(copied from upstream tracker)

In production environment, we have a problem: one standby-replay's md_log_replay thread is hanged.

1,The reason:

line1:    while (!journaler->is_readable() &&
  line2:       journaler->get_read_pos() < journaler->get_write_pos() &&
  line3:       !journaler->get_error()) {
  line4:        C_SaferCond readable_waiter;
  line5:        journaler->wait_for_readable(&readable_waiter);
  line6:        r = readable_waiter.wait();
  line7:    }
This code is from void MDLog::_replay_thread().
(1), If the code enter the while and this thread ("md_log_replay") is switched to the MR_Finisher thread between line3 and line5.  (HERE: journaler->get_read_pos() < journaler->get_write_pos())
  (2), Then the MR_Finisher thread calls Journaler::C_Read: finish ls->_finish_read() -> _assimilate_prefetch().
    a) In _assimilate_prefetch(), journaler->get_write_pos() maybe set to be equal to journaler->get_read_pos().
    b) Because the variable on_readable is 0, the f->complete() will not be called!
        if (on_readable) {
          C_OnFinisher *f = on_readable;
          on_readable = 0;
          f->complete(0);
        }
  (3),Then the MR_Finisher thread is switched to the md_log_replay thread, it will hang on line6 forever !!