Where are we on this?


Tom Dunstan wrote:
> Hi all
> Here is a patch that provides an initial implementation of the module
> idea that was kicked around over the last few days. While there
> certainly wasn't consensus on list, enough people seemed interested in
> the idea of database-owner-installable modules that I thought it was
> worth having a play with.
> The general idea, to recap, is to have modules, whether included in
> the distribution a la contrib or installed separately, installed under
> a directory such as $pkglib_dir/modules/foo. A typical module
> directory might contain:
>  - foo.so/foo.dll
>  - install.sql
>  - uninstall.sql
>  - foo.conf
>  - some-other-file-needed-by-foo-module.dat
> The module would be installed on the system, but the necessary scripts
> to install it in a particular database have not been run. In
> particular, the modules would not usually be install in template1.
> Database owners themselves can then opt to enable a particular
> installed module in their own database - they do not have to hassle a
> sysadmin to do it for them.
> Features of the patch:
>  - A database owner can issue the command "INSTALL MODULE foo", and
> pgsql will look for a $pkglib_dir/modules/foo/install.sql file to run,
> and run it.
>  - The install script can do pretty much anything - the user is
> treated as the superuser for the duration of the script. The main and
> obvious usage is to create C language functions required by the
> module.
>  - An entry is created in a new pg_module catalog. This is mainly to
> guard against someone trying to install a module twice at this point,
> but it may have other uses in the future (see below).
>  - "UNINSTALL MODULE foo" looks for and executes
> $pkglib_dir/modules/foo/uninstall.sql and cleans up the catalog.
> Here is a list of things that are either still to do before I'd
> consider it worthy of inclusion (should the general approach be
> considered acceptable), or which I'd like some guidance on:
>  - Currently the script is executed in one big SPI_execute call, and
> so errors and NOTICEs print out the entire script as context. I'm not
> sure how to break it up without writing a full parser - we must have
> something available in the backend to break a string up into multiple
> statements to execute, but I'm not sure where to look. Also, is there
> a better way to do this than SPI?
>  - I've hacked in a bit of an end-run around permissions checks to
> make the current user look like a super-user while a module script is
> running. Is there a better way to do this?
>  - I can't create C language functions from dlls under the modules
> dir. I'd like to be able to specify 'modules/foo/foo' as the library
> name, but the backend sees a slash and decides that must mean the path
> is absolute. I see two ways to fix this: change the existing code in
> dfmgr.c to *really* check for absolute/relative paths rather than the
> current hack, or I could stick in a special case for when it starts
> with "modules/". I thought I'd get some guidance on-list. Do people
> think that sticking the dll in with other resources for the module
> under $pkglib_dir is a bad thing? (I think having everything in one
> place is a very good thing myself).Is the existing check written the
> way it is for a particular reason, or is it just "good enough"?
>  - It would be nice to create the empty modules dir when we install
> pgsql, but while I suppose hacking a Makefile to install it is the way
> to go, I'm not sure which one would be appropriate.
>  - Hack pgxs to install stuff into a modules dir if we give it some
> appropriate flag.
>  - I'd like to add pg_depend entries for stuff installed by the module
> on the pd_module entry, so that you can't drop stuff required by the
> module without uninstalling the module itself. There would have to be
> either a function or more syntax to allow a script to do that, or some
> sort of module descriptor that let the backend do it itself.
>  - Once the issue of loading a dll from inside the module's directory
> is done, I'd like to look for an e.g. module_install() function inside
> there, and execute that rather than the install.sql if found. Ditto
> for uninstall.
>  - Maybe a basic mechanism to allow a module to require another one.
> Even just a "SELECT require_module('bar')" call at the top of a
> script.
>  - It would be nice to suppress NOTICEs when installing stuff - the
> user almost certainly doesn't care.
>  - Pick up config files in module directories, so that a module can
> install and pick up config for itself rather than getting the sysadmin
> to hack the global custom_variable_classes setting.
>  - Should plperl etc be done as modules so that their config can live
> independently as well? And to allow modules to "require" them?
> Some other nice to haves for some point in the future:
>  - Have some sort of install module privilege, rather than just a
> check for database ownership
>  - Allow looking for modules under some module path for e.g.
> /usr/local module installs
>  - Convert existing contrib to modules where appropriate :)
>  - I really have no idea what happens if non-ascii characters are in
> an install script at the moment. What happens if funky characters are
> passed to an SPI_execute call?
> Very far future:
>  - Have pgxs auto-generate rpm .spec files for modules, plus e.g. .deb
> equivalent, wix files for windows etc etc.
>  - Versioning on modules?
> General discussion:
> I see this work as orthogonal to both the CPAN-style distribution /
> repository discussion, and the fate-of-contrib discussion. If contrib
> modules are reworked as this sort of module and left in the
> distribution, they'll be easier to use and more likely to be installed
> than they are now. If, as Tom suggested, they're mostly moved out of
> the pgsql source tree and to e.g. pgfoundry or whatever, this
> mechanism should make them (and every other extension out there) easy
> to package, install and enable in a user's database.
> Similarly, I don't personally care for a CPAN-style distribution setup
> - on my preferred unix-like system I use yum and on windows I prefer
> installers. Nonetheless, a standardised system to install and
> enable/disable modules acts as an enabler for all packaging and
> distribution systems.
> I'm not sure about the command names - there was already a tendency in
> the recent discussion to mix the notion of installation of code on the
> filesystem versus installation into a particular user's database. The
> convention for doing stuff in a db is CREATE/DROP, but CREATE MODULE
> doesn't feel right to me, just as I don't really like CREATE LANGUAGE.
> How about ENABLE/DISABLE MODULE? Makes it clear that the module is
> installed, it's just not available in this database yet. Thoughts?
> Anyway, discussion and feedback hereby solicited!
> Cheers
> Tom

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  Bruce Momjian  <[EMAIL PROTECTED]>        http://momjian.us
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