Re: failure in 019_replslot_limit

At Wed, 5 Apr 2023 11:55:14 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in
> Hi,
>
> On 2023-04-05 11:48:53 -0700, Andres Freund wrote:
> > Note that a checkpoint started at "17:50:23.787", but didn't finish before the
> > database was shut down. As far as I can tell, this can not be caused by
> > checkpoint_timeout, because by the time we get to invalidating replication
> > slots, we already did CheckPointBuffers(), and that's the only thing that
> > delays based on checkpoint_timeout.
> >
> > ISTM that this indicates that checkpointer got stuck after signalling
> > 344783.
> >
> > Do you see any other explanation?
>
> This all sounded vaguely familiar. After a bit bit of digging I found this:
>
> https://postgr.es/m/20220223014855.4lsddr464i7mymk2%40alap3.anarazel.de
>
> Which seems like it plausibly explains the failed test?

As my understanding, ConditionVariableSleep() can experience random
wake-ups and ReplicationSlotControlLock doesn't prevent slot
release. So, I can imagine a situation where that blocking might
happen. If the call ConditionVariableSleep(&s->active_cv) wakes up
unexpectedly due to a latch set for reasons other than the CV
broadcast, and the target process releases the slot between fetching
active_pid in the loop and the following call to
ConditionVariablePrepareToSleep(), the CV broadcast triggered by the
slot release might be missed. If that's the case, we'll need to check
active_pid again after the calling ConditionVariablePrepareToSleep().

Does this make sense?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center