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

On 2013-12-03 15:46:09 -0500, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > I'd rather have an readily-verifiable fix that changes WAL format than a
> > tricky fix that avoids doing so.  So, modulo not having seen the change, +1.
>
> Yeah, same here.

I am afraid it won't be *that* simple. We still need code to look into
multis, check whether all members are ok wrt. cutoff_xid and replace
them, either by the contained xid, or by a new multi with the still
living members. Ugly.

There's currently also the issue that heap_freeze_tuple() modifies the
tuple inplace without a critical section. We're executing a
HeapTupleSatisfiesVacuum() before we get to WAL logging things, that has
plenty of rope to hang itself on. So that doesn't really seem ok to me?

Attached is a pre-pre alpha patch for this. To fix the issue with the
missing critical section it splits freezing into
heap_prepare_freeze_tuple() and heap_execute_freeze_tuple(). The former
doesn't touch tuples and is executed on the primary, and the second
actually peforms the modifications and is executed both, during normal
processing and recovery.

Needs a fair bit of work:
* Should move parts of the multixact processing into multixact.c,
  specifically it shouldn't require CreateMultiXactId() to be exported.
* it relies on forward-declaring a struct in heapam.h that's actually
  defined heapam_xlog.h - that's pretty ugly.
* any form of testing but make check/isolationcheck across SR.
* lots of the comments around need to be added/reworked
* has a simpler version of Alvaro's patch to HTSV in there

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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