Fix panic in tailer due to race condition between send() and close()

pkg/ingester/tailer.go

@@ -25,8 +25,11 @@ type tailer struct {
 	expr     logql.Expr
 
 	sendChan chan *logproto.Stream
-	done     chan struct{}
-	closeMtx sync.Mutex
+
+	// Signaling channel used to notify once the tailer gets closed
+	// and the loop and senders should stop
+	closeChan chan struct{}
+	closeOnce sync.Once
 
 	blockedAt      *time.Time
 	blockedMtx     sync.RWMutex
@@ -54,7 +57,7 @@ func newTailer(orgID, query string, conn logproto.Querier_TailServer) (*tailer,
 		conn:           conn,
 		droppedStreams: []*logproto.DroppedStream{},
 		id:             generateUniqueID(orgID, query),
-		done:           make(chan struct{}),
+		closeChan:      make(chan struct{}),
 		expr:           expr,
 	}, nil
 }
@@ -75,7 +78,7 @@ func (t *tailer) loop() {
 				t.close()
 				return
 			}
-		case <-t.done:
+		case <-t.closeChan:
 			return
 		case stream, ok = <-t.sendChan:
 			if !ok {
@@ -147,26 +150,23 @@ func (t *tailer) isWatchingLabels(metric model.Metric) bool {
 
 func (t *tailer) isClosed() bool {
 	select {
-	case <-t.done:
+	case <-t.closeChan:
 		return true
 	default:
 		return false
 	}
 }
 
 func (t *tailer) close() {
-	if t.isClosed() {
-		return
-	}
-
-	t.closeMtx.Lock()
-	defer t.closeMtx.Unlock()
-
-	if t.isClosed() {
-		return
-	}
-	close(t.done)
-	close(t.sendChan)
+	t.closeOnce.Do(func() {
+		// Signal the close channel
+		close(t.closeChan)
+
+		// We intentionally do not close sendChan in order to avoid a panic on
+		// send to a just-closed channel. It's OK not to close a channel, since
+		// it will be eventually garbage collected as soon as no goroutine
+		// references it anymore, whether it has been closed or not.
+	})
 }
 
 func (t *tailer) blockedSince() *time.Time {

pkg/ingester/tailer_test.go

@@ -0,0 +1,47 @@
+package ingester
+
+import (
+	"math/rand"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/grafana/loki/pkg/logproto"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTailer_sendRaceConditionOnSendWhileClosing(t *testing.T) {
+	runs := 100
+
+	stream := logproto.Stream{
+		Labels: `{type="test"}`,
+		Entries: []logproto.Entry{
+			{Timestamp: time.Unix(int64(1), 0), Line: "line 1"},
+			{Timestamp: time.Unix(int64(2), 0), Line: "line 2"},
+		},
+	}
+
+	for run := 0; run < runs; run++ {
+		tailer, err := newTailer("org-id", stream.Labels, nil)
+		require.NoError(t, err)
+		require.NotNil(t, tailer)
+
+		routines := sync.WaitGroup{}
+		routines.Add(2)
+
+		go assert.NotPanics(t, func() {
+			defer routines.Done()
+			time.Sleep(time.Duration(rand.Intn(1000)) * time.Microsecond)
+			tailer.send(stream)
+		})
+
+		go assert.NotPanics(t, func() {
+			defer routines.Done()
+			time.Sleep(time.Duration(rand.Intn(1000)) * time.Microsecond)
+			tailer.close()
+		})
+
+		routines.Wait()
+	}
+}