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>

proxy/masque/client_test.go

@@ -0,0 +1,54 @@
+package masque
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/xtls/xray-core/transport/internet/splithttp"
+)
+
+func newTestClient(newConn func(c *Client) splithttp.XmuxConn) *Client {
+	c := &Client{ctx: context.Background()}
+	c.xmux = splithttp.NewXmuxManager(splithttp.XmuxConfig{
+		MaxConnections: &splithttp.RangeConfig{From: 3, To: 3},
+	}, func() splithttp.XmuxConn {
+		return newConn(c)
+	})
+	return c
+}
+
+func TestGetTunnelFallsBack(t *testing.T) {
+	refused := errors.New("refused")
+	healthy := &tunnel{done: make(chan struct{})}
+	dials := 0
+	c := newTestClient(func(c *Client) splithttp.XmuxConn {
+		if dials++; dials == 1 {
+			return healthy
+		}
+		c.lastErr, c.lastErrAt = refused, time.Now()
+		return failedTunnel(refused)
+	})
+	for i := 0; i < 5; i++ {
+		if _, got, err := c.getTunnel(context.Background(), nil); err != nil || got != healthy {
+			t.Fatalf("flow %d: got %p, %v instead of the healthy tunnel", i, got, err)
+		}
+	}
+	if dials != 2 {
+		t.Errorf("dialed %d times, a failed tunnel should not be retried within %v", dials, retryInterval)
+	}
+}
+
+func TestGetTunnelFailsWithoutTunnels(t *testing.T) {
+	refused := errors.New("refused")
+	c := newTestClient(func(c *Client) splithttp.XmuxConn {
+		c.lastErr, c.lastErrAt = refused, time.Now()
+		return failedTunnel(refused)
+	})
+	for i := 0; i < 2; i++ {
+		if _, _, err := c.getTunnel(context.Background(), nil); err != refused {
+			t.Fatalf("flow %d: got %v instead of %v", i, err, refused)
+		}
+	}
+}