Re: "unexpected duplicate for tablespace" problem in logical replication

On Thu, Sep 18, 2025 at 5:53 PM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2025-09-18 08:17:49 -0400, Andres Freund wrote:
> > On 2025-09-18 17:37:10 +0530, Ashutosh Bapat wrote:
> > > From 6a3562b4ac8917c8b577797e5468416a90cc04f5 Mon Sep 17 00:00:00 2001
> > > From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
> > > Date: Thu, 18 Sep 2025 17:24:09 +0530
> > > Subject: [PATCH] Negative RelfilenumberMap cache entries from
> > >  pg_filenode_relation()
> > >
> > > RelidByRelfilenumber() adds negative entries to the cache. It has three
> > > users, logical replication, autoprewarm and pg_filenode_relation(). The
> > > first two need negative entries in the cache in case they happen to
> > > lookup non-existent mapping again and again.  However such mappings will
> > > be smaller in number and usually come from some database object e.g. WAL
> > > or autoprewarm metadata.
> > >
> > > But pg_filenode_relation(), which is SQL callable, may be invoked many
> > > times with invalid tablespace and relfilenode pairs, causing the cache
> > > to be bloated with negative cache entries. This can be used as a denial
> > > of service attack since any user can execute it. This commit avoids such
> > > a bloat.
> >
> > I don't really understand why this is worth fixing for the relfilenode stuff
> > specifically - isn't this true for just about *all* of our caches? Many, if
> > not most, can be reached via SQL?
>
> Example:
>
> postgres[315631][1]=# SELECT count(*), sum(total_bytes) total_bytes, sum(total_nblocks) total_nblocks,
sum(free_bytes)free_bytes, sum(free_chunks) free_chunks, sum(used_bytes) used_bytes FROM pg_backend_memory_contexts
WHEREpath @> (SELECT path FROM pg_backend_memory_contexts WHERE name = 'CacheMemoryContext');
 
> ┌───────┬─────────────┬───────────────┬────────────┬─────────────┬────────────┐
> │ count │ total_bytes │ total_nblocks │ free_bytes │ free_chunks │ used_bytes │
> ├───────┼─────────────┼───────────────┼────────────┼─────────────┼────────────┤
> │    89 │      747200 │           187 │     130336 │         216 │     616864 │
> └───────┴─────────────┴───────────────┴────────────┴─────────────┴────────────┘
> (1 row)
>
> Time: 1.540 ms
> postgres[315631][1]=# SELECT to_regclass(g.i::text||'.'||g.i::text) is NULL, count(*) FROM generate_series(1,
10000000)g(i) GROUP BY 1;
 
> ┌──────────┬──────────┐
> │ ?column? │  count   │
> ├──────────┼──────────┤
> │ t        │ 10000000 │
> └──────────┴──────────┘
> (1 row)
>

Interesting! I didn't know that cat cache could have negative entries
in it. But SearchCatCacheMiss says so explicitly
/*
* Tuple was not found in cache, so we have to try to retrieve it directly
* from the relation. If found, we will add it to the cache; if not
* found, we will add a negative cache entry instead.
*/

That settles it. Use of negative entries spread wider than I thought
and in places where they may not even be useful. Thanks for the
example. I don't see any reason to pursue this patch specifically.

-- 
Best Wishes,
Ashutosh Bapat