common/signal/timer.go: Refator to use sync.Once (#5052)

Fixes https://github.com/XTLS/Xray-core/issues/5051

common/common.go

@@ -7,6 +7,7 @@ import (
 	"go/build"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strings"
 
 	"github.com/xtls/xray-core/common/errors"
@@ -153,3 +154,14 @@ func GetModuleName(pathToProjectRoot string) (string, error) {
 	}
 	return moduleName, fmt.Errorf("no `go.mod` file in every parent directory of `%s`", pathToProjectRoot)
 }
+
+// CloseIfExists call obj.Close() if obj is not nil.
+func CloseIfExists(obj any) error {
+	if obj != nil {
+		v := reflect.ValueOf(obj)
+		if !v.IsNil() {
+			return Close(obj)
+		}
+	}
+	return nil
+}