Patch to implement pg_current_logfile() function

Hi,

Here is a patch that is supposed to solve the remaining problem to
find the current log file used by the log collector after a rotation.
There is lot of external command to try to find this information but
it seems useful to have an internal function to retrieve the name
of the current log file from the log collector.

There is a corresponding item in the TODO list at "Administration"
section. The original thread can be reach at the following link
http://archives.postgresql.org/pgsql-general/2008-11/msg00418.php
The goal is to provide a way to query the log collector subprocess
to determine the name of the currently active log file.

This patch implements the pg_current_logfile() function that can be
used as follow. The function returns NULL when logging_collector
is not active and outputs a warning.

postgres=# \pset null *
postgres=# SELECT pg_current_logfile();
WARNING: current log can not be reported because log collection is not
active
pg_current_logfile
--------------------
*
(1 line)

So a better query should be:

postgres=# SELECT CASE WHEN current_setting('logging_collector')='on'
THEN pg_current_logfile()
ELSE current_setting('log_destination')
END;
current_setting
-----------------
syslog
(1 line)

Same query with log collection active and, for example,
log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log'

postgres=# SELECT CASE WHEN current_setting('logging_collector')='on'
THEN pg_current_logfile()
ELSE current_setting('log_destination')
END;
current_setting
-----------------------------------------
pg_log/postgresql-2016-03-09_152827.log
(1 line)

Then after a log rotation:

postgres=# SELECT pg_rotate_logfile();
pg_rotate_logfile
-------------------
t
(1 line)

postgres=# select pg_current_logfile();
pg_current_logfile
-----------------------------------------
pg_log/postgresql-2016-03-09_152908.log
(1 line)

I choose to allow the log collector to write his current log file name
into the lock file 'postmaster.pid'. This allow simple access to this
information through system commands, for example:

postgres(at)W230ST:~$ tail -n1 /usr/local/pgql-devel/data/postmaster.pid
pg_log/postgresql-2016-03-09_152908.log

Log filename is written at the 8th line position when log collection
is active and all other information have been written to lock file.

The function pg_current_logfile() use in SQL mode read the lock file
to report the information.

I don't know if there's any limitation on using postmaster.pid file to
do that but it seems to me a bit weird to log this information to an
other file. My first attempt was to use a dedicated file and save it
to global/pg_current_logfile or pg_stat_tmp/pg_current_logfile but I
think it is better to use the postmaster.pid file for that. I also
though about a communication protocol or notification with the log
collector subprocess to query and retrieve the name of the currently
active log file. But obviously, it would be too much work for just this
simple function and I can't see any other feature that need such a
work.

Any though? Should I add this patch to the commit fest? If the use
of the postmater.pid file is a problem I can easily modify the patch
to use an alternate file.

Best regards,

--
Gilles Darold
Consultant PostgreSQL
http://dalibo.com - http://dalibo.org