Date: Thu, 26 Mar 2020 23:44:24 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Andres Freund <andres@anarazel.de>,
	Michael Paquier <michael@paquier.xyz>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: error context for vacuum to include block number
Message-ID: <20200327044424.GD20103@telsasoft.com>
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On Fri, Mar 27, 2020 at 09:49:29AM +0530, Amit Kapila wrote:
> On Fri, Mar 27, 2020 at 3:47 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> >
> > > Hm, I was just wondering what happens if an error happens *during*
> > > update_vacuum_error_cbarg().  It seems like if we set
> > > errcbarg->phase=VACUUM_INDEX before setting errcbarg->indname=indname, then an
> > > error would cause a crash.
> > >
> 
> Can't that be avoided if you check if cbarg->indname is non-null in
> vacuum_error_callback as we are already doing for
> VACUUM_ERRCB_PHASE_TRUNCATE?
> 
> > >  And if we pfree and set indname before phase, it'd
> > > be a problem when going from an index phase to non-index phase.
> 
> How is it possible that we move to the non-index phase without
> clearing indname as we always revert back the old phase information?

The crash scenario I'm trying to avoid would be like statement_timeout or other
asynchronous event occurring between two non-atomic operations.

I said that there's an issue no matter what order we set indname/phase;
If we wrote:
|cbarg->indname = indname;
|cbarg->phase = phase;
..and hit a timeout (or similar) between setting indname=NULL but before
setting phase=VACUUM_INDEX, then we can crash due to null pointer.

But if we write:
|cbarg->phase = phase;
|if (cbarg->indname) {pfree(cbarg->indname);}
|cbarg->indname = indname ? pstrdup(indname) : NULL;
..then we can still crash if we timeout between freeing cbarg->indname and
setting it to null, due to acccessing a pfreed allocation.

> > >  So maybe we
> > > have to set errcbarg->phase=VACUUM_ERRCB_PHASE_UNKNOWN while in the function,
> > > and errcbarg->phase=phase last.
> 
> I find that a bit ad-hoc, if possible, let's try to avoid it.

I think we can do what you suggesting, if the callback checks if (cbarg->indname!=NULL).

We'd have to write:
// Must set indname *before* updating phase, in case an error occurs before
// phase is set, to avoid crashing if we're going from an index phase to a
// non-index phase (which should not read indname).  Must not free indname
// until it's set to null.
char *tmp = cbarg->indname;
cbarg->indname = indname ? pstrdup(indname) : NULL;
cbarg->phase = phase;
if (tmp){pfree(tmp);}

Do you think that's better ?

-- 
Justin