Re: pgsql: Improve accounting for memory used by shared hash tables

On 4/4/25 01:43, Tomas Vondra wrote:
> On 4/4/25 00:57, David Rowley wrote:
>> On Thu, 3 Apr 2025 at 04:16, Tomas Vondra <tomas(dot)vondra(at)postgresql(dot)org> wrote:
>>> Improve accounting for memory used by shared hash tables
>>
>> I've not looked into why, but this is causing an issue in the
>> join_rel_hash during add_join_rel(). See the attached script.
>>
>> ERROR: pfree called with invalid pointer 0x60a15edc44e0 (header
>> 0x0000002000000008)
>>
>
> Thanks for the report and reproducer. I'll take a look tomorrow.
>

I took a quick look, and I think the reason is fairly simple - the
commit allocates the header and the directory as a single chunk. And for
shared hash tables that's fine, because those have non-expandable
directory. But the patch does the same thing for non-shared hash tables
(not intentionally), which means that if we end up expanding the hash,
it fails in dir_realloc(). Because hashp->dir is not a separately
allocated chunk.

This is clearly a bug in the patch, I should have caught this during a
review. But I'm also quite surprised none of the regression tests seems
to expand the hash table ...

I'll think about a way to fix this tomorrow.

--
Tomas Vondra