Re: Fix race condition in SSI when reading PredXact->SxactGlobalXmin

Thanks for taking a look.

I tried adding a reproduction with injection points, but had some trouble
with them. I'll give it another go this weekend.

I guess one of the commitfest entries should be deleted? I had already made
one already. I don't see a way to do that though.
https://commitfest.postgresql.org/patch/6037/

Josh

On Mon, Oct 20, 2025 at 5:55 PM Mihail Nikalayeu <mihailnikalayeu(at)gmail(dot)com>
wrote:

> Hello!
>
> Josh Curtis <jcurtis825(at)gmail(dot)com>:
> > This is definitely a bit more complex. It requires that
> SetNewSxactGlobalXmin is never called when SxactGlobalXmin is invalid to
> prevent readers from seeing an invalid transaction ID when they should see
> a valid one -- I think this is the case now since before
> SetNewSxactGlobalXmin is called postgres checks that
> PredXact->SxactGlobalXminCount > 0. I assume this entails that
> SxactGlobalXmin is valid, but I have not checked every place the two
> variables are modified.
>
> Such logic feels fragile to me. Maybe add a special flag like
> 'PredXact->SxactGlobalSkipAllowed' which will be updated to 'true' by
> SetNewSxactGlobalXmin, and dropped to 'false' by functions affecting
> PredXact->activeList?
> But I prefer the first solution anyway.
>
> Best regards,
> Mikhail.
>