Add runtime warnings

common/warning/warning.go

@@ -0,0 +1,31 @@
+package warning
+
+import (
+	"sync"
+
+	"github.com/sagernet/sing-box/log"
+)
+
+type Warning struct {
+	logger    log.Logger
+	check     CheckFunc
+	message   string
+	checkOnce sync.Once
+}
+
+type CheckFunc = func() bool
+
+func New(checkFunc CheckFunc, message string) Warning {
+	return Warning{
+		check:   checkFunc,
+		message: message,
+	}
+}
+
+func (w *Warning) Check() {
+	w.checkOnce.Do(func() {
+		if w.check() {
+			log.Warn(w.message)
+		}
+	})
+}