From: Bossart, Nathan <bossartn@amazon.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Magnus Hagander <magnus@hagander.net>
Subject: Re: archive modules
Date: Tue, 2 Nov 2021 16:39:48 +0000
Message-ID: <ED59AEC5-AE88-41CF-BC5E-34B32722A398@amazon.com> (raw)
In-Reply-To: <CA+TgmobM+uOL+qiuv0xDep+KeV9wkdCbkFnSeaDtaHyostqj1g@mail.gmail.com>
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<CA+TgmoYx1hhvmm4BLSJ15XM3ALrM6wyQ4XTesgtdgofSkHXqhA@mail.gmail.com>
<9970BE22-2BC3-49C9-AD68-51983118B701@amazon.com>
<CA+TgmoZfft6oCGYP3agn_Y_v3KPBauBZG60tfTm=_kke+xfvaw@mail.gmail.com>
<EEE89F96-BF41-4DE5-A0EA-8C8BD23BDEE5@amazon.com>
<CA+TgmobM+uOL+qiuv0xDep+KeV9wkdCbkFnSeaDtaHyostqj1g@mail.gmail.com>
On 11/2/21, 9:17 AM, "Robert Haas" <robertmhaas@gmail.com> wrote:
> On Tue, Nov 2, 2021 at 12:10 PM Bossart, Nathan <bossartn@amazon.com> wrote:
>> Yes, that seems doable. My point is that I've intentionally chosen to
>> preload the libraries at the moment so that it's possible to define
>> PGC_POSTMASTER GUCs and to use RegisterBackgroundWorker(). If we
>> think that switching archive modules without restarting is more
>> important, I believe we will need to take on a few restrictions.
>
> I guess I'm failing to understand what the problem is. You can set
> GUCs of the form foo.bar in postgresql.conf anyway, right?
I must not be explaining it well, sorry. I'm mainly thinking about
the following code snippets.
In guc.c:
/*
* Only allow custom PGC_POSTMASTER variables to be created during shared
* library preload; any later than that, we can't ensure that the value
* doesn't change after startup. This is a fatal elog if it happens; just
* erroring out isn't safe because we don't know what the calling loadable
* module might already have hooked into.
*/
if (context == PGC_POSTMASTER &&
!process_shared_preload_libraries_in_progress)
elog(FATAL, "cannot create PGC_POSTMASTER variables after startup");
In bgworker.c:
if (!process_shared_preload_libraries_in_progress &&
strcmp(worker->bgw_library_name, "postgres") != 0)
{
if (!IsUnderPostmaster)
ereport(LOG,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
worker->bgw_name)));
return;
}
You could still introduce GUCs in _PG_init(), but they couldn't be
defined as PGC_POSTMASTER. Also, you could still use
RegisterDynamicBackgroundWorker() to register a background worker, but
you couldn't use RegisterBackgroundWorker(). These might be
acceptable restrictions if swapping archive libraries on the fly seems
more important, but I wanted to bring that front and center to make
sure everyone understands the tradeoffs.
It's also entirely possible I'm misunderstanding something here...
Nathan
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To: pgsql-hackers@postgresql.org
Cc: bossartn@amazon.com, robertmhaas@gmail.com, michael@paquier.xyz, masao.fujii@oss.nttdata.com, sfrost@snowman.net, magnus@hagander.net
Subject: Re: archive modules
In-Reply-To: <ED59AEC5-AE88-41CF-BC5E-34B32722A398@amazon.com>
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