Fix queue worker incorrectly stopped when there are still more items in the queue (#29532) (#29546)

Backport #29532

Without `case <-t.C`, the workers would stop incorrectly, the test won't
pass. For the worse case, there might be only one running worker
processing the queue items for long time because other workers are
stopped. The root cause is related to the logic of doDispatchBatchToWorker.
It isn't a serious problem at the moment, so keep it as-is.

(cherry picked from commit 86cd94cba6d63c84528f6f8d52b1ec22b44ac2f8)

modules/queue/workergroup.go

@@ -60,6 +60,9 @@ func (q *WorkerPoolQueue[T]) doDispatchBatchToWorker(wg *workerGroup[T], flushCh
 		full = true
 	}
 
+	// TODO: the logic could be improved in the future, to avoid a data-race between "doStartNewWorker" and "workerNum"
+	// The root problem is that if we skip "doStartNewWorker" here, the "workerNum" might be decreased by other workers later
+	// So ideally, it should check whether there are enough workers by some approaches, and start new workers if necessary.
 	q.workerNumMu.Lock()
 	noWorker := q.workerNum == 0
 	if full || noWorker {
@@ -143,7 +146,11 @@ func (q *WorkerPoolQueue[T]) doStartNewWorker(wp *workerGroup[T]) {
 		log.Debug("Queue %q starts new worker", q.GetName())
 		defer log.Debug("Queue %q stops idle worker", q.GetName())
 
+		atomic.AddInt32(&q.workerStartedCounter, 1) // Only increase counter, used for debugging
+
 		t := time.NewTicker(workerIdleDuration)
+		defer t.Stop()
+
 		keepWorking := true
 		stopWorking := func() {
 			q.workerNumMu.Lock()
@@ -158,13 +165,18 @@ func (q *WorkerPoolQueue[T]) doStartNewWorker(wp *workerGroup[T]) {
 			case batch, ok := <-q.batchChan:
 				if !ok {
 					stopWorking()
-				} else {
-					q.doWorkerHandle(batch)
-					t.Reset(workerIdleDuration)
+					continue
+				}
+				q.doWorkerHandle(batch)
+				// reset the idle ticker, and drain the tick after reset in case a tick is already triggered
+				t.Reset(workerIdleDuration)
+				select {
+				case <-t.C:
+				default:
 				}
 			case <-t.C:
 				q.workerNumMu.Lock()
-				keepWorking = q.workerNum <= 1
+				keepWorking = q.workerNum <= 1 // keep the last worker running
 				if !keepWorking {
 					q.workerNum--
 				}

modules/queue/workerqueue.go

@@ -40,6 +40,8 @@ type WorkerPoolQueue[T any] struct {
 	workerMaxNum    int
 	workerActiveNum int
 	workerNumMu     sync.Mutex
+
+	workerStartedCounter int32
 }
 
 type flushType chan struct{}

modules/queue/workerqueue_test.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -175,11 +176,7 @@ func testWorkerPoolQueuePersistence(t *testing.T, queueSetting setting.QueueSett
 }
 
 func TestWorkerPoolQueueActiveWorkers(t *testing.T) {
-	oldWorkerIdleDuration := workerIdleDuration
-	workerIdleDuration = 300 * time.Millisecond
-	defer func() {
-		workerIdleDuration = oldWorkerIdleDuration
-	}()
+	defer test.MockVariableValue(&workerIdleDuration, 300*time.Millisecond)()
 
 	handler := func(items ...int) (unhandled []int) {
 		time.Sleep(100 * time.Millisecond)
@@ -250,3 +247,25 @@ func TestWorkerPoolQueueShutdown(t *testing.T) {
 	q, _ = newWorkerPoolQueueForTest("test-workpoolqueue", qs, handler, false)
 	assert.EqualValues(t, 20, q.GetQueueItemNumber())
 }
+
+func TestWorkerPoolQueueWorkerIdleReset(t *testing.T) {
+	defer test.MockVariableValue(&workerIdleDuration, 10*time.Millisecond)()
+
+	handler := func(items ...int) (unhandled []int) {
+		time.Sleep(50 * time.Millisecond)
+		return nil
+	}
+
+	q, _ := newWorkerPoolQueueForTest("test-workpoolqueue", setting.QueueSettings{Type: "channel", BatchLength: 1, MaxWorkers: 2, Length: 100}, handler, false)
+	stop := runWorkerPoolQueue(q)
+	for i := 0; i < 20; i++ {
+		assert.NoError(t, q.Push(i))
+	}
+
+	time.Sleep(500 * time.Millisecond)
+	assert.EqualValues(t, 2, q.GetWorkerNumber())
+	assert.EqualValues(t, 2, q.GetWorkerActiveNumber())
+	// when the queue never becomes empty, the existing workers should keep working
+	assert.EqualValues(t, 2, q.workerStartedCounter)
+	stop()
+}