Repository: pion/webrtc (go)

Issue:
PeerConnection should drop reference to datachannel when it's closed

### Your environment.
  - Version: 3.x


### What did you do?
libp2p is a peer to peer networking library used in IPFS among other things. We are using webrtc in go-libp2p to provide connectivity to nodes running in the browser. For our use, we create a peer connection and open lots of data channels over the life time of the connection. 

Currently the reference to datachannels is not dropped when a datachannel is closed. `PeerConnection.sctpTransport.dataChannels` always has a reference to the existing data channels which has a reference to the sctp stream. Over the life time of the connection this leads to a lot of memory usage. 

### What did you expect?

I expected the memory used by the datachannel to be garbage collected. 


### Proposed fix

`Datachannel` has an OnClose handler. This handler should remove the datachannel from the sctpTransport
To preserve existing behaviour we should:
1. Only keep the reference to the ID that was used on the datachannel
2. Create a `DataChannelStats` object for the stats on the datachannel to preserve collectStats behaviour. 




Pull request description:
Drop reference to detached datachannels
This allows users of detached datachannels to garbage collect resources associated with the datachannel and the sctp stream. There is no functional change here.

Fixes #2672 


Patch:
diff --git a/datachannel.go b/datachannel.go
index 96ae9121..a846eb58 100644
--- a/datachannel.go
+++ b/datachannel.go
@@ -420,7 +420,6 @@ func (d *DataChannel) ensureOpen() error {
 // resulting DataChannel object.
 func (d *DataChannel) Detach() (datachannel.ReadWriteCloser, error) {
 	d.mu.Lock()
-	defer d.mu.Unlock()
 
 	if !d.api.settingEngine.detach.DataChannels {
 		return nil, errDetachNotEnabled
@@ -432,7 +431,28 @@ func (d *DataChannel) Detach() (datachannel.ReadWriteCloser, error) {
 
 	d.detachCalled = true
 
-	return d.dataChannel, nil
+	dataChannel := d.dataChannel
+	d.mu.Unlock()
+
+	// Remove the reference from SCTPTransport so that the datachannel
+	// can be garbage collected on close
+	d.sctpTransport.lock.Lock()
+	n := len(d.sctpTransport.dataChannels)
+	j := 0
+	for i := 0; i < n; i++ {
+		if d == d.sctpTransport.dataChannels[i] {
+			continue
+		}
+		d.sctpTransport.dataChannels[j] = d.sctpTransport.dataChannels[i]
+		j++
+	}
+	for i := j; i < n; i++ {
+		d.sctpTransport.dataChannels[i] = nil
+	}
+	d.sctpTransport.dataChannels = d.sctpTransport.dataChannels[:j]
+	d.sctpTransport.lock.Unlock()
+
+	return dataChannel, nil
 }
 
 // Close Closes the DataChannel. It may be called regardless of whether
diff --git a/peerconnection.go b/peerconnection.go
index ea1e2f47..dfafbf10 100644
--- a/peerconnection.go
+++ b/peerconnection.go
@@ -2018,6 +2018,9 @@ func (pc *PeerConnection) CreateDataChannel(label string, options *DataChannelIn
 
 	pc.sctpTransport.lock.Lock()
 	pc.sctpTransport.dataChannels = append(pc.sctpTransport.dataChannels, d)
+	if d.ID() != nil {
+		pc.sctpTransport.dataChannelIDsUsed[*d.ID()] = struct{}{}
+	}
 	pc.sctpTransport.dataChannelsRequested++
 	pc.sctpTransport.lock.Unlock()
 
diff --git a/sctptransport.go b/sctptransport.go
index 26cee487..800de1df 100644
--- a/sctptransport.go
+++ b/sctptransport.go
@@ -52,6 +52,7 @@ type SCTPTransport struct {
 
 	// DataChannels
 	dataChannels          []*DataChannel
+	dataChannelIDsUsed    map[uint16]struct{}
 	dataChannelsOpened    uint32
 	dataChannelsRequested uint32
 	dataChannelsAccepted  uint32
@@ -65,10 +66,11 @@ type SCTPTransport struct {
 // meant to be used together with the basic WebRTC API.
 func (api *API) NewSCTPTransport(dtls *DTLSTransport) *SCTPTransport {
 	res := &SCTPTransport{
-		dtlsTransport: dtls,
-		state:         SCTPTransportStateConnecting,
-		api:           api,
-		log:           api.settingEngine.LoggerFactory.NewLogger("ortc"),
+		dtlsTransport:      dtls,
+		state:              SCTPTransportStateConnecting,
+		api:                api,
+		log:                api.settingEngine.LoggerFactory.NewLogger("ortc"),
+		dataChannelIDsUsed: make(map[uint16]struct{}),
 	}
 
