[Nix-dev] Python bindings to librsvg or gnome-desktop

[Cillian de Róiste]

On Tue, Jun 19, 2012 at 10:51 AM, Alexander Foremny
<alexanderforemny at googlemail.com> wrote:
> Hello,
>
> I am trying to create a Nix expression for solfege [1] and got it
> building fine using the  Nix expression available as Gist [2].
> However, running the application fails with the following error,
> cryptographic hashes replaced by dots.
>
>> Traceback (most recent call last):
>>   File "/nix/store/...-solfege-3.20.4/share/solfege/solfege/startup.py", line 159, in start_gui
>>     solfege.win = w = MainWin(options, datadir)
>>   File "/nix/store/...-solfege-3.20.4/share/solfege/solfege/mainwin.py", line 119, in __init__
>>     stock.SolfegeIconFactory(self, datadir)
>>   File "/nix/store/...-solfege-3.20.4/share/solfege/solfege/stock.py", line 71, in __init__
>>     self.add_icons(d)
>>   File "/nix/store/...-solfege-3.20.4/share/solfege/solfege/stock.py", line 33, in add_icons
>>     iconset = gtk.IconSet(gtk.gdk.pixbuf_new_from_file(os.path.join(self.datadir, filename)))
>> glib.GError: Couldn't recognize the image file format for file '/nix/store/..-solfege-3.20.4/share/solfege/graphics/solfege.svg'
>
> This issue has been addressed [3] and appears to be an issue not
> caused by solfege but by a missing python dependency [4]. It is solved
> by making solfege depend on python-rsvg. However, I cannot find this
> package in Nixpkgs. Further research revealed that this binding is now
> part of the Python bindings to Gnome [5]. As observable in the current
> NIx expression I added the dependency gnome_python hoping this would
> solve the problem. However, it does not.
>
> Is it correct that gnome_python are the Python bindings to Gnome
> Desktop which should contain pyrsvg? If not, is pyrsvg already
> packaged? If not, where can I find the sources of python-gnome-desktop
> and where would be a good place to place a Nix expression for this?
>
> Or could it be the case that this functionality should be supplied by
> pygtk, but this (optional?) feature is disabled due to missing
> dependencies?

I ran into the same issue trying to package key-mon [1], your
debugging helped a lot. I've packaged python-rsvg, and I had to change
python-gnome so that python-rsvg could be built and I also needed to
enable the librsvg pixbuf-loader (previously disabled):

https://github.com/cillianderoiste/nixpkgs-goibhniu/tree/python-rsvg

This should work with solfege too since I got exactly the same error,
but you'll need to wrap it with something like:

    wrapProgram $out/bin/solfege --prefix GDK_PIXBUF_MODULE_FILE :
${librsvg}/lib/gdk-pixbuf/loaders.cache

Since I use buildPythonPackage for key-mon, this results in a wrapper
which wraps another wrapper. It all seems to work fine though.

One concern is that if you wrap a program as described, then it will
only be able to open svg files and not other formats. If this is a
problem, the loaders.cache file could be concatenated with the
gdk-pixbuf loaders.cache [2] and then that could then be used as the
GDK_PIXBUF_MODULE_FILE. I bet there are smarter, nixian ways to do
such a thing!

On a typical distro these loaders are added to a global directory like
/usr/lib/gdk-pixbuf-2.0/2.10.0/loaders/ then gdk-pixbuf-query-loaders
is used to update /usr/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache when a
new loader is added. In the case of pygtk (and presumably for gtk in
general [3]) gtk.gdk.pixbuf_new_from_file uses this file to find the
loaders for the various formats.

These changes would cause a massive rebuild (1110 packages, based on
comparing the drv-paths), so I won't even add them as a pull request
yet. It would be great to hear if there are more elegant ways to
approach this.

All the best,
Cillian

[1] http://code.google.com/p/key-mon/
[2] e.g. /nix/store/fimfm5ybk2m7iklsm78b84v7sy8flvgw-gdk-pixbuf-2.24.1/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache
[3] http://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-query-loaders.html

> [1] http://solfege.org
> [2] https://gist.github.com/2953069
> [3] http://code.google.com/p/solfege/issues/detail?id=255
> [4] https://bugs.launchpad.net/ubuntu/+source/solfege/+bug/1004485
> [5] http://www.rittau.org/blog/20060418-00