Re: Catalog version access

Thanks for taking a look!

On 1/23/22, 7:31 PM, "Michael Paquier" <michael(at)paquier(dot)xyz> wrote:
> On Mon, Aug 16, 2021 at 06:12:54PM +0000, Bossart, Nathan wrote:
>> I was looking at the --check switch for the postgres binary recently
>> [0], and this sounds like something that might fit in nicely there.
>> In the attached patch, --check will also check the control file if one
>> exists.
>
> This would not work on a running postmaster as CreateDataDirLockFile()
> is called beforehand, but we want this capability, no?

I was not under the impression this was a requirement, based on the
use-case discussed upthread [0].

> Abusing a bootstrap option for this purpose does not look like a good
> idea, to be honest, especially for something only used internally now
> and undocumented, but we want to use something aimed at an external
> use with some documentation. Using a separate switch would be more
> adapted IMO.

This is the opposite of what Magnus proposed earlier [1]. Do we need
a new pg_ctl option for this? It seems like it is really only
intended for use by PostgreSQL developers, but perhaps there are other
use-cases I am not thinking of. In any case, the pg_ctl option would
probably end up using --check (or another similar mode) behind the
scenes.

> Also, I think that we should be careful with the read of
> the control file to avoid false positives? We can rely on an atomic
> read/write thanks to its maximum size of 512 bytes, but this looks
> like a lot what has been done recently with postgres -C for runtime
> GUCs, that *require* a read of the control file before grabbing the
> values we are interested in.

Sorry, I'm not following this one. In my proposed patch, the control
file (if one exists) is read after CreateDataDirLockFile(), just like
PostmasterMain().

Nathan

[0] https://postgr.es/m/20210222022407.ecaygvx2ise6uwyz%40alap3.anarazel.de
[1] https://postgr.es/m/CABUevEySovaEDci7c0DXOrV6c7JzWqYzfVwOiRUJxMao%3D9seEw%40mail.gmail.com