Re: Autoprewarm workers terminated due to a segmentation fault

On 6/10/26 00:25, Matheus Alcantara wrote:
> On Tue Jun 9, 2026 at 6:44 PM -03, Tomas Vondra wrote:
>> So how does it get advanced past the prewarm_stop_idx? I've been unable
>> to reproduce it locally, maybe it's platform-specific. The original
>> report was from ARM, are you on ARM too, Matheus?
>>
>
> Yes, I'm also on ARM. I also set pg_prewarm.autoprewarm_interval=10s on
> postgresql.conf, not sure if it make more easier to reproduce.
>
>> But AFAIK the code may not account for read stream callback updating the
>> pos to prewarm_stop_idx? The callback may end with (p->pos =
>> apw_state->prewarm_stop_idx), and that seems to be past the end of the
>> array.
>>
>
> Yes, this is my understanding.
>
>> That'd mean the proposed check is generally the correct way to fix this.
>> TBH it's not clear to me why this needs to set the *next* entry at the
>> end of the loop. Well, it does that so that the loop condition can use
>> 'blk', but that seems a bit fragile / confusing, and no one noticed the
>> issue.
>>
>> Maybe this would be a better way to write the while loop?
>>
>> while (i < apw_state->prewarm_stop_idx)
>> {
>> blk = block_info[i];
>>
>> if (blk.tablespace != tablespace ||
>> blk.filenumber != filenumber)
>> break;
>>
>> ...
>> }
>>
>>
>
> Is attached patch what are you sugesting? If yes, I agree that looks
> better, it's more safe and easier to understand.
>

Yeah, that's roughly what I had in mind.

After looking a bit closer, I think this bug was introduced by

commit 6acab8bdbcda735ef47b1bb0ba2284d6c465cd88
Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Date: Fri Apr 4 15:25:27 2025 -0400

Refactor autoprewarm_database_main() in preparation for read stream

which happens to advance to the next block in a couple places

blk = block_info[++i];

before we know it the incremented "i" is a valid element. The following
commit (d9c7911e1a5f adding the read stream) ends up doing the same
thing, except the index is incremented in a callback.

Melanie, do you agree with the proposed fix?

regards

--
Tomas Vondra