drop incoming sessions when the accept queue fills up (#24)

p2p/transport/webtransport/listener.go

@@ -158,8 +158,13 @@ func (l *listener) httpHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// TODO: think about what happens when this channel fills up
-	l.queue <- newConn(l.transport, sess, sconn, connScope)
+	select {
+	case l.queue <- newConn(l.transport, sess, sconn, connScope):
+	default:
+		log.Debugw("accept queue full, dropping incoming connection", "peer", sconn.RemotePeer(), "addr", r.RemoteAddr, "error", err)
+		sess.Close()
+		connScope.Done()
+	}
 }
 
 func (l *listener) Accept() (tpt.CapableConn, error) {

p2p/transport/webtransport/transport_test.go

@@ -24,6 +24,7 @@ import (
 	"github.com/libp2p/go-libp2p/core/network"
 	mocknetwork "github.com/libp2p/go-libp2p/core/network/mocks"
 	"github.com/libp2p/go-libp2p/core/peer"
+	tpt "github.com/libp2p/go-libp2p/core/transport"
 
 	"github.com/golang/mock/gomock"
 	ma "github.com/multiformats/go-multiaddr"
@@ -510,3 +511,34 @@ func TestStaticTLSConf(t *testing.T) {
 		defer conn.Close()
 	})
 }
+
+func TestAcceptQueueFilledUp(t *testing.T) {
+	serverID, serverKey := newIdentity(t)
+	tr, err := libp2pwebtransport.New(serverKey, nil, network.NullResourceManager)
+	require.NoError(t, err)
+	defer tr.(io.Closer).Close()
+	ln, err := tr.Listen(ma.StringCast("/ip4/127.0.0.1/udp/0/quic/webtransport"))
+	require.NoError(t, err)
+	defer ln.Close()
+
+	newConn := func() (tpt.CapableConn, error) {
+		t.Helper()
+		_, key := newIdentity(t)
+		cl, err := libp2pwebtransport.New(key, nil, network.NullResourceManager)
+		require.NoError(t, err)
+		defer cl.(io.Closer).Close()
+		return cl.Dial(context.Background(), ln.Multiaddr(), serverID)
+	}
+
+	for i := 0; i < 16; i++ {
+		conn, err := newConn()
+		require.NoError(t, err)
+		defer conn.Close()
+	}
+
+	conn, err := newConn()
+	if err == nil {
+		_, err = conn.AcceptStream()
+	}
+	require.Error(t, err)
+}