libpod: shutdown Stop waits for handlers completion

wait for handlers currently being processed.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/shutdown/handler.go

@@ -91,6 +91,11 @@ func Stop() error {
 		return nil
 	}
 
+	// if the signal handler is running, wait that it terminates
+	handlerLock.Lock()
+	defer handlerLock.Unlock()
+	// it doesn't need to be in the critical section, but staticcheck complains if
+	// the critical section is empty.
 	cancelChan <- true
 
 	return nil