[Devel] What is a feature? (extension_data)

[David Robillard]

On Sat, 2011-10-29 at 16:19 +0300, Stefano D'Angelo wrote:
> 2011/10/28 David Robillard <d at drobilla.net>:
> > The current release of LV2 has some confusing comments about
> > extension_data.  It's not really clear just what a "feature" is, and how
> > the plugin data would indicate that the plugin provides some
> > extension_data (despite the comment saying that they must).
> 
> Do we actually need that the plugin data says anything about
> extension_data being not NULL/doing anything useful? Anyway, I can't
> find the comment you are referring to...

For some value of "need", yes (at the very least it's useful to know
what plugins support what), but it's not critical like for features
passed to instantiate (i.e. it's not really so bad to use
extension_data() as a discovery mechanism for these)

> However, I have checked the current extensions on lv2plug.in and the
> only ones making use of extension_data() are persist, data-access and
> ui-resize that uses the equivalent extension_data() for UIs. Now, in
> all of these cases I don't think it's really possible to determine
> whether a feature is a host feature or a plugin feature, to me it
> looks it's always somehow a combination of the two, and also I see no
> need to make such distinctions.

There has been an unfortunate history in the past of using extension
URIs themselves as feature URIs.  This was a very bad idea (extensions
could have several features) and isn't the way things will be done from
now one.  Event is the most obvious example of why this is wrong.

I am more interested in how it should be than trying to classify past
behaviour.  Past behaviour is inconsistent, that's the problem.

> P.S.: for the record, I think there is at the very least some overlap
> with the LV2_Feature array in instantiate(), I guess we could just
> have avoided having extension_data() in the first place.

Huh?  The LV2_Feature array is the the host passing things to the
plugin, extension_data is for the plugin returning things to the host.
Pretty clear distinction.

-dr