Extension Templates S03E11 - Mailing list pgsql-hackers

Hi,

Please find attached to this email the latest and greatest version of
in-line SQL only extensions support, known as "Extension Templates" and
which could be renamed "In-Catalog Extension Templates".

I've included a high-level description of the patch in a style that
targets the detailed commit messages for features of that source code
impact level.

The attached patch is known to address all points raised in the previous
reviews and to implement the best design we could come up with, thanks
to immense helping from Tom, Heikki and Markus. Of course, bugs are all
my precious.

I'm going to register that patch to the next commitfest. It's not the
only patch I intend to register for september though, as I want to get
to a usable situation with Event Triggers, so you can expect a series of
patches for that, covering what couldn't make it previously.

As I think this WIP is about as ready-for-committer as it will ever be,
it would be fantastic if we could do a single committer review before
CF2013-09 so that I know that it's going to be accepted… or not. Well at
least it's in the queue already, we'll see what can be done.

Regards,

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Implement in-catalog Extension Template facility.

Previously, the only way to CREATE EXTENSION involved installing file
system templates in a place generally owned by root: creation scripts,
upgrade scripts, main control file and auxilliary control files. This
patch implements a way to upload all those resources into the catalogs,
so that a PostgreSQL connection is all you need to make an extension
available.

By design and for security concerns the current Extension Template
facility is not able to deal with extensions that need to load a DSO
module into the backend. Using any other PL is supported though.

An extension created from a template depends on it, and the templates
are part of any backup script taken with pg_dump. So that at pg_restore
time, when CREATE EXTENSION is executed the templates are already in
place.

To be able to do that, though, we need a difference in behavior in
between the classic file system level templates and the catalog
templates: there's no dependency tracking happening at all with file
system templates and those can be changed at will even if an extension
has been already instanciated from the templates, or even removed.

Apart from the dependency tracking, the only other difference between
file system templates and catalog templates for extensions is that the
later are managed per-database. The file system level templates being
managed per major version of PostgreSQL is considered a drawback of that
method and not to be immitated by the in-catalog system, more flexible
by design.

At CREATE EXTENSION time, the file system templates are always prefered
to the catalog templates. Also, it's prohibited to make available an
extension in the catalogs if an extension of the same name is already
available from file system templates. That said, some "race conditions"
make it still possible to have the same extension name available as a
file system template and a catalog template. Even if only the former
will ever get installed, it's been deemed prudent to restrict the
in-catalog templates for extensions to superusers only.

--
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support