Re: Bug in MultiXact replay compat logic for older minor version after crash-recovery

On 20/03/2026 15:39, Andrey Borodin wrote:
>> On 20 Mar 2026, at 16:19, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>>
>> Hmm, after startup, before we have zeroed any pages, it still works though. So I think my patch works, but it means that tracking the latest page we have zeroed is not merely an optimization to avoid excessive SimpleLruDoesPhysicalPageExist() calls, it's needed for correctness. Need to adjust the comments for that.
>
> If we are sure buffers have no this page we can detect it via FS.
> Otherwise... nothing bad can happen, actually. We might get false positive and zero the page once more.

Zeroing the page again is dangerous because the CREATE_ID records can be
out of order. The page might already contain some later multixids, and
zeroing will overwrite them.

> If we got init_needed==false, maybe cache it for this page and set last_initialized_offsets_page = pageno?
> Or, perhaps, XLOG_MULTIXACT_ZERO_OFF_PAGE will do it for us anyway, but a bit later.

My patch does set last_initialized_offsets_page = pageno, if it
initializes the page, so yeah I think we're good there.

- Heikki