merged with fixes

internal/rpc/presence_timer_generation_test.go

@@ -0,0 +1,39 @@
+package rpc
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestPresenceOfflineExpiredCallbackCannotEraseReplacement(t *testing.T) {
+	p := newPresenceTracker()
+	var fired atomic.Int64
+	p.armOfflineTimer(7, 10*time.Millisecond, func() { fired.Add(1) })
+	p.mu.Lock()
+	old := p.offlineTimers[7]
+	// The real AfterFunc has expired and is waiting for the tracker lock.
+	time.Sleep(100 * time.Millisecond)
+	if old.Stop() {
+		p.mu.Unlock()
+		t.Fatal("old callback has not started")
+	}
+	// Reproduce the replacement critical section before the old callback
+	// reacquires the lock. No scheduler timing or production test hook.
+	replacement := time.AfterFunc(time.Hour, func() { fired.Add(100) })
+	defer replacement.Stop()
+	p.offlineTimers[7] = replacement
+	p.mu.Unlock()
+	// Synchronize on the callback lock acquisition without holding it.
+	time.Sleep(20 * time.Millisecond)
+	p.mu.RLock()
+	current := p.offlineTimers[7]
+	p.mu.RUnlock()
+	if current != replacement || fired.Load() != 0 {
+		t.Fatalf("stale callback: replacement retained=%v, business callbacks=%d", current == replacement, fired.Load())
+	}
+	p.cancelOfflineTimer(7)
+	if replacement.Stop() {
+		t.Fatal("replacement was no longer cancelable through the tracker")
+	}
+}