[Libreoffice] Extension dependencies

Hi all,

As already mentioned briefly in my talk at LOCon, we need to make up our 
mind how to handle LO extension dependencies.

Each .oxt extension can carry any number of dependencies, specifying 
conditions that need to be met by the hosting LO installation for the 
extension to be successfully deployable (see 
<http://wiki.services.openoffice.org/wiki/Documentation/DevGuide/Extensions/Dependencies>).

The mechanism was designed to allow for any number of fine-grained 
dependency specifications ("this extension requires availability of 
com.sun.star.whatever.Service"), but only a coarse-grained 
OpenOffice.org-minimal-version ever got defined (and also a 
maximal-version one).  After all, it is hard (or at least tedious) for 
extension developers to come up with a precise list of features they 
depend on, which would be needed to specify a precise list of 
fine-grained dependencies.  A single minimal-version dependency appeared 
so much easier to use.

The OpenOffice.org-minimal-version dependency worked fine as long as OOo 
was the de-facto standard, with derivatives tracking OOo development 
rather closely and producing new versions in sync with upstream OOo. 
The minimal-version dependency's version value was to be interpreted as 
the version of the underlying OOo reference version, so even if a 
derivative used a different versioning scheme everything worked out.  An 
extension developed for one product could also be deployed in a 
different one (at least in theory; of course there are always minor 
glitches that spoil the picture).

Now, LO and OOo (now AOOo) are no longer in such close relation.  LO is 
heading towards its version 3.5 already while AOOo did not yet publish 
its version 3.4, and new features are routinely added to LO that are 
unlikely to be included into AOOo.  This leads to the following items:

** How shall LO handle the OpenOffice.org-minimal-version dependency?

Strictly speaking, LO 3.4/3.5 should still check against an 
OpenOffice.org-minimal-version value of 3.3, as that's the latest 
available (A)OOo version.  However, for LO 3.4, this check has (probably 
accidentally) been changed to "3.4" (and with the current code towards 
LO 3.5 it has yet again changed to "3.5").  I would argue to leave it at 
"3.4" for both LO 3.4 and LO 3.5.  (For one, it can obviously no longer 
be changed back to "3.3" for LO 3.4, and changing it back to "3.3" for 
LO 3.5 would probably cause more confusion than its worth.  For another, 
semantically the check will likely be more-or-less correct, matching the 
features an AOOo 3.4 will presumably come out with.)  If no one objects, 
I will adapt LO 3.5 so that it still checks for "3.4," not "3.5."

In the future, when AOOo releases new versions after AOOo 3.4, we would 
need to check whether it semantically makes sense for LO to bump its 
OpenOffice.org-minimal-version check (i.e., whether the features of that 
new AOOo version are also available in LO).

** How to specify dependencies for newly introduced LO features?

One option would be to add an additional LibreOffice-minimal-version 
dependency.  Another option would be to actually make use of 
fine-grained dependencies and introduce them as needed (i.e., whenever 
an extension developer wants to make use of a new LO feature, a 
corresponding dependency would have to be added).

The former has as pro its ease of use.  The latter has as pro that it 
shows a way out of the problems caused by OpenOffice.org-minimal-version 
and for an opportunity of continued sharing of extensions across 
products:  If each extension only lists the features it requires, 
regardless of any product's version numbers, it becomes much easier to 
match that extension against a given product's capabilities, regardless 
of brand.

I'd tend to give the fine-grained dependencies a try.  That would 
require cooperation from extension developers (they would need to voice 
their demand for dependencies for specific features, and they would need 
to make accurate use of those dependencies in their extensions).  To 
achieve cross-product extension compatibility, it would also require 
some sort of advertising of LO's newly introduced fine-grained 
dependencies (so that other products---where possible---would pick them 
up together with the corresponding features), but we would need a place 
to document any newly introduced dependencies, anyway.

What do other people think here?

** What happens when LO 4 becomes incompatible?

In that case, any OpenOffice.org-minimal-version dependency, regardless 
of version value, should be considered as not satisfied by LO 4. 
Similarly, if we did introduce a LibreOffice.org-minimal-version 
dependency, any version value of that dependency less than 4 should be 
considered as not satisfied.  On the other hand, treatment of any 
fine-grained dependencies would have to be handled on a case-by-case basis.

Stephan