Re: [PATCH] Stop ALTER SYSTEM from making bad assumptions

Hi,

On 2019-08-02 20:27:25 -0400, Stephen Frost wrote:
> On Fri, Aug 2, 2019 at 18:47 Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Stephen Frost <sfrost@snowman.net> writes:
> > > On Fri, Aug 2, 2019 at 18:27 Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > >>> The proposal seems to be to run through the .auto.conf file, remove any
> > >>> duplicates, and append the new entry at the end. That seems reasonable.
> >
> > >> +1
> >
> > > I disagree that this should only be addressed in alter system, as I’ve
> > said
> > > before and as others have agreed with.  Having one set of code that can
> > be
> > > used to update parameters in the auto.conf and then have that be used by
> > > pg_basebackup, alter system, and external tools, is the right approach.
> >
> > I don't find that to be necessary or even desirable.  Many (most?) of the
> > situations where this would be important wouldn't have access to a running
> > backend, and maybe not to any PG code at all --- what if your tool isn't
> > written in C?
>
>
> What if you want to access PG and your tool isn’t written in C?  You use a
> module, extension, package, whatever, that provides the glue between what
> your language wants and what the C library provides.  There’s psycopg2 for
> python, DBD::Pg for Perl, et al, and they use libpq. There’s languages that
> like to write their own too, like the JDBC driver, the Golang driver, but
> that doesn’t mean we shouldn’t provide libpq or that non-C tools can’t
> leverage libpq.  This argument is just not sensible.

Oh, comeon. Are you seriously suggesting that a few commands to add a a
new config setting to postgresql.auto.conf will cause a lot of people to
write wrappers around $new_config_library in their language of choice,
because they did the same for libpq? And that we should design such a
library, for v12?

> I think it's perfectly fine to say that external tools need only append
> > to the file, which will require no special tooling.  But then we need
> > ALTER SYSTEM to be willing to clean out duplicates, if only so you don't
> > run out of disk space after awhile.

> Uh, if you don’t ever run ALTER SYSTEM then you could also “run out of disk
> space” due to external tools modifying by just adding to the file.

That was commented upon in the emails you're replying to? It seems
hardly likely that you'd get enough config entries to make that
problematic while postgres is not running. While running it's a
different story.

> Personally, I don’t buy the “run out of disk space” argument but if we are
> going to go there then we should apply it appropriately.
>
> Having the history of changes to auto.conf would actually be quite useful,
> imv, and worth a bit of disk space (heck, it’s not exactly uncommon for
> people to keep their config files in git repos..). I’d suggest we also
> include the date/time of when the modification was made.

That just seems like an entirely different project. It seems blindlingly
obvious that we can't keep the entire history in the file that we're
going to be parsing on a regular basis. Having some form of config
history tracking might be interesting, but I think it's utterly and
completely independent from what we need to fix for v12.