Re: Somebody has not thought through subscription locking considerations

Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com> writes:
> On 31/03/17 19:35, Tom Lane wrote:
>> At the very least, it would be a good idea to exclude the system
>> catalogs from logical replication, wouldn't it?

> They are excluded.

Well, the exclusion is apparently not checked early enough. I would say
that touches of system catalogs should never result in any attempts to
access the logical relation infrastructure, but what we have here is
such an access.

> The problematic part is that the pg_subscription_rel manipulation
> functions acquire ShareRowExclusiveLock and not the usual
> RowExclusiveLock because there were some worries about concurrency.

No, the problematic part is that there is any heap_open happening at
all. That open could very easily result in a recursive attempt to read
pg_class, for example, which is going to be fatal if we're in the midst
of vacuum full'ing or reindex'ing pg_class. It's frankly astonishing
to me that this patch seems to have survived testing under
CLOBBER_CACHE_ALWAYS, because it's only the catalog caches that are
preventing such recursive lookups.

regards, tom lane