Re: Extension upgrade and GUCs

On 20 August 2015 at 13:21, Paul Ramsey <pramsey(at)cleverelephant(dot)ca> wrote:

> On August 20, 2015 at 2:17:31 AM, Simon Riggs (simon(at)2ndquadrant(dot)com
> (mailto:simon(at)2ndquadrant(dot)com)) wrote:
>
> > On 18 August 2015 at 21:03, Paul Ramsey wrote:
> >
> > > So I need a way to either (a) notice when I already have a (old) copy
> > > of the library loaded and avoid trying to setup the GUC in that case
> > > or (b) set-up the GUC in a somewhat less brittle way than
> > > DefineCustomStringVariable() allows, something that can overwrite
> > > things instead of just erroring out.
> >
> > Are you trying to preserve the in-memory state across upgrade as well?
> It sounds unlikely we can support that directly in the general case.
>
> I’m not sure what you mean by this.
>

The value of the global variable can't be maintained across upgrade.

> > Sounds like we need RedefineCustomStringVariable()
>
> Yes, if that had existed we would not have had any problems (as long as it
> delegated back to Define..() in the case where the variable hadn’t been
> created yet…, since one of the problems is knowing if/to-what-extent a
> custom variable has already been defined).
>
> We do now have a fix, which involved copying about 100 lines of core code
> (find_option, guc_var_compare, guc_name_compare) up, that does a low level
> search to see if there is a config_generic for a particular variable name,
> and if so whether it’s a placeholder or not. The presence of a
> non-placeholding definition is used as a “uh oh, there’s already a library
> in here” warning which keeps us from re-defining the variable and causing
> trouble.
>

I'm sure we all agree PostGIS is an important use case. Core is the right
place to put such code.

Please submit a patch that does that - better than someone else trying to
get it right for you. Thanks

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services