Fix detached datachannels handling

#2696 introduced removing
datachannels from the sctptransport for better garbage collection.

That PR introduced a race condition for data channels created before
connection establishment. When an out of band negotiated data channel,
created before peerconnection establishment is detached, there's a race
between the data channel being removed from `r.dataChannels` and it
being copied in to the existing data channel slice in the
acceptDataChannels goroutine.

This PR fixes this race by copying the slice before any datachannels
could be detached.

sctptransport.go

@@ -142,7 +142,7 @@ func (r *SCTPTransport) Start(_ SCTPCapabilities) error {
 	r.dataChannelsOpened += openedDCCount
 	r.lock.Unlock()
 
-	go r.acceptDataChannels(sctpAssociation)
+	go r.acceptDataChannels(sctpAssociation, dataChannels)
 
 	return nil
 }
@@ -163,10 +163,9 @@ func (r *SCTPTransport) Stop() error {
 	return nil
 }
 
-func (r *SCTPTransport) acceptDataChannels(a *sctp.Association) {
-	r.lock.RLock()
-	dataChannels := make([]*datachannel.DataChannel, 0, len(r.dataChannels))
-	for _, dc := range r.dataChannels {
+func (r *SCTPTransport) acceptDataChannels(a *sctp.Association, existingDataChannels []*DataChannel) {
+	dataChannels := make([]*datachannel.DataChannel, 0, len(existingDataChannels))
+	for _, dc := range existingDataChannels {
 		dc.mu.Lock()
 		isNil := dc.dataChannel == nil
 		dc.mu.Unlock()
@@ -175,8 +174,6 @@ func (r *SCTPTransport) acceptDataChannels(a *sctp.Association) {
 		}
 		dataChannels = append(dataChannels, dc.dataChannel)
 	}
-	r.lock.RUnlock()
-
 ACCEPT:
 	for {
 		dc, err := datachannel.Accept(a, &datachannel.Config{

sctptransport_test.go

@@ -8,6 +8,7 @@ package webrtc
 
 import (
 	"bytes"
+	"sync"
 	"testing"
 	"time"
 
@@ -126,3 +127,127 @@ func TestSCTPTransportOnClose(t *testing.T) {
 		t.Fatal("timed out")
 	}
 }
+
+func TestSCTPTransportOutOfBandNegotiatedDataChannelDetach(t *testing.T) {
+	// create lots of peer connections
+	done := make(chan struct{}, 50)
+	const N = 50
+	for i := 0; i < N; i++ {
+		go func() {
+			// Use Detach data channels mode
+			s := SettingEngine{}
+			s.DetachDataChannels()
+			api := NewAPI(WithSettingEngine(s))
+
+			// Set up two peer connections.
+			config := Configuration{}
+			offerPC, err := api.NewPeerConnection(config)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			answerPC, err := api.NewPeerConnection(config)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+
+			defer closePairNow(t, offerPC, answerPC)
+			defer func() { done <- struct{}{} }()
+
+			negotiated := true
+			id := uint16(0)
+			readDetach := make(chan struct{})
+			dc1, err := offerPC.CreateDataChannel("", &DataChannelInit{
+				Negotiated: &negotiated,
+				ID:         &id,
+			})
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			dc1.OnOpen(func() {
+				_, _ = dc1.Detach()
+				close(readDetach)
+			})
+
+			writeDetach := make(chan struct{})
+			dc2, err := answerPC.CreateDataChannel("", &DataChannelInit{
+				Negotiated: &negotiated,
+				ID:         &id,
+			})
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			dc2.OnOpen(func() {
+				_, _ = dc2.Detach()
+				close(writeDetach)
+			})
+
+			var wg sync.WaitGroup
+			wg.Add(2)
+			go func() {
+				defer wg.Done()
+				connestd := make(chan struct{}, 1)
+				offerPC.OnConnectionStateChange(func(state PeerConnectionState) {
+					if state == PeerConnectionStateConnected {
+						connestd <- struct{}{}
+					}
+				})
+				select {
+				case <-connestd:
+				case <-time.After(10 * time.Second):
+					t.Error("conn establishment timed out")
+					return
+				}
+				<-readDetach
+				err := dc1.dataChannel.SetReadDeadline(time.Now().Add(10 * time.Second))
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				buf := make([]byte, 10)
+				n, err := dc1.dataChannel.Read(buf)
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				if string(buf[:n]) != "hello" {
+					t.Error("invalid read")
+				}
+			}()
+			go func() {
+				defer wg.Done()
+				connestd := make(chan struct{}, 1)
+				answerPC.OnConnectionStateChange(func(state PeerConnectionState) {
+					if state == PeerConnectionStateConnected {
+						connestd <- struct{}{}
+					}
+				})
+				select {
+				case <-connestd:
+				case <-time.After(10 * time.Second):
+					t.Error("connection establishment timed out")
+					return
+				}
+				<-writeDetach
+				n, err := dc2.dataChannel.Write([]byte("hello"))
+				if err != nil || n != len("hello") {
+					t.Error(err)
+				}
+			}()
+			err = signalPair(offerPC, answerPC)
+			require.NoError(t, err)
+			wg.Wait()
+		}()
+	}
+
+	for i := 0; i < N; i++ {
+		select {
+		case <-done:
+		case <-time.After(20 * time.Second):
+			t.Fatal("timed out")
+		}
+	}
+}