[sheepdog] Is it necessary for outstanding io block leave/join event?

[Yibin Shen]

static inline void process_event_queue(void)
{
        struct event_struct *cevent;
        /*
         * we need to serialize events so we don't call queue_work
         * if one event is running by executing event_fn() or event_done().
         */
        if (event_running || sys->nr_outstanding_io)
                return;

        cevent = list_first_entry(&sys->event_queue,
                        struct event_struct, event_list);
        list_del(&cevent->event_list);
        sys->cur_cevent = cevent;

        event_running = 1;

        event_work.fn = event_fn;
        event_work.done = event_done;

        unregister_event(cdrv_fd);
        queue_work(sys->event_wqueue, &event_work);
}

given some simple cases, there is a sheep cluster with node A, B, C,
and node C down.

1.1 case 1
===========
1. A have infly IO(A->B)
2. C leave
3. switchs
   1. B have no infly IO, so B start to handle leave event, epoch++.
      1. B start to handle IO(A->B), epoch is not indentical, so return fail.
      2. A re-queue IO(A->B).
      3. A start to handle leave event. epoch++.
      4. resend IO.
   2. B have infly IO(^A->B)
      1. B finish infly IO(^A->B)
      2. B start to handle leave event , epoch++
      3. B start to handle IO(A->B), but epoch is not indentical, return fail.
      4. A re-queue IO(A->B).
      5. A start to handle leave event. epoch++.
      6. resend IO(A->B).
   3. B have infly IO(A->B)
      1. B finish infly IO(A->B).
      2. B start to handle leave event, epoch++.
      3. A finish infly IO(A->B).
      4. A start to handle leave event, epoch++.

1.2 case 2
===========
1. infly IO(A->C)
2. C down
3. C is unreachable, so return fail after socket dead
4. A re-queue IO(A->C)
5. A start to handle leave event, epoch++.

join event is similar. so in all cases I can consider,
seems harmless for leave/join event be handled while sheep still have
outstanding IO,
that will accelerate cluster recovery effectively.

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