[gentoo-dev] [RFC] gtk/gtk2/gtk3 USE flag situation

[Mart Raudsepp]

Hello,

Despite it being 2016 and gtk2 pretty much dead, buried and forgotten
upstream, many applications still support only gtk2, have subtle issues
with their gtk3 port, or support both, with some of our userbase
clinging to gtk2 for dubious political or aesthetical reasons.

For the latter cases, despite GNOME teams policy and strong preference
on not providing a choice and just choosing gtk2 or gtk3 (gtk3 if it's
working as good as gtk2), some cases exist where the maintainers want
to provide such choice. In some cases it is understandable for a short
while during transition, e.g firefox. In other cases, it is purely for
the sake of providing the choice of working with a deprecated toolkit,
apparently.

My highly biased essay aside, we need to finally globally agree on what
we do in this situation. If we allow this choice at all, only for
special cases, or widespread. And if this choice is provided, how do we
name the USE flag.

Historically, for very good reasons in past and present GNOME team
members opinion, USE=gtk has always meant to mean to provide support
for gtk in general, not any particular version. This is opposite to
what the Qt team has been doing.
In our opinion, in a perfect world, only USE=gtk would exist, and no
USE=gtk2 or USE=gtk3 would be necessary. But as we don't live in a
perfect world, we have made use of USE=gtk3 for providing gtk3 support
from library packages to mean to build gtk3 support. Sadly that
overloads USE=gtk in many cases to then mean to build gtk2 support.
This would ideally not be needed, as the package would instead be
slotted and parallel installable for gtk2 and gtk3, which should be
theoretically possible in all cases, because gtk2 and gtk3 may not live
in the same process, so not the same library either.
Due to some packages needing too much manpower effort to do such a
split, USE flags are used in such a case.
Good examples of such slot splits existing are for example the
libappindicator stack. This used to be the case with almost all GNOME
libraries as well, but most of them only provide gtk3 now, as gtk2 is,
well, dead.
Bad examples would be e.g avahi and gtk-vnc, which deemed too hard to
split up into separate SLOTs. In some cases it might have been meant as
a transitional thing, until all consumers are ported to gtk3, but it
has been lingering on due to consumer apps not being ported or we
haven't yet noticed to remove the gtk2 support in the library package.

Now these are libraries, and despite some USE flag confusion, it's not
a huge issue, because consumers are USE depending on what is required.
This all is written out in
https://wiki.gentoo.org/wiki/Project:GNOME/Gnome_Team_Ebuild_Policies#gtk3
since the GNOME project pages moving to wiki, and also long before that
in GuideXML era, and we've pointed people towards that.

And then we have applications that support building against either gtk2
or gtk3.
In most cases, any requests to provide the choice to have an
application use gtk2 instead of gtk3 gets instantly marked as duplicate
of https://bugs.gentoo.org/374057 but in some cases the maintainer has
chosen to provide this choice for now, and here is the problem - we
don't really have a good agreed on way to name such a choice in USE
flags, if we should provide such a choice at all.

USE=gtk2 is not good, due to the confusion issues with USE=gtk3 and
USE=gtk and it being problematic. The GNOME team shall probably veto
such USE flag usage if we are deemed to have such an authority as gtk+
maintainers, unless we rework it all in expectations of gtk2 corpse
being carried around for a decade as well... I have quite a few bugs
against packages to file already for this, afair.

I kind of like what firefox did there, going in the spirit of the
force-openrc flag we have for avoiding systemd dependency, even if it
currently means worse user experience. So if we provide such a choice
for apps at all, I might agree to USE=force-gtk2 for this for apps. And
we would like to eventually (or immediately) p.use.mask this and once
it's 2017 and gtk2 truly dead and buried and full of known security
holes, get rid of it again.
But this highlighted the inconsistency we are having, ending up with QA
initiated bug https://bugs.gentoo.org/581662

tl;dr and my proposal would be the following:

* USE=gtk means providing support for GTK+; because we don't have a
USE=gui, this also means "provide a GUI version built on top of gtk+"
for packages where a GUI is optional.

* USE=gtk3 may be used only for controlling extra libraries to be
shipped for gtk3 support (the extra library file will link to gtk3),
_in addition_ to gtk2 version. This is a temporarily measure until gtk2
support can be dropped and it will only ship gtk3 version of the
library. This gives a flag to be able to USE depend on by gtk3 apps.
This leaves the question about the opposite open, however. This is why
USE=gtk2 would be bad for apps, maybe we need to use it for this
library case, when gtk3 version is primary and we just have 1 app
remaining that needs the gtk2 version or something.
The concept of library is broad here, covering also gtk theme engines
(x11-themes/gtk-engine-*, but they shouldn't be hard to split) and
modules (e.g caribou, libcanberra)

* Applications may only use a gtk2 based stack or gtk3 based stack in a
given version/revision. gtk3 is strongly preferred when it is deemed to
not have any regressions compared to gtk2 build, but the choice is
ultimately with the maintainer. Once the application converts to using
gtk3 in our distribution, it should try hard to stay that way in
upcoming versions as well.

* Some exceptions to the above may exist under heavy consideration,
especially in cases where the toolkit usage is complex and may have
some issues for some, but in general gtk3 support is deemed good by
upstream. Most notable here would be browsers like firefox and
chromium, which are using gtk dependency more for emulating the theme
it uses, rather than using it as its real toolkit. If such exceptions
are allowed, the USE flag naming here must be consistent amongst the
exceptions. My proposal would be USE=force-gtk2 then, as I have no
better ideas without stomping on the USE=gtk{2,3} historical meaning.


When arguing in favor of supporting gtk2 builds more for apps, please
do keep in mind that gtk2 really is pretty much dead. And no, MATE,
XFCE and others are NOT continuing its support; they are just slow in
fully converting to gtk3, but they are doing so and I expect both of
those to be fully done this year, around autumn.
If the issue is political or just a general gnome3 or gtk3 hate, then I
would ask you to keep your political opinions or hate outside this
thread and go contemplate on more important life issues.
If the issue is lack of themes, then I would like you to help package
more gtk3 themes. gtk3.20 now has a stable CSS based theme API and
themes shouldn't be breaking anymore beyond this point, theoretically.
And gtk3 theme packages should pretty much just be CSS files and some
metadata. Though we have yet to get over that bumpy thing yet (a main
reason gtk3.20 isn't in main tree yet).

Thoughts? Agreements? Suggestions?
I'm particularly interested in QA opinion here. I believe WilliamH
wanted to spearhead this from their side.


Regards,
Mart Raudsepp
Gentoo developer, GNOME team