Hi Bertrand, On Wed, Jul 1, 2026 at 3:20 PM Bertrand Drouvot <[email protected]> wrote: > > Hi hackers, > > While reviewing [1], I got segfault(s) because I created a custom statistics > extension that I forgot to add to shared_preload_libraries. Then using one of > its function produced: > > " > Core was generated by `postgres: postgres postgres [local] SELECT > '. > Program terminated with signal SIGSEGV, Segmentation fault. > #0 pgstat_init_entry (kind=kind@entry=24, > shhashent=shhashent@entry=0x73f6c341a740) at pgstat_shmem.c:335 > 335 chunk = dsa_allocate_extended(pgStatLocal.dsa, > " > > Indeed, if a custom statistics extension is loaded via CREATE EXTENSION > without > being listed in shared_preload_libraries, its _PG_init() skips the call to > pgstat_register_kind(). The SQL functions are still created, and calling them > invokes pgstat functions with a kind that was never registered. > > pgstat_get_kind_info() returns NULL in this case. The existing code only > checked this via Assert() in some paths, so non-assert builds would > dereference > NULL and segfault. > > The attached patch adds runtime checks in all public-facing pgstat functions > that > accept a PgStat_Kind and dereference the returned kind info: > > - pgstat_prep_pending_entry() > - pgstat_fetch_entry() > - pgstat_reset() > - pgstat_reset_of_kind() > - pgstat_have_entry() > - pgstat_snapshot_fixed() > - pgstat_init_entry() > - pgstat_reset_entry() > > Each now raises ERROR with ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE when > the kind is not known or registered.
Thanks for the patch — nice catch, and the diagnosis looks right. One small thing: in pgstat_snapshot_fixed(), the existing Assert(pgstat_is_kind_valid(kind)); becomes redundant after the new NULL check. A non-NULL kind_info already implies the kind is valid (that's the only way pgstat_get_kind_info() returns non-NULL), so the assert can never fire. Might as well drop it and keep just the fixed_amount one. > > [1]: https://postgr.es/m/akSi2txzLZWQL31Q%40bdtpg > > Regards, > > -- > Bertrand Drouvot > PostgreSQL Contributors Team > RDS Open Source Databases > Amazon Web Services: https://aws.amazon.com -- Regards, Ewan Young
