Re: Config reload/restart preview

I started working on it yesterday after Thom proposed this idea internally at EDB. The discussion until now doesn't seem to be hostile to a SQL interface, so attached is a hack attempt, which hopefully will serve at least as a POC. A sample session is shown below, while changing a few values in postgresql.conf files.

The central idea is to use the SIGHUP processing function to do the work for us and report potential changes via DEBUG2, instead of having to write a new parsing engine. The (GUC-nesting + PGC_S_TEST) is nice to have since it avoids the current session from adopting the values that are different in conf file. This approach is susceptible to the fact that the connected superuser may have its GUC values picked up from user/database/session level settings (ALTER USER/DATABASE .. SET ; or SET param TO val;).

$ pgsql
Expanded display is used automatically.
psql (9.4devel)
Type "help" for help.

postgres=# show work_mem;
 work_mem
----------
 1MB
(1 row)

postgres=# set client_min_messages = debug2;
SET
postgres=# select pg_test_reload_conf();
DEBUG:  parameter "work_mem" changed to "70MB"
 pg_test_reload_conf
---------------------
 t
(1 row)

postgres=# show work_mem;
 work_mem
----------
 1MB
(1 row)

postgres=# select pg_test_reload_conf();
DEBUG:  parameter "shared_buffers" cannot be changed without restarting the server
DEBUG:  configuration file "/home/gurjeet/dev/pgdbuilds/report_guc_chanege_pre_reload/db/data/postgresql.conf" contains errors; unaffected changes were applied
 pg_test_reload_conf
---------------------
 t
(1 row)

postgres=# select pg_test_reload_conf();
DEBUG:  parameter "log_min_messages" removed from configuration file, reset to default
 pg_test_reload_conf
---------------------
 t
(1 row)