Fire OnBufferedAmountLow in a goroutine

If a user blocks this routine it would stop inbound message handling in
SCTP. To reduce the sharp edge fire it in a goroutine so users don't
need to worry about blocking.

`data-channels-flow-control` exhibited no differences in throughput from
this change.

Resolves #846

datachannel.go

@@ -707,9 +707,13 @@ func (d *DataChannel) OnBufferedAmountLow(f func()) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
-	d.onBufferedAmountLow = f
+	d.onBufferedAmountLow = func() {
+		go f()
+	}
 	if d.dataChannel != nil {
-		d.dataChannel.OnBufferedAmountLow(f)
+		d.dataChannel.OnBufferedAmountLow(func() {
+			go f()
+		})
 	}
 }
 

datachannel_go_test.go

@@ -277,7 +277,7 @@ func TestDataChannelBufferedAmount(t *testing.T) { //nolint:cyclop
 		report := test.CheckRoutines(t)
 		defer report()
 
-		var nCbs int
+		var nCbs uint32
 		buf := make([]byte, 1000)
 
 		offerPC, answerPC, err := newPair()
@@ -315,7 +315,7 @@ func TestDataChannelBufferedAmount(t *testing.T) { //nolint:cyclop
 			dc.SetBufferedAmountLowThreshold(1500)
 			// The callback function should directly be passed to sctp
 			dc.OnBufferedAmountLow(func() {
-				nCbs++
+				atomic.AddUint32(&nCbs, 1)
 			})
 
 			for i := 0; i < 10; i++ {
@@ -332,7 +332,7 @@ func TestDataChannelBufferedAmount(t *testing.T) { //nolint:cyclop
 
 		closePair(t, offerPC, answerPC, done)
 
-		assert.True(t, nCbs > 0, "callback should be made at least once")
+		assert.True(t, atomic.LoadUint32(&nCbs) > 0, "callback should be made at least once")
 	})
 }
 
@@ -813,3 +813,45 @@ func TestDataChannelMessageSize(t *testing.T) {
 	<-messagesSent.Done()
 	closePairNow(t, offerPC, answerPC)
 }
+
+func TestOnBufferedAmountLowDeadlock(t *testing.T) {
+	offerPC, answerPC, err := newPair()
+	assert.NoError(t, err)
+
+	offerDataChannel, err := offerPC.CreateDataChannel("", nil)
+	assert.NoError(t, err)
+
+	assert.NoError(t, signalPair(offerPC, answerPC))
+
+	gotAllMessages, gotAllMessagesCancel := context.WithCancel(context.Background())
+	offerDataChannel.OnOpen(func() {
+		for {
+			select {
+			case <-gotAllMessages.Done():
+				return
+			case <-time.After(5 * time.Millisecond):
+				assert.NoError(t, offerDataChannel.Send([]byte{0xBE, 0xEF}))
+			}
+		}
+	})
+
+	answerPC.OnDataChannel(func(dataChannel *DataChannel) {
+		dataChannel.SetBufferedAmountLowThreshold(1)
+
+		var onBufferedAmountLowFired atomic.Bool
+		dataChannel.OnBufferedAmountLow(func() {
+			onBufferedAmountLowFired.Store(true)
+			<-gotAllMessages.Done()
+		})
+
+		var onMessageCount uint32
+		dataChannel.OnMessage(func(msg DataChannelMessage) {
+			if onBufferedAmountLowFired.Load() && atomic.AddUint32(&onMessageCount, 1) == 10 {
+				gotAllMessagesCancel()
+			}
+		})
+	})
+
+	<-gotAllMessages.Done()
+	closePairNow(t, offerPC, answerPC)
+}

examples/data-channels-flow-control/main.go

@@ -86,8 +86,6 @@ func createOfferer() *webrtc.PeerConnection {
 
 	// This callback is made when the current bufferedAmount becomes lower than the threshold
 	dataChannel.OnBufferedAmountLow(func() {
-		// Make sure to not block this channel or perform long running operations in this callback
-		// This callback is executed by pion/sctp. If this callback is blocking it will stop operations
 		select {
 		case sendMoreCh <- struct{}{}:
 		default: