Direct/Freedom outbound: Prefer IPv4 for finalRules' "AsIs" (#6075)

https://github.com/XTLS/Xray-core/pull/6075#issuecomment-4391684629
https://github.com/XTLS/Xray-core/pull/6075#issuecomment-4392629645
https://github.com/XTLS/Xray-core/pull/6075#issuecomment-4395258425

common/utils/probe_routes.go

@@ -0,0 +1,73 @@
+package utils
+
+import (
+	"net"
+	"os"
+	"runtime"
+	"sync"
+	"time"
+)
+
+func probeRoutes() (ipv4 bool, ipv6 bool) {
+	if conn, err := net.Dial("udp4", "192.33.4.12:53"); err == nil {
+		ipv4 = true
+		conn.Close()
+	}
+	if conn, err := net.Dial("udp6", "[2001:500:2::c]:53"); err == nil {
+		ipv6 = true
+		conn.Close()
+	}
+	return
+}
+
+var routeCache struct {
+	sync.Once
+	sync.RWMutex
+	expire     time.Time
+	ipv4, ipv6 bool
+}
+
+func CheckRoutes() (bool, bool) {
+	if !isGUIPlatform {
+		routeCache.Once.Do(func() {
+			routeCache.ipv4, routeCache.ipv6 = probeRoutes()
+		})
+		return routeCache.ipv4, routeCache.ipv6
+	}
+
+	routeCache.RWMutex.RLock()
+	now := time.Now()
+	if routeCache.expire.After(now) {
+		routeCache.RWMutex.RUnlock()
+		return routeCache.ipv4, routeCache.ipv6
+	}
+	routeCache.RWMutex.RUnlock()
+
+	routeCache.RWMutex.Lock()
+	defer routeCache.RWMutex.Unlock()
+
+	now = time.Now()
+	if routeCache.expire.After(now) { // double-check
+		return routeCache.ipv4, routeCache.ipv6
+	}
+	routeCache.ipv4, routeCache.ipv6 = probeRoutes()    // ~2ms
+	routeCache.expire = now.Add(100 * time.Millisecond) // ttl
+	return routeCache.ipv4, routeCache.ipv6
+}
+
+var isGUIPlatform = detectGUIPlatform()
+
+func detectGUIPlatform() bool {
+	switch runtime.GOOS {
+	case "android", "ios", "windows", "darwin":
+		return true
+	case "linux", "freebsd", "openbsd":
+		if t := os.Getenv("XDG_SESSION_TYPE"); t == "wayland" || t == "x11" {
+			return true
+		}
+		if os.Getenv("DISPLAY") != "" || os.Getenv("WAYLAND_DISPLAY") != "" {
+			return true
+		}
+	}
+	return false
+}