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From: Noah Misch <noah@leadboat.com>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Amit Kapila <amit.kapila16@gmail.com>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Michael Paquier <michael.paquier@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Reviewing freeze map code
Date: Sun, 26 Jun 2016 17:39:08 -0400
Message-ID: <20160626213908.GA1317196@tornado.leadboat.com> (raw)
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On Tue, Jun 21, 2016 at 10:59:25AM +1200, Thomas Munro wrote:
> On Fri, Jun 17, 2016 at 3:36 PM, Noah Misch <noah@leadboat.com> wrote:
> > I agree the non-atomic, unlogged change is a problem.  A related threat
> > doesn't require a torn page:
> >
> >   AssignTransactionId() xid=123
> >   heapam.c:3931 HeapTupleHeaderSetXmax(oldtup.t_data, 123);
> >   some ERROR before heap_update() finishes
> >   rollback;  -- xid=123
> >   some backend flushes the modified page
> >   immediate shutdown
> >   AssignTransactionId() xid=123
> >   commit;  -- xid=123
> >
> > If nothing wrote an xlog record that witnesses xid 123, the cluster can
> > reassign it after recovery.  The failed update is now considered a successful
> > update, and the row improperly becomes dead.  That's important.
> 
> I wonder if that was originally supposed to be handled with the
> HEAP_XMAX_UNLOGGED flag which was removed in 11919160.  A comment in
> the heap WAL logging commit f2bfe8a2 said that tqual routines would
> see the HEAP_XMAX_UNLOGGED flag in the event of a crash before logging
> (though I'm not sure if the tqual routines ever actually did that).

HEAP_XMAX_UNLOGGED does appear to have originated in contemplation of this
same hazard.  Looking at the three commits in "git log -S HEAP_XMAX_UNLOGGED"
(f2bfe8a b58c041 1191916), nothing ever completed the implementation by
testing for that flag.


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