Re: subscriptionCheck failures on nightjar

From: Andres Freund <andres(at)anarazel(dot)de>
To: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: subscriptionCheck failures on nightjar
Date: 2019-02-13 21:51:47
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On 2019-02-14 09:52:33 +1300, Thomas Munro wrote:
> On Thu, Feb 14, 2019 at 8:11 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Andres Freund <andres(at)anarazel(dot)de> writes:
> > > I was kinda pondering just open coding it. I am not yet convinced that
> > > my idea of just using an open FD isn't the least bad approach for the
> > > issue at hand. What precisely is the NFS issue you're concerned about?
> >
> > I'm not sure that fsync-on-FD after the rename will work, considering that
> > the issue here is that somebody might've unlinked the file altogether
> > before we get to doing the fsync. I don't have a hard time believing that
> > that might result in a failure report on NFS or similar. Yeah, it's
> > hypothetical, but the argument that we need a repeat fsync at all seems
> > equally hypothetical.
> >
> > > Right now fsync_fname_ext isn't exposed outside fd.c...
> >
> > Mmm. That makes it easier to consider changing its API.
> Just to make sure I understand: it's OK for the file not to be there
> when we try to fsync it by name, because a concurrent checkpoint can
> remove it, having determined that we don't need it anymore? In other
> words, we really needed either missing_ok=true semantics, or to use
> the fd we already had instead of the name?

I'm not yet sure that that's actually something that's supposed to
happen, I got to spend some time analysing how this actually
happens. Normally the contents of the slot should actually prevent it
from being removed (as they're newer than
ReplicationSlotsComputeLogicalRestartLSN()). I kind of wonder if that's
a bug in the drop logic in newer releases.


Andres Freund

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