Re: Config reload/restart preview

On Thu, Jun 20, 2013 at 9:33 AM, Magnus Hagander <magnus@hagander.net> wrote:

On Thu, Jun 20, 2013 at 2:54 PM, Dimitri Fontaine
<dimitri@2ndquadrant.fr> wrote:
> Magnus Hagander <magnus@hagander.net> writes:
>>> Should we have a way of previewing changes that would be applied if we
>>> reloaded/restarted the server?
>>
>> Yes, we should.
>
> +1
>
>> This would go well with something I started working on some time ago
>> (but haven't actually gotten far on at all), which is the ability for
>> pg_ctl to be able to give feedback at all. Meaning a "pg_ctl reload"
>> should also be able to tell you which parameters were changed, without
>> having to go to the log. Obviously that's almost exactly the same
>> feature.
>
> It could probably connect to the server and issue the SQL command to
> reload, and that one could probably get enhanced to return modified
> variable as NOTICE, or be run with the right client_min_message:
>
>     SELECT pg_reload_conf();
>
> The pg_ctl client would then have to know to display the messages sent
> back by the server.

The problem with that is that now you must *always* have your system
set up to allow the postgres user to log in in pg_hba.conf or it
fails.

But yes, one option would be to use SQL instead of opening a socket.
Maybe that's a better idea - have pg_ctl try to use that if available,
and if not send a regular signal and not try to collect the output.

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)