Re: mvcc catalo gsnapshots and TopTransactionContext

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On February 2, 2014 5:52:22 PM CET, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> More to the point, changing the Assert so it doesn't fire
>> doesn't do one damn thing to ameliorate the fact that cache reload
>> during transaction abort is wrong and unsafe.

> And, as upthread, I still don't think that's correct. I don't have
> sources available right now, but IIRC we already have aborted out of the
> transaction. Released locks, the xid and everything.

Nope ... the case exhibited in the example is dying in AtEOSubXact_Inval,
which is in the very midst of subxact abort.

I've been thinking about this for the past little while, and I believe
that it's probably okay to have RelationClearRelation leave the relcache
entry un-rebuilt, but with rd_isvalid = false so it will be rebuilt when
next opened. The rationale is explained in the comments in the attached
patch. I've checked that this fixes Noah's test case and still passes
the existing regression tests.

regards, tom lane