 	res.updateMessageSize()
@@ -287,6 +289,13 @@ func (r *SCTPTransport) onDataChannel(dc *DataChannel) (done chan struct{}) {
 	r.lock.Lock()
 	r.dataChannels = append(r.dataChannels, dc)
 	r.dataChannelsAccepted++
+	if dc.ID() != nil {
+		r.dataChannelIDsUsed[*dc.ID()] = struct{}{}
+	} else {
+		// This cannot happen, the constructor for this datachannel in the caller
+		// takes a pointer to the id.
+		r.log.Errorf("accepted data channel with no ID")
+	}
 	handler := r.onDataChannelHandler
 	r.lock.Unlock()
 
@@ -393,21 +402,12 @@ func (r *SCTPTransport) generateAndSetDataChannelID(dtlsRole DTLSRole, idOut **u
 	r.lock.Lock()
 	defer r.lock.Unlock()
 
-	// Create map of ids so we can compare without double-looping each time.
-	idsMap := make(map[uint16]struct{}, len(r.dataChannels))
-	for _, dc := range r.dataChannels {
-		if dc.ID() == nil {
-			continue
-		}
-
-		idsMap[*dc.ID()] = struct{}{}
-	}
-
 	for ; id < max-1; id += 2 {
-		if _, ok := idsMap[id]; ok {
+		if _, ok := r.dataChannelIDsUsed[id]; ok {
 			continue
 		}
 		*idOut = &id
+		r.dataChannelIDsUsed[id] = struct{}{}
 		return nil
 	}
 


Test patch:
diff --git a/datachannel_go_test.go b/datachannel_go_test.go
index 72be159b..baea1a9e 100644
--- a/datachannel_go_test.go
+++ b/datachannel_go_test.go
@@ -692,3 +692,57 @@ func TestDataChannel_Dial(t *testing.T) {
 		closePair(t, offerPC, answerPC, done)
 	})
 }
+
+func TestDetachRemovesDatachannelReference(t *testing.T) {
+	// Use Detach data channels mode
+	s := SettingEngine{}
+	s.DetachDataChannels()
+	api := NewAPI(WithSettingEngine(s))
+
+	// Set up two peer connections.
+	config := Configuration{}
+	pca, err := api.NewPeerConnection(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pcb, err := api.NewPeerConnection(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer closePairNow(t, pca, pcb)
+
+	dcChan := make(chan *DataChannel, 1)
+	pcb.OnDataChannel(func(d *DataChannel) {
+		d.OnOpen(func() {
+			if _, detachErr := d.Detach(); detachErr != nil {
+				t.Error(detachErr)
+			}
+
+			dcChan <- d
+		})
+	})
+
+	if err = signalPair(pca, pcb); err != nil {
+		t.Fatal(err)
+	}
+
+	attached, err := pca.CreateDataChannel("", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	open := make(chan struct{}, 1)
+	attached.OnOpen(func() {
+		open <- struct{}{}
+	})
+	<-open
+
+	d := <-dcChan
+	d.sctpTransport.lock.RLock()
+	defer d.sctpTransport.lock.RUnlock()
+	for _, dc := range d.sctpTransport.dataChannels[:cap(d.sctpTransport.dataChannels)] {
+		if dc == d {
+			t.Errorf("expected sctpTransport to drop reference to datachannel")
+		}
+	}
+}
diff --git a/sctptransport_test.go b/sctptransport_test.go
index 74313deb..9943e8f0 100644
--- a/sctptransport_test.go
+++ b/sctptransport_test.go
@@ -10,11 +10,15 @@ import "testing"
 
 func TestGenerateDataChannelID(t *testing.T) {
 	sctpTransportWithChannels := func(ids []uint16) *SCTPTransport {
-		ret := &SCTPTransport{dataChannels: []*DataChannel{}}
+		ret := &SCTPTransport{
+			dataChannels:       []*DataChannel{},
+			dataChannelIDsUsed: make(map[uint16]struct{}),
+		}
 
 		for i := range ids {
 			id := ids[i]
 			ret.dataChannels = append(ret.dataChannels, &DataChannel{id: &id})
+			ret.dataChannelIDsUsed[id] = struct{}{}
 		}
 
 		return ret
@@ -46,5 +50,8 @@ func TestGenerateDataChannelID(t *testing.T) {
 		if *idPtr != testCase.result {
 			t.Errorf("Wrong id: %d expected %d", *idPtr, testCase.result)
 		}
+		if _, ok := testCase.s.dataChannelIDsUsed[*idPtr]; !ok {
+			t.Errorf("expected new id to be added to the map: %d", *idPtr)
+		}
 	}
 }