Re: pg_rewind: warn when checkpoint hasn't happened after promotion

On Sat, Jun 4, 2022 at 6:29 PM James Coleman <jtc331@gmail.com> wrote:
>
> A few weeks back I sent a bug report [1] directly to the -bugs mailing
> list, and I haven't seen any activity on it (maybe this is because I
> emailed directly instead of using the form?), but I got some time to
> take a look and concluded that a first-level fix is pretty simple.
>
> A quick background refresher: after promoting a standby rewinding the
> former primary requires that a checkpoint have been completed on the
> new primary after promotion. This is correctly documented. However
> pg_rewind incorrectly reports to the user that a rewind isn't
> necessary because the source and target are on the same timeline.
>
> Specifically, this happens when the control file on the newly promoted
> server looks like:
>
>     Latest checkpoint's TimeLineID:       4
>     Latest checkpoint's PrevTimeLineID:   4
>     ...
>     Min recovery ending loc's timeline:   5
>
> Attached is a patch that detects this condition and reports it as an
> error to the user.
>
> In the spirit of the new-ish "ensure shutdown" functionality I could
> imagine extending this to automatically issue a checkpoint when this
> situation is detected. I haven't started to code that up, however,
> wanting to first get buy-in on that.
>
> 1: https://www.postgresql.org/message-id/CAAaqYe8b2DBbooTprY4v=BiZEd9qBqVLq+FD9j617eQFjk1KvQ@mail.gmail.com

Thanks. I had a quick look over the issue and patch - just a thought -
can't we let pg_rewind issue a checkpoint on the new primary instead
of erroring out, maybe optionally? It might sound too much, but helps
pg_rewind to be self-reliant i.e. avoiding external actor to detect
the error and issue checkpoint the new primary to be able to
successfully run pg_rewind on the pld primary and repair it to use it
as a new standby.

Regards,
Bharath Rupireddy.