Re: [Patch] Make pg_checksums skip foreign tablespace directories

Hi,

Am Freitag, den 31.01.2020, 13:53 +0900 schrieb Michael Paquier:
> On Thu, Jan 30, 2020 at 06:11:22PM +0100, Michael Banck wrote:
> Having a past tablespace version left
> around after an upgrade is a pilot error in my opinion because
> pg_upgrade generates a script to cleanup past tablespaces, no? So
> your patch does not look like a good idea to me.

Not sure I agree with it, but if that (i.e. after pg_upgrade in copy
mode, you have no business to use the old cluster as well as the new
one) is project policy, fair enough.

However, Postgres does not disallow to just create tablespaces in the
same location from two different versions, so you don't need the
pg_upgade scenario to get into this (pg_checksums checking the wrong
cluster's data) problem:

postgres(at)kohn:~$ psql -p 5437 -c "CREATE TABLESPACE bar LOCATION '/var/lib/postgresql/bar'"
CREATE TABLESPACE
postgres(at)kohn:~$ psql -p 5444 -c "CREATE TABLESPACE bar LOCATION '/var/lib/postgresql/bar'"
CREATE TABLESPACE
postgres(at)kohn:~$ ls bar
PG_11_201809051 PG_12_201909212
postgres(at)kohn:~$ touch bar/PG_11_201809051/pg_internal.init.123
postgres(at)kohn:~$ pg_ctlcluster 12 main stop
sudo systemctl stop postgresql(at)12-main
postgres(at)kohn:~$ LANG=C /usr/lib/postgresql/12/bin/pg_checksums -D /var/lib/postgresql/12/main
pg_checksums: error: invalid segment number 0 in file name
"/var/lib/postgresql/12/main/pg_tblspc/16396/PG_11_201809051/pg_internal
.init.123"

I believe this is in order to allow pg_upgrade to run in the first
place. But is this pilot error as well? In any case, it is a situation
we allow to happen so IMO we should fix pg_checksums to skip the foreign
tablespaces.

As an aside, I would advocate to just skip files which fail the segment
number determination step (and maybe log a warning), not bail out.

Michael

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