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From: Justin Pryzby <pryzby@telsasoft.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: pgsql-hackers@postgresql.org
Subject: Re: error context for vacuum to include block number
Date: Fri, 27 Mar 2020 14:04:29 -0500
Message-ID: <20200327190428.GS20103@telsasoft.com> (raw)
In-Reply-To: <CAA4eK1LKYnDV8bcx59TdR5-s2Cfk75g_wfAjRyy-cL8S+WvHXw@mail.gmail.com>
References: <20200326044115.GB28385@telsasoft.com>
	<CAA4eK1LBtp+qpsz8uR-oj2h6jzhLEVo9TEdm46anKM-n=8nS0g@mail.gmail.com>
	<CAA4eK1L33gvQ1z-CHctzTWng2HfmFn0cDJif_mpBOvG47Mymow@mail.gmail.com>
	<CA+fd4k61uPzeAWT1EZOwPWq7DkhgVTrC_wmUNCzVW+MbP9Wjfg@mail.gmail.com>
	<20200326150457.GB17431@telsasoft.com>
	<20200326221752.GR17431@telsasoft.com>
	<CAA4eK1+b5-aJcbpvqCFB7Qi0zLKTZNJmA+Js644M2YeiEyTZtw@mail.gmail.com>
	<20200327044424.GD20103@telsasoft.com>
	<20200327061601.GE20103@telsasoft.com>
	<CAA4eK1LKYnDV8bcx59TdR5-s2Cfk75g_wfAjRyy-cL8S+WvHXw@mail.gmail.com>

On Fri, Mar 27, 2020 at 11:50:30AM +0530, Amit Kapila wrote:
> > > The crash scenario I'm trying to avoid would be like statement_timeout or other
> > > asynchronous event occurring between two non-atomic operations.
> > >
> > +if (errinfo->phase==VACUUM_ERRCB_PHASE_VACUUM_INDEX && errinfo->indname==NULL)
> > +{
> > +kill(getpid(), SIGINT);
> > +pg_sleep(1); // that's needed since signals are delivered asynchronously
> > +}
> > I'm not sure if those are possible outside of "induced" errors.  Maybe the
> > function is essentially atomic due to no CHECK_FOR_INTERRUPTS or similar?
> 
> Yes, this is exactly the point.  I think unless you have
> CHECK_FOR_INTERRUPTS in that function, the problems you are trying to
> think won't happen.

Hm, but I caused a crash *without* adding CHECK_FOR_INTERRUPTS, just
kill+sleep.  The kill() could come from running pg_cancel_backend().  And the
sleep() just encourages a context switch, which can happen at any time.  I'm
not convinced that the function couldn't be interrupted by a signal.

-- 
Justin





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  Cc: pryzby@telsasoft.com, amit.kapila16@gmail.com, masahiko.sawada@2ndquadrant.com, alvherre@2ndquadrant.com, andres@anarazel.de, michael@paquier.xyz
  Subject: Re: error context for vacuum to include block number
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