Thread

Re: Accessing an invalid pointer in BufferManagerRelation structure

Daniil Davydov <3danissimo@gmail.com> — 2025-10-20T19:03:35Z

Hi,

On Sun, Oct 19, 2025 at 6:32 PM Álvaro Herrera <alvherre@kurilemu.de> wrote:
>
> I edited your comments a bit.  What do you think of this version?
>

Thanks for looking into this! All added changes LGTM.

> I don't think this is backpatchable material, mainly because you said
> you don't see any situations in which relcache invalidations would
> occur in the middle of this.  Can you can cause a problem to occur by
> adding an untimely invalidation somewhere without this patch, which is
> fixed by this patch?

Yep, by now (if I did not miss anything) described problem cannot occur even
with debug_discard_caches parameter on. But one of our clients had faced
this problem, because we have some differences in source code. In the first
letter I described scenario, when the occurrence of a problem becomes possible :
***
if (just for example) we add acquiring of new lock into ExtendBufferedRelLocal
or ExtendBufferedRelShared, relation cache will be invalidated (inside
AcceptInvalidationMessages).
***

>
> BTW how does this interact with SMgrRelation pinning?

Hm, maybe I don't fully understand the question. Smgr pinning is one of the side
effects of BMR_GET_SMGR (in case we use this macro with
invalidated/uninitialized
rd_smgr entry).
Without patch pincount cannot save us from undesirable invalidation,
because it guards
from smgr_destroy, not from smgr_release.

--
Best regards,
Daniil Davydov