Re: race condition when writing pg_control

Hi,

On 2024-05-16 14:50:50 -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > I suspect it will be difficult to investigate this one too much further
> > unless we can track down a copy of the control file with the bad checksum.
> > Other than searching for any new code that isn't doing the appropriate
> > locking, maybe we could search the buildfarm for any other occurrences.  I
> > also seem some threads concerning whether the way we are reading/writing
> > the control file is atomic.
> 
> The intention was certainly always that it be atomic.  If it isn't
> we have got *big* trouble.

We unfortunately do *know* that on several systems e.g. basebackup can read a
partially written control file, while the control file is being
updated. Thomas addressed this partially for frontend code, but not yet for
backend code. See
https://postgr.es/m/CA%2BhUKGLhLGCV67NuTiE%3Detdcw5ChMkYgpgFsa9PtrXm-984FYA%40mail.gmail.com

Greetings,

Andres Freund