[patch] GUC source file and line number

Hi,

Here's a patch to add source file and line numbers to GUC variables.
Basically this makes pg_settings look like this:

alvherre=# select name, setting, sourcefile, sourceline from pg_settings where name like '%work%';
name | setting | sourcefile | sourceline
------------------------+---------+--------------------------------------------+------------
autovacuum_max_workers | 3 | |
maintenance_work_mem | 32768 | /pgsql/install/00head/data/postgresql.conf | 119
work_mem | 2048 | /pgsql/install/00head/data/workmem.conf | 4
(3 lignes)

This should make life easier for tools trying to find the config file
each setting is from (and possibly edit it).

It was initially authored by Magnus, but I had to hit it with the clue
stick until it understood reset sources, i.e. to make it behave sanely
when a session has a SET value and then a SIGHUP causes the backend to
read a config file that has changed underneath it:

-- initial value was 1MB

alvherre=# select name, setting, sourcefile, sourceline from pg_settings where name = 'work_mem';
name | setting | sourcefile | sourceline
------------------------+---------+--------------------------------------------+------------
work_mem | 1024 | /pgsql/install/00head/data/workmem.conf | 1
(1 ligne)

alvherre=# set work_mem to 200;
SET
alvherre=# select name, setting, sourcefile, sourceline from pg_settings where name = 'work_mem';
name | setting | sourcefile | sourceline
------------------------+---------+--------------------------------------------+------------
work_mem | 200 | |
(1 ligne)

-- SIGHUP happened here, file was changed to 2MB and the setting moved some lines below

alvherre=# select name, setting, sourcefile, sourceline from pg_settings where name = 'work_mem';
name | setting | sourcefile | sourceline
------------------------+---------+--------------------------------------------+------------
work_mem | 200 | |
(1 ligne)

alvherre=# reset work_mem ;
RESET
alvherre=# select name, setting, sourcefile, sourceline from pg_settings where name = 'work_mem';
name | setting | sourcefile | sourceline
------------------------+---------+--------------------------------------------+------------
work_mem | 2048 | /pgsql/install/00head/data/workmem.conf | 4
(1 ligne)

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support