incus/server/events: Fix issue with race condition

AddHandler is checking if the listener is not nil before starting it.
However, when the last handler is removed, the listener could be set to
nil asynchronously in a goroutine. If a new handler was added immediately
after, there was a chance the listener would never be started again,
leading to dropped events. This fix ensures that the listener is properly
restarted in such cases.

Signed-off-by: Piotr Resztak <[email protected]>

Author: presztak

internal/server/events/internalListener.go

@@ -18,6 +18,7 @@ type InternalListener struct {
 	listenerCtx    context.Context
 	listenerCancel context.CancelFunc
 	lock           sync.Mutex
+	wg             sync.WaitGroup
 }
 
 // NewInternalListener returns an InternalListener.
@@ -49,7 +50,9 @@ func (l *InternalListener) startListener() {
 		l.listener = nil
 	}(l.listenerCtx)
 
+	l.wg.Add(1)
 	go func(ctx context.Context, handlers map[string]EventHandler) {
+		defer l.wg.Done()
 		for {
 			select {
 			case <-ctx.Done():
@@ -75,6 +78,7 @@ func (l *InternalListener) startListener() {
 func (l *InternalListener) stopListener() {
 	if l.listenerCancel != nil {
 		l.listenerCancel()
+		l.wg.Wait()
 	}
 }