MASQUE client: Support XMUX ("maxConnections": 1 by default); Use Chrome's User-Agent by default (#7121)

https://github.com/XTLS/Xray-core/pull/6810#issuecomment-5842441136
https://github.com/XTLS/Xray-core/pull/6878#issuecomment-6095112915
https://github.com/XTLS/Xray-core/pull/7121#issuecomment-6117636043

---------

Co-authored-by: SVLAVR <skugorov.s@gmail.com>

transport/internet/splithttp/mux.go

@@ -79,6 +79,14 @@ func (m *XmuxManager) newXmuxClient() *XmuxClient {
 }
 
 func (m *XmuxManager) GetXmuxClient(ctx context.Context) *XmuxClient { // when locking
+	return m.getXmuxClient(ctx, true)
+}
+
+func (m *XmuxManager) GetExistingXmuxClient(ctx context.Context) *XmuxClient {
+	return m.getXmuxClient(ctx, false)
+}
+
+func (m *XmuxManager) getXmuxClient(ctx context.Context, create bool) *XmuxClient {
 	for i := 0; i < len(m.xmuxClients); {
 		xmuxClient := m.xmuxClients[i]
 		if xmuxClient.XmuxConn.IsClosed() ||
@@ -99,11 +107,14 @@ func (m *XmuxManager) GetXmuxClient(ctx context.Context) *XmuxClient { // when l
 	}
 
 	if len(m.xmuxClients) == 0 {
+		if !create {
+			return nil
+		}
 		errors.LogDebug(ctx, "XMUX: creating xmuxClient because xmuxClients is empty")
 		return m.newXmuxClient()
 	}
 
-	if m.connections > 0 && len(m.xmuxClients) < int(m.connections) {
+	if create && m.connections > 0 && len(m.xmuxClients) < int(m.connections) {
 		errors.LogDebug(ctx, "XMUX: creating xmuxClient because maxConnections was not hit, xmuxClients = ", len(m.xmuxClients))
 		return m.newXmuxClient()
 	}
@@ -120,6 +131,9 @@ func (m *XmuxManager) GetXmuxClient(ctx context.Context) *XmuxClient { // when l
 	}
 
 	if len(xmuxClients) == 0 {
+		if !create {
+			return nil
+		}
 		errors.LogDebug(ctx, "XMUX: creating xmuxClient because maxConcurrency was hit, xmuxClients = ", len(m.xmuxClients))
 		return m.newXmuxClient()
 	}

transport/internet/splithttp/mux_test.go

@@ -90,3 +90,23 @@ func TestDefault(t *testing.T) {
 		t.Error("did not get 1 distinct clients, got ", len(xmuxClients))
 	}
 }
+
+func TestGetExistingXmuxClient(t *testing.T) {
+	xmuxConfig := XmuxConfig{
+		MaxConnections: &RangeConfig{From: 2, To: 2},
+	}
+
+	xmuxManager := NewXmuxManager(xmuxConfig, func() XmuxConn {
+		return &fakeRoundTripper{}
+	})
+
+	if xmuxManager.GetExistingXmuxClient(context.Background()) != nil {
+		t.Error("got a client without creating one")
+	}
+	xmuxClient := xmuxManager.GetXmuxClient(context.Background())
+	for i := 0; i < 4; i++ {
+		if xmuxManager.GetExistingXmuxClient(context.Background()) != xmuxClient {
+			t.Error("did not reuse the existing client")
+		}
+	}
+}