mds: relax certain asserts in mdlog replay thread

mds: relax certain asserts in mdlog replay thread

The calls to journaler->is_readable() and journaler->get_error()
in MDLog::_replay_thread() will drop Journaler::lock between
invocations, so, theoretically, its possible that the initial check:

  // loop
  int r = 0;
  while (1) {
    // wait for read?
    while (!journaler->is_readable() &&
       journaler->get_read_pos() < journaler->get_write_pos() &&
       !journaler->get_error()) {
      C_SaferCond readable_waiter;
      journaler->wait_for_readable(&readable_waiter);
      r = readable_waiter.wait();
    }
    if (journaler->get_error()) {
      r = journaler->get_error();
      dout(0) << "_replay journaler got error " << r << ", aborting" << dendl;

journaler->is_readable() returned true, thereby breaking out of
the (inner) while loop and by passing the journaler->get_error()
check, and by the time this hits the next set of checks:

    if (!journaler->is_readable() &&
    journaler->get_read_pos() == journaler->get_write_pos())
      break;

    ceph_assert(journaler->is_readable() || mds->is_daemon_stopping());

It's possible that the journal is unreadable due to some error that
happened during prefetch. In short, these checks are racy.

So, remove these racy assert check along with journaler->is_readable()
check when validating the journal end and rely on the next iteration
of reading the journal for error handling.

Fixes: http://tracker.ceph.com/issues/57048
Signed-off-by: Venky Shankar <vshankar@redhat.com>