Re: Missing pg_control crashes postmaster

On 7/23/18 7:00 PM, Tom Lane wrote:
> Brian Faherty <anothergenericuser(at)gmail(dot)com> writes:
>
>> There does not really seem to be a need for this behavior as all the
>> information postgres needs is in memory at this point. I propose with
>> a patch to just recreate pg_control on updates if it does not exist.
>
> I would vote to reject any such patch; it's too likely to cause more
> problems than it solves. Generally, if critical files like that one
> have disappeared, trying to write new data isn't going to be enough
> to fix it and could well result in more corruption.
>
> As an example, imagine that you do "rm -rf $PGDATA; initdb" without
> remembering to shut down the old postmaster first. Currently, the
> old postmaster will panic/quit fairly promptly and no harm done.
> The more aggressive it is at trying to "recover" from the situation,
> the more likely it is to corrupt the new installation.

It seems much more likely that a missing/modified postmaster.pid will
cause postgres to panic than it is for a missing pg_control to do so.

Older versions of postgres don't panic until the next checkpoint and
newer versions won't panic at all on an idle system since we fixed
redundant checkpoints in 9.6 (6ef2eba3). An idle postgres 11 cluster
seems happy enough to run without a pg_control file indefinitely (or at
least 10 minutes, which is past the default checkpoint time). As soon
as I write data or perform a checkpoint it does panic, of course.

Conversely, removing/modifying postmaster.pid causes postgres to panic
very quickly on the versions I tested, 9.4 and 11.

It seems to me that doing the postmaster.pid test at checkpoint time (if
we don't already) would be enough to protect pg_control against
unintentionally replaced clusters.

Or perhaps writing to an alternate file as David J suggests would do the
trick.

It seems like an easy win if we can find a safe way to do it, though I
admit that this is only a benefit in corner cases.

Regards,
--
-David
david(at)pgmasters(dot)net