[maemo-developers] Non-experimental QT versioning in Autobuilder (was: QT Packages, Repositories and PR1.2)

[From: Javier S. Pedro]

Attila Csipa wrote:

> In theory you just depend on the proper version of libqt4-dev, but it's
> tricky as you might indirectly reference libs that are also newer in the
> SDK/new PR so end up with a 4.6 dependency (or broken build)

Seems that we need to follow on this, as it is clearly desirable that
developers could make use of Qt 4.5 on Extras packages _now_.
(And Qt 4.6 if we ever update again the autobuilder to 4.7;
not that I am saying we should).

Basically, these are the problems I've heard that happen when targeting
libqt4-dev and submitting to the autobuilder:
1. Even though your application does not use Qt 4.6 API, autobuilder
creates a dependency to Qt 4.6 instead of Qt 4.5. A possible explanation
is that some macros, templates, MOC, etc. when using QT 4.6 headers expand
to Qt 4.6 newly introduced symbols thus causing the final binary to have
dependencies to Qt 4.6 libs.
2. There's at least one package that uses only 4.5 functions save for one
single network-related function which comes from 4.6. Thus the final
dependencies are detected as: libqt4-core >= 4.5, libqt4-gui >= 4.5, ... 
libqt4-network >= _4.6_ . This could cause potential problems with certain 
very Maemo specific ;) package manager's algorithms.
3. Qt Desktop guarantees that an application built under Qt 4.5 headers
is binary compatible with Qt 4.6 libs. I'd like to know if we can guarantee
that an application built under Qt 4.6 headers but using no Qt 4.5 introduced
APIs would work under Qt 4.5 libs.

If we can find fixes for all the above problems, we could use the same system
we currently use for versioning Gtk+ packages dependencies for Qt applications
(aka squeeze devkit .symbols files). 
Problem 2 at least seems to have a relatively easy workaround.

Comments? Extra problems? WORKSFORME? Rants? Everything welcome.

Javier.