Re: Issue with PGC_BACKEND parameters

> I had observed one problem with PGC_BACKEND parameters while testing patch
> for ALTER SYSTEM command.

> Problem statement: If I change PGC_BACKEND parameters directly in
> postgresql.conf and then do pg_reload_conf() and reconnect, it will
>                   still show the old value.
> Detailed steps
> 1. Start server with default settings
> 2. Connect Client
> 3. show log_connections; -- it will show as off, this is correct.
> 4. Change log_connections in postgresql.conf to on
> 5. issue command select pg_reload_conf() in client (which is started in step-2)
> 6. Connect a new client
> 7. show log_connections; -- it will show as off, this is "in-correct".

> The problem is in step-7, it should show as on.

> This problem occur only in Windows.

> The reason for this problem is that in WINDOWS, when a new session is
> started it will load the changed parameters in new backend by
> global/config_exec_params file. The flow is in
> SubPostmasterMain()->read_nondefault_variables()->set_config_option().

> In below code in function set_config_option(), it will not allow to change
> PGC_BACKEND variable and even in comments it has mentioned that only
> postmaster will be allowed to change and the same will propagate to
> subsequently started backends, but this is not TRUE for Windows.

> switch (record->context)
> {
> ..
> ..
> case PGC_BACKEND:
>                       if (context == PGC_SIGHUP)
>                       {
>                               /*
>                                * If a PGC_BACKEND parameter is changed in
> the config file,
>                                * we want to accept the new value in the
> postmaster (whence
>                                * it will propagate to subsequently-started
> backends), but
>                                * ignore it in existing backends.
> This is a tad klugy, but
>                                * necessary because we don't re-read the
> config file during
>                                * backend start.
>                                */
>                               if (IsUnderPostmaster)
>                                       return -1;
>                       }

>}


> I think to fix the issue we need to pass the information whether PGC_BACKEND
> parameter is allowed to change in set_config_option() function.
> One way is to pass a new parameter.

Yesterday, I again thought about this issue and found that we can handle it by
checking IsInitProcessingMode() which will be True only during backend startup
which is what we need here.

Please find the attached patch to fix this issue.

I think that this issue should be fixed in PostgreSQL, because
currently PGC_BACKEND
parameters doesn't work on Windows.

I will upload this patch to next CF, so that it can be tracked.
Kindly let me know your suggestions or if you have any objections?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com