2.FakePendingResult{}
+	fakePendingResult.SetResult(result)
+	// Start with waiter count above the cap (2 > 1), so the condition fails
+	fakePendingResult.WaiterCount = 2
+
+	fakeConsolidator.CreateReturn = &sync2.FakeConsolidatorCreateReturn{
+		Created:       false, // Simulate identical query already running
+		PendingResult: fakePendingResult,
+	}
+
+	// Set up database query/response for fallback execution
+	db.AddQuery(input, result)
+
+	qre := newTestQueryExecutor(context.Background(), tsv, input, 0)
+	qre.options = &querypb.ExecuteOptions{Consolidator: querypb.ExecuteOptions_CONSOLIDATOR_ENABLED}
+
+	// Execute query
+	actualResult, err := qre.Execute()
+	require.NoError(t, err)
+	require.NotNil(t, actualResult)
+
+	// Verify we got the correct result (not empty)
+	require.Equal(t, result.Fields, actualResult.Fields)
+	require.Equal(t, result.Rows, actualResult.Rows)
+
+	// Verify consolidator was attempted
+	require.Len(t, fakeConsolidator.CreateCalls, 1)
+
+	// Verify we did NOT wait (because waiter cap was exceeded)
+	require.Equal(t, 0, fakePendingResult.WaitCalls)
+
+	// Verify we did NOT broadcast (because we're not the original)
+	require.Equal(t, 0, fakePendingResult.BroadcastCalls)
+
+	// Verify AddWaiterCounter was called: once with 0 (to check count), once with -1 (cleanup)
+	require.Len(t, fakePendingResult.AddWaiterCounterCalls, 2)
+	require.Equal(t, int64(0), fakePendingResult.AddWaiterCounterCalls[0])  // Check current count
+	require.Equal(t, int64(-1), fakePendingResult.AddWaiterCounterCalls[1]) // Decrement
+
+	// Verify fallback executed the query independently
+	require.Equal(t, 1, db.GetQueryCalledNum(input))
+
+	db.VerifyAllExecutedOrFail()
+}
+
 func TestGetConnectionLogStats(t *testing.T) {
 	db := setUpQueryExecutorTest(t)
 	defer db.Close()