decorator factory for dbus-python methods

This is a crazy idea I had; that I want to share with people.

When you're implementing an object in dbus-python, you decorate your published method calls like this:

class ExampleObserver(dbus.service.Object):
    ...

    @dbus.service.method(dbus_interface=telepathy.interfaces.CLIENT_OBSERVER,
                         in_signature='ooa(oa{sv})oaoa{sv}',
                         out_signature='')
    def ObserveChannels(self, account, connection, channels, dispatch_operation,
                        requests_satisfied, observer_info):
        ...

The input and output signatures are incredibly easy to get wrong. The thing is, most D-Bus APIs (e.g. Telepathy) have a specification that contains these arguments. Some APIs (e.g. Telepathy-Python) provide generated code including interface names and constants. So why can't we do something more like?

class ExampleObserver(dbus.service.Object):
    ...

    @telepathy.decorators.Telepathy.Client.Observer.ObserveChannels
    def ObserveChannels(self, account, connection, channels, dispatch_operation,
                        requests_satisfied, observer_info):
        ...

With a decorator factory that looks up the parameters and then wraps the dbus.service.method factory.

Well, I just wrote a proof-of-concept. It looks something like this:

class Decorators(object):
    methods = {
        'org.freedesktop.DBus.Properties.GetAll': [ 's', 'a{sv}' ],
        'org.freedesktop.DBus.Properties.Get': [ 'ss', 'v' ],
        'org.freedesktop.Telepathy.Client.Observer.ObserveChannels': [ 'ooa(oa{sv})oaoa{sv}', '' ],
    }

    def __init__(self, namespace):
        self._namespace = namespace

    def __getattr__(self, key):
        return Decorators('%s.%s' % (self._namespace, key))

    def __call__(self, func):
        iface = self._namespace.rsplit('.', 1)[0]
        in_sig, out_sig = self.methods[self._namespace]
        return dbus.service.method(dbus_interface=iface,
                                   in_signature=in_sig,
                                   out_signature=out_sig)(func)

    def __str__(self):
        return self._namespace

decorators = Decorators('org.freedesktop')

Obviously in the real version, it would have a generated map of functions, or map of interfaces each with a map of functions, and a way to handle signals, but neat huh?

streaming to Airtunes (Airport Express) with PulseAudio

So, being back in Australia, the sun is shining, I'm sat on my sofa and I can't help but feel like I'm on summer holidays. So I want to crank some Uh Huh Her on the stereo.

We have an Airport Express, but using would normally require using Steph's computer, or the computer downstairs. So instead I thought I'd give the support in PulseAudio a go.

It's pretty easy to set up. Install the (Debian/Ubuntu) packages paprefs pavucontrol pulseaudio-module-raop pulseaudio-module-zeroconf (Fedora packages might have different names). Run paprefs and tick the option to discover Airtunes devices.

paprefs
Then using pavucontrol you can set the output device for an application.

pavucontrol
It's not perfect yet, it can sometimes get stuck skipping, and you have to move the stream to your local computer and back to the Airport (apparently the buffering is just implemented using windowing, but maybe needs to be smarter?), but it's working pretty good. Still, it's insanely neat, and because it's in PulseAudio, you can redirect any stream you like (not just from your music player).

It's not just Airtunes either. You can stream to other PulseAudio servers (discovered over mDNS) as well as UPnP media renderers.

Upgrading to Ubuntu Karmic

Upgraded to Ubuntu Karmic pre-release today on my Thinkpad X200s. Had two issues:

  1. machine wouldn't prompt for the key to unencrypt the hard disk so it didn't boot. This seems to be Launchpad bug 446591. I followed this fix.
  2. forgot that I had downgraded my Intel graphics driver to v2.4, causing my consoles to break, suspend to RAM to break and only support a broken version of EXA. apt-get install xserver-xorg-video-intel fixed this.

Things seem much better now than the mess I had this morning.

improving your code by fixing warnings

I don't know why, but there is a class of software programmer who seem to believe that compiler and runtime warnings are somehow optional, or something that can be cleaned up at a later date. This class of programmer is very common in the commercial software world.

I've never really been sure why people think these warnings can be ignored. Compilers are pretty smart these days, and if a the compiler says you have a type-mismatch for your pointer assignment… there's a good chance you did something silly.

Library developers don't just add all of those pesky assertions to their methods for fun. If your code is outputting critical warnings, you probably shouldn't be surprised that it's crashing shortly afterwards. Something clearly went wrong.

So what can you do? Compiling your code with warnings-as-errors when developing it is a very good step (-Werror in gcc). For runtime warnings, you can probably set an environment variable to make the program abort when reached (e.g. for GLib you can export G_DEBUG=fatal-warnings or fatal-criticals) — this allows you to attach a debugger when the warning is reached.

Simple steps for much better code.

and now for something from the school of pure evil (setting a background on a GtkTextView)

This is something that comes up from time to time, someone wants to set some attractive background on a GtkTextView. Well, unfortunately for you, the theme engine isn't able to help you here because GtkTextView never calls gtk_paint_box() or friends. Still, today I stared at the code for a bit and came up with a particularly hacky solution: see for yourself. You have to handle everything yourself, resizing (hook into size-allocate), scaling whatever is needed. You could probably cheat though and call something like gtk_paint_box() and then let the theme engine do that bit of the work for you… I've not tried.

Just to prove it works:


This depends on what is basically an implementation detail of the widget, and I make no promises that it will continue working in the future. With that warning in mind, do what you will with the code.

P.S. the background image is a photo of the art in my hotel room. If I was Google, I'd totally sue.

key maintenance

For those who care about things like GPG keys, I generated a new GPG key today. My old key wasn't compromised, but it's been around for a long time and is full of lots of old subkeys and my old name and stuff. I've signed my new key with my old key and set the expiry for my old key to tomorrow (I've not revoked it — should I?).

Update: it turns out that everyone in Debian is now generating 4096 RSA+RSA