Re: [HACKERS] WAL logging problem in 9.4.3?

Hello, this is a fix for the defect 1 of 3.

At Thu, 26 Dec 2019 12:46:39 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> Thank you for the findings.
>
> At Wed, 25 Dec 2019 16:15:21 -0800, Noah Misch <noah(at)leadboat(dot)com> wrote in
> > By improving AssertPendingSyncs_RelationCache() and by testing with
> > -DRELCACHE_FORCE_RELEASE, I now know of three defects in the attached v30nm.
> > Would you fix these?
>
> I'd like to do that, please give me som time.
>
> > === Defect 1: Forgets to skip WAL after SAVEPOINT; DROP TABLE; ROLLBACK TO
> >
> > A test in transactions.sql now fails in AssertPendingSyncs_RelationCache(),
> > when running "make check" under wal_level=minimal. I test this way:
> >
> > printf '%s\n%s\n' 'wal_level = minimal' 'max_wal_senders = 0' >$PWD/minimal.conf
> > make check TEMP_CONFIG=$PWD/minimal.conf
> >
> > Self-contained demonstration:
> > begin;
> > create table t (c int);
> > savepoint q; drop table t; rollback to q; -- forgets table is skipping wal
> > commit; -- assertion failure

This is complex than expected. The DROP TABLE unconditionally removed
relcache entry. To fix that, I tried to use rd_isinvalid but it failed
because there's a state that a relcache invalid but the corresponding
catalog entry is alive.

In the attached patch 0002, I added a boolean in relcache that
indicates that the relation is already removed in catalog but not
committed. I needed to ignore invalid relcache entries in
AssertPendingSyncs_RelationCache but I think it is the right thing to
do.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center