Re: replace plugins directory with GUC

On 10.10.2012 03:45, Peter Eisentraut wrote:
> About that plugins directory ($libdir/plugins) ... I don't think we ever
> really got that to work sensibly.  I don't remember the original design
> discussion, but I have seen a number of explanations offered over the
> years.  It's not clear who decides what to put in there (plugin author,
> packager, DBA?), how to put it there (move it, copy it, symlink it? --
> no support in pgxs), and based on what criteria.

Yeah, it would be good to clarify that. It was originally added for the 
pldebugger module 
(http://archives.postgresql.org/pgsql-hackers/2006-07/msg00803.php), but 
I couldn't find any discussion on the decision to create a new 'plugins' 
directory.

It was never quite enough for pldebugger, as you still needed to add it 
to shared_preload_libraries for it to work, at least if you wanted to 
get the full functionality. The installation procedure in the README 
clearly instructs to add it to shared_preload_libraries, it doesn't say 
anything about local_preload_libraries. I recently refactored pldebugger 
to not install in the plugins directory anymore, it now just drops it in 
$libdir.

> It would seem to be much more in the spirit of things to simply list the
> allowed plugins in a GUC variable, like
>
> some_clever_name_here = $libdir/this, $libdir/that
>
> but there is probably a reason why this wasn't done that way in the
> first place.

I think the idea was that plugins directory would be easier for 
users/admins. I agree that a GUC like above feels more natural.

Now that we support include-directories in postgresql.conf, you could 
put a "mylib.conf" file in the include directory that contains the above 
line, if you want to enable/disable a module just by moving things 
around in the filesystem (after configuring an include directory in 
postgresql.conf). But actually, you can't, because there's no way to 
append to a setting, you can only override. That's an obvious missing 
feature in the include mechanism. Even ignoring the plugins directory, 
it would be nice to be able to append libraries to shared_preload_libraries.

- Heikki