Re: PANIC during crash recovery of a recently promoted standby

On Thu, Jun 07, 2018 at 07:58:29PM +0900, Kyotaro HORIGUCHI wrote:
> (I believe that) By definition recovery doesn't end until the
> end-of-recovery check point ends so from the viewpoint I think it
> is wrong to clear ControlFile->minRecoveryPoint before the end.
>
> Invalid-page checking during crash recovery is hamful rather than
> useless. It is done by CheckRecoveryConsistency even in crash
> recovery against its expectation because there's a case where
> minRecoveryPoint is valid but InArchiveRecovery is false. The two
> variable there seems to be in contradiction with each other.
>
> The immediate cause of the contradition is that StartXLOG wrongly
> initializes local minRecoveryPoint from control file's value even
> under crash recovery. updateMinRecoveryPoint also should be
> turned off during crash recovery. The case of crash after last
> checkpoint end has been treated in UpdateMinRecoveryPoint but
> forgot to consider this case, where crash recovery has been
> started while control file is still holding valid
> minRecoveryPoint.

I have been digesting your proposal and I reviewed it, and I think that
what you are proposing here is on the right track. However, the updates
around minRecoveryPoint and minRecoveryPointTLI ought to be more
consistent because that could cause future issues. I have spotted two
bug where I think the problem is not fixed: when replaying a WAL record
XLOG_PARAMETER_CHANGE, minRecoveryPoint and minRecoveryPointTLI would
still get updated from the control file values which can still lead to
failures as CheckRecoveryConsistency could still happily trigger a
PANIC, so I think that we had better maintain those values consistent as
long as crash recovery runs. And XLogNeedsFlush() also has a similar
problem.

Note that there is as well the case where the startup process switches
from crash recovery to archive recovery, in which case the update of the
local copies have to happen once the switch is done. Your patch covers
that with just updating updateMinRecoveryPoint each time crash recovery
happens but that's not completely consistent, but it seems that it also
missed the fact that updateMinRecoveryPoint needs to be switched back to
true as the startup process can update the control file. Actually,
updateMinRecoveryPoint needs to be switched back to true in that case or
the startup process would not be able to update minRecoveryPoint when it
calls XLogFlush for example.

There is the point of trying to get rid of updateMinRecoveryPoint which
has crossed my mind, but that's not wise as it's default value allows
the checkpointer minRecoveryPoint when started, which also has to happen
once the startup process gets out of recovery and tells the postmaster
to start the checkpointer. For backends as well that's a sane default.

There is also no point in taking ControlFileLock when checking if the
local copy of minRecoveryPoint is valid or not, so this can be
bypassed.

The assertion in CheckRecoveryConsistency is definitely a good idea as
this should only be called by the startup process, so we can keep it.

In the attached, I have fixed the issues I found and added the test case
which should be included in the final commit. Its concept is pretty
simple, the idea is to keep the local copy of minRecoveryPoint to
InvalidXLogRecPtr as long as crash recovery runs, and is switched back
to normal if there is a switch to archive recovery after a crash
recovery.

This is not really a complicated patch, and it took a lot of energy from
me the last couple of days per the nature of the many scenarios to think
about... So an extra pair of eyes from another committer would be
welcome. I am letting that cool down for a couple of days now.
--
Michael