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

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

common/signal/timer.go

@@ -3,6 +3,7 @@ package signal
 import (
 	"context"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/xtls/xray-core/common"
@@ -14,10 +15,12 @@ type ActivityUpdater interface {
 }
 
 type ActivityTimer struct {
-	sync.RWMutex
+	mu        sync.RWMutex
 	updated   chan struct{}
 	checkTask *task.Periodic
 	onTimeout func()
+	consumed  atomic.Bool
+	once      sync.Once
 }
 
 func (t *ActivityTimer) Update() {
@@ -37,39 +40,39 @@ func (t *ActivityTimer) check() error {
 }
 
 func (t *ActivityTimer) finish() {
-	t.Lock()
-	defer t.Unlock()
+	t.once.Do(func() {
+		t.consumed.Store(true)
+		t.mu.Lock()
+		defer t.mu.Unlock()
 
-	if t.onTimeout != nil {
+		common.CloseIfExists(t.checkTask)
 		t.onTimeout()
-		t.onTimeout = nil
-	}
-	if t.checkTask != nil {
-		t.checkTask.Close()
-		t.checkTask = nil
-	}
+	})
 }
 
 func (t *ActivityTimer) SetTimeout(timeout time.Duration) {
+	if t.consumed.Load() {
+		return
+	}
 	if timeout == 0 {
 		t.finish()
 		return
 	}
 
-	checkTask := &task.Periodic{
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	// double check, just in case
+	if t.consumed.Load() {
+		return
+	}
+	newCheckTask := &task.Periodic{
 		Interval: timeout,
 		Execute:  t.check,
 	}
-
-	t.Lock()
-
-	if t.checkTask != nil {
-		t.checkTask.Close()
-	}
-	t.checkTask = checkTask
+	common.CloseIfExists(t.checkTask)
+	t.checkTask = newCheckTask
 	t.Update()
-	common.Must(checkTask.Start())
-	t.Unlock()
+	common.Must(newCheckTask.Start())
 }
 
 func CancelAfterInactivity(ctx context.Context, cancel context.CancelFunc, timeout time.Duration) *ActivityTimer {