innodb purge--srv_worker_thread

1、将trh放到srv_sys->tasks队列

srv_que_task_enqueue_low：
    UT_LIST_ADD_LAST(queue, srv_sys->tasks, thr);
    srv_release_threads(SRV_WORKER, 1);
        slot->suspended = FALSE;
        释放一个srv_slot：os_event_set(slot->event);
        slot = &srv_sys->sys_threads[i];，i从0开始，找一个挂起的slot。

2、srv_worker_thread

srv_worker_thread：
do {
    srv_suspend_thread(slot);// slot->suspended = TRUE;
    os_event_wait(slot->event);//等待os_event_set
    if (srv_task_execute()) {//fetch task from queue and run purge
      /* If there are tasks in the queue, wakeup the purge coordinator thread. */
      srv_wake_purge_thread_if_not_active();
    }
    /* Note: we are checking the state without holding the purge_sys->latch here. */
  } while (purge_sys->state != PURGE_STATE_EXIT);

purge_sys->state在协调线程退出时置成PURGE_STATE_EXIT，所以worker线程是在协调线程退出后才退出