Re: Patch to implement pg_current_logfile() function

On Sun, 27 Nov 2016 21:54:46 +0100
Gilles Darold <gilles(dot)darold(at)dalibo(dot)com> wrote:

> I've attached the v15 of the patch

> I've not applied patch patch_pg_current_logfile-v14.diff.backoff to
> prevent constant call of logfile_writename() on a busy system (errno =
> ENFILE | EMFILE).

I don't think it should be applied and included in the basic
functionality patch in any case. I think it needs to be submitted as a
separate patch along with the basic functionality patch. Backing off
the retry of the current_logfiles write could be overly fancy and
simply not needed.

> I think this can be done quite simply by testing if
> log rotate is still enabled. This is possible because function
> logfile_rotate() is already testing if errno = ENFILE | EMFILE and in
> this case rotation_disabled is set to true. So the following test
> should do the work:
>
> if (log_metainfo_stale && !rotation_disabled)
> logfile_writename();
>
> This is included in v15 patch.

I don't see this helping much, if at all.

First, it's not clear just when rotation_disabled can be false
when log_metainfo_stale is true. The typical execution
path is for logfile_writename() to be called after rotate_logfile()
has already set rotataion_disabled to true. logfile_writename()
is the only place setting log_metainfo_stale to true and
rotate_logfile() the only place settig rotation_disabled to false.

While it's possible
that log_metainfo_stale might be set to true when logfile_writename()
is called from within open_csvlogfile(), this does not help with the
stderr case. IMO better to just test for log_metaifo_stale at the
code snippet above.

Second, the whole point of retrying the logfile_writename() call is
to be sure that the current_logfiles file is updated before the logs
rotate. Waiting until logfile rotation is enabled defeats the purpose.

Regards,

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein