Release the task queue lock while running cancel handlers

em_task_queue_cancel ran cancel handlers with the queue lock held. A
handler can block, and a blocked thread processes its dlopen queue, which
can be the queue being cancelled: a thread exiting during dlopen() then
deadlocks on its own lock. em_task_queue_execute already unlocks around
tasks.

See #18887.

Author: clementperon

system/lib/pthread/em_task_queue.c

@@ -188,7 +188,12 @@ void em_task_queue_cancel(em_task_queue* queue) {
   while (!em_task_queue_is_empty(queue)) {
     task t = em_task_queue_dequeue(queue);
     if (t.cancel) {
+      // Release the lock while running the handler, as in
+      // em_task_queue_execute.  The handler can block, and a blocked thread
+      // processes its dlopen queue, which can be this queue.
+      pthread_mutex_unlock(&queue->mutex);
       t.cancel(t.arg);
+      pthread_mutex_lock(&queue->mutex);
     }
   }
   pthread_mutex_unlock(&queue->mutex);