Fix cancel issue between Query Frontend and Query Schdeduler

pkg/lokifrontend/frontend/v2/frontend.go

@@ -192,21 +192,15 @@ func (f *Frontend) RoundTripGRPC(ctx context.Context, req *httpgrpc.HTTPRequest)
 	retries := f.cfg.WorkerConcurrency + 1 // To make sure we hit at least two different schedulers.
 
 enqueueAgain:
+	var cancelCh chan<- uint64
 	select {
 	case <-ctx.Done():
 		return nil, ctx.Err()
 
 	case f.requestsCh <- freq:
 		// Enqueued, let's wait for response.
-	}
-
-	var cancelCh chan<- uint64
-
-	select {
-	case <-ctx.Done():
-		return nil, ctx.Err()
+		enqRes := <-freq.enqueue
 
-	case enqRes := <-freq.enqueue:
 		if enqRes.status == waitForResponse {
 			cancelCh = enqRes.cancelCh
 			break // go wait for response.
@@ -228,6 +222,7 @@ enqueueAgain:
 				// cancellation sent.
 			default:
 				// failed to cancel, ignore.
+				level.Warn(f.log).Log("msg", "failed to send cancellation request to scheduler, queue full")
 			}
 		}
 		return nil, ctx.Err()

pkg/lokifrontend/frontend/v2/frontend_scheduler_worker.go

@@ -191,7 +191,8 @@ func newFrontendSchedulerWorker(conn *grpc.ClientConn, schedulerAddr string, fro
 		schedulerAddr: schedulerAddr,
 		frontendAddr:  frontendAddr,
 		requestCh:     requestCh,
-		cancelCh:      make(chan uint64),
+		// Allow to enqueue enough cancellation requests. ~ 8MB memory size.
+		cancelCh: make(chan uint64, 1000000),
 	}
 	w.ctx, w.cancel = context.WithCancel(context.Background())
 
@@ -322,6 +323,8 @@ func (w *frontendSchedulerWorker) schedulerLoop(loop schedulerpb.SchedulerForFro
 						Body: []byte("too many outstanding requests"),
 					},
 				}
+			default:
+				req.enqueue <- enqueueResult{status: failed}
 			}
 
 		case reqID := <-w.cancelCh:

pkg/lokifrontend/frontend/v2/frontend_test.go

@@ -176,6 +176,44 @@ func TestFrontendCancellation(t *testing.T) {
 	})
 }
 
+// If FrontendWorkers are busy, cancellation passed by Query frontend may not reach
+// all the frontend workers thus not reaching the scheduler as well.
+// Issue: https://github.com/grafana/loki/issues/5132
+func TestFrontendWorkerCancellation(t *testing.T) {
+	f, ms := setupFrontend(t, nil)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
+
+	// send multiple requests > maxconcurrency of scheduler. So that it keeps all the frontend worker busy in serving requests.
+	reqCount := testFrontendWorkerConcurrency + 5
+	var wg sync.WaitGroup
+	for i := 0; i < reqCount; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			resp, err := f.RoundTripGRPC(user.InjectOrgID(ctx, "test"), &httpgrpc.HTTPRequest{})
+			require.EqualError(t, err, context.DeadlineExceeded.Error())
+			require.Nil(t, resp)
+		}()
+	}
+
+	wg.Wait()
+
+	// We wait a bit to make sure scheduler receives the cancellation request.
+	// 2 * reqCount because for every request, should also be corresponding cancel request
+	test.Poll(t, 5*time.Second, 2*reqCount, func() interface{} {
+		ms.mu.Lock()
+		defer ms.mu.Unlock()
+
+		return len(ms.msgs)
+	})
+
+	ms.checkWithLock(func() {
+		require.Equal(t, 2*reqCount, len(ms.msgs))
+	})
+}
+
 func TestFrontendFailedCancellation(t *testing.T) {
 	f, ms := setupFrontend(t, nil)