fix(api): send to closed channel in mergeLogStreams (#7006)

server/application/logs.go

@@ -120,16 +120,22 @@ func mergeLogStreams(streams []chan logEntry, bufferingDuration time.Duration) c
 	var sentAt time.Time
 
 	ticker := time.NewTicker(bufferingDuration)
+	done := make(chan struct{})
 	go func() {
-		for range ticker.C {
-			sentAtLock.Lock()
-			// waited long enough for logs from each streams, send everything accumulated
-			if sentAt.Add(bufferingDuration).Before(time.Now()) {
-				_ = send(true)
-				sentAt = time.Now()
-			}
+		for {
+			select {
+			case <-done:
+				return
+			case <-ticker.C:
+				sentAtLock.Lock()
+				// waited long enough for logs from each streams, send everything accumulated
+				if sentAt.Add(bufferingDuration).Before(time.Now()) {
+					_ = send(true)
+					sentAt = time.Now()
+				}
 
-			sentAtLock.Unlock()
+				sentAtLock.Unlock()
+			}
 		}
 	}()
 
@@ -145,6 +151,11 @@ func mergeLogStreams(streams []chan logEntry, bufferingDuration time.Duration) c
 		_ = send(true)
 
 		ticker.Stop()
+		// ticker.Stop() does not close the channel, and it does not wait for the channel to be drained. So we need to
+		// explicitly prevent the gorountine from leaking by closing the channel. We also need to prevent the goroutine
+		// from calling `send` again, because `send` pushes to the `merged` channel which we're about to close.
+		// This describes the approach nicely: https://stackoverflow.com/questions/17797754/ticker-stop-behaviour-in-golang
+		done <- struct{}{}
 		close(merged)
 	}()
 	return merged

server/application/logs_test.go

@@ -75,3 +75,33 @@ func TestMergeLogStreams(t *testing.T) {
 
 	assert.Equal(t, []string{"1", "2", "3", "4"}, lines)
 }
+
+func TestMergeLogStreams_RaceCondition(t *testing.T) {
+	// Test for regression of this issue: https://github.com/argoproj/argo-cd/issues/7006
+	for i := 0; i < 5000; i++ {
+		first := make(chan logEntry)
+		second := make(chan logEntry)
+
+		go func() {
+			parseLogsStream("first", io.NopCloser(strings.NewReader(`2021-02-09T00:00:01Z 1`)), first)
+			time.Sleep(time.Duration(i%3) * time.Millisecond)
+			close(first)
+		}()
+
+		go func() {
+			parseLogsStream("second", io.NopCloser(strings.NewReader(`2021-02-09T00:00:02Z 2`)), second)
+			time.Sleep(time.Duration((i+1)%3) * time.Millisecond)
+			close(second)
+		}()
+
+		merged := mergeLogStreams([]chan logEntry{first, second}, 1*time.Millisecond)
+
+		// Drain the channel
+		for range merged {
+		}
+
+		// This test intentionally doesn't test the order of the output. Under these intense conditions, the test would
+		// fail often due to out of order entries. This test is only meant to reproduce a race between a channel writer
+		// and channel closer.
+	}
+}