Re: pgsql: Fix a couple of bugs in MultiXactId freezing

Hi,

On 2013-12-03 10:29:54 -0500, Noah Misch wrote:
> On Tue, Dec 03, 2013 at 04:08:23PM +0100, Andres Freund wrote:
> > On 2013-12-03 09:16:18 -0500, Noah Misch wrote:
> > > On Tue, Dec 03, 2013 at 11:56:07AM +0100, Andres Freund wrote:
> > > > On 2013-12-03 00:47:07 -0500, Noah Misch wrote:
> > > > > On Sat, Nov 30, 2013 at 01:06:09AM +0000, Alvaro Herrera wrote:
> > > > Any idea how to cheat our way out of that one given the current way
> > > > heap_freeze_tuple() works (running on both primary and standby)? My only
> > > > idea was to MultiXactIdWait() if !InRecovery but that's extremly grotty.
> > > > We can't even realistically create a new multixact with fewer members
> > > > with the current format of xl_heap_freeze.
> > >
> > > Perhaps set HEAP_XMAX_LOCK_ONLY on the tuple? We'd then ensure all update XID
> > > consumers check HEAP_XMAX_IS_LOCKED_ONLY() first, much like xmax consumers are
> > > already expected to check HEAP_XMAX_INVALID first. Seems doable, albeit yet
> > > another injection of complexity.
> >
> > I think its pretty much checked that way already, but the problem seems
> > to be how to avoid checks on xid commit/abort in that case. I've
> > complained in 20131121200517(dot)GM7240(at)alap2(dot)anarazel(dot)de that the old
> > pre-condition that multixacts aren't checked when they can't be relevant
> > (via OldestVisibleM*) isn't observed anymore.
> > So, if we re-introduce that condition again, we should be on the safe
> > side with that, right?
>
> What specific commit/abort checks do you have in mind?

MultiXactIdIsRunning() does a TransactionIdIsInProgress() for each
member which in turn does TransactionIdDidCommit(). Similar when locking
a tuple that's locked/updated without a multixact where we go for a
TransactionIdIsInProgress() in XactLockTableWait().

Greetings,

Andres Freund

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