Adding OnDial handler for datachannels

Adds an `OnDial` handler to be fired once the dialing side has sent
the DCEP OPEN message over the data channel.

datachannel.go

@@ -49,6 +49,8 @@ type DataChannel struct {
 	onMessageHandler    func(DataChannelMessage)
 	openHandlerOnce     sync.Once
 	onOpenHandler       func()
+	dialHandlerOnce     sync.Once
+	onDialHandler       func()
 	onCloseHandler      func()
 	onBufferedAmountLow func()
 	onErrorHandler      func(error)
@@ -175,6 +177,7 @@ func (d *DataChannel) open(sctpTransport *SCTPTransport) error {
 	dc.OnBufferedAmountLow(d.onBufferedAmountLow)
 	d.mu.Unlock()
 
+	d.onDial()
 	d.handleOpen(dc, false, d.negotiated)
 	return nil
 }
@@ -228,6 +231,30 @@ func (d *DataChannel) onOpen() {
 	}
 }
 
+// OnDial sets an event handler which is invoked when the
+// peer has been dialed, but before said peer has responsed
+func (d *DataChannel) OnDial(f func()) {
+	d.mu.Lock()
+	d.dialHandlerOnce = sync.Once{}
+	d.onDialHandler = f
+	d.mu.Unlock()
+
+	if d.ReadyState() == DataChannelStateOpen {
+		// If the data channel is already open, call the handler immediately.
+		go d.dialHandlerOnce.Do(f)
+	}
+}
+
+func (d *DataChannel) onDial() {
+	d.mu.RLock()
+	handler := d.onDialHandler
+	d.mu.RUnlock()
+
+	if handler != nil {
+		go d.dialHandlerOnce.Do(handler)
+	}
+}
+
 // OnClose sets an event handler which is invoked when
 // the underlying data transport has been closed.
 func (d *DataChannel) OnClose(f func()) {

datachannel_go_test.go

@@ -33,12 +33,17 @@ func TestDataChannel_EventHandlers(t *testing.T) {
 	api := NewAPI()
 	dc := &DataChannel{api: api}
 
+	onDialCalled := make(chan struct{})
 	onOpenCalled := make(chan struct{})
 	onMessageCalled := make(chan struct{})
 
 	// Verify that the noop case works
 	assert.NotPanics(t, func() { dc.onOpen() })
 
+	dc.OnDial(func() {
+		close(onDialCalled)
+	})
+
 	dc.OnOpen(func() {
 		close(onOpenCalled)
 	})
@@ -48,10 +53,12 @@ func TestDataChannel_EventHandlers(t *testing.T) {
 	})
 
 	// Verify that the set handlers are called
+	assert.NotPanics(t, func() { dc.onDial() })
 	assert.NotPanics(t, func() { dc.onOpen() })
 	assert.NotPanics(t, func() { dc.onMessage(DataChannelMessage{Data: []byte("o hai")}) })
 
 	// Wait for all handlers to be called
+	<-onDialCalled
 	<-onOpenCalled
 	<-onMessageCalled
 }
@@ -578,3 +585,74 @@ func TestDataChannel_NonStandardSessionDescription(t *testing.T) {
 	<-onDataChannelCalled
 	closePairNow(t, offerPC, answerPC)
 }
+
+func TestDataChannel_Dial(t *testing.T) {
+	t.Run("handler should be called once, by dialing peer only", func(t *testing.T) {
+		report := test.CheckRoutines(t)
+		defer report()
+
+		dialCalls := make(chan bool, 2)
+		wg := new(sync.WaitGroup)
+		wg.Add(2)
+
+		offerPC, answerPC, err := newPair()
+		if err != nil {
+			t.Fatalf("Failed to create a PC pair for testing")
+		}
+
+		answerPC.OnDataChannel(func(d *DataChannel) {
+			if d.Label() != expectedLabel {
+				return
+			}
+
+			d.OnDial(func() {
+				// only dialing side should fire OnDial
+				t.Fatalf("answering side should not call on dial")
+			})
+
+			d.OnOpen(wg.Done)
+		})
+
+		d, err := offerPC.CreateDataChannel(expectedLabel, nil)
+		assert.NoError(t, err)
+		d.OnDial(func() {
+			dialCalls <- true
+			wg.Done()
+		})
+
+		assert.NoError(t, signalPair(offerPC, answerPC))
+
+		wg.Wait()
+		closePairNow(t, offerPC, answerPC)
+
+		assert.Len(t, dialCalls, 1)
+	})
+
+	t.Run("handler should be called immediately if already dialed", func(t *testing.T) {
+		report := test.CheckRoutines(t)
+		defer report()
+
+		done := make(chan bool)
+
+		offerPC, answerPC, err := newPair()
+		if err != nil {
+			t.Fatalf("Failed to create a PC pair for testing")
+		}
+
+		d, err := offerPC.CreateDataChannel(expectedLabel, nil)
+		assert.NoError(t, err)
+		d.OnOpen(func() {
+			// when the offer DC has been opened, its guaranteed to have dialed since it has
+			// received a response to said dial. this test represents an unrealistic usage,
+			// but its the best way to guarantee we "missed" the dial event and still invoke
+			// the handler.
+			d.OnDial(func() {
+				done <- true
+			})
+		})
+
+		assert.NoError(t, signalPair(offerPC, answerPC))
+
+		closePair(t, offerPC, answerPC, done)
+	})
+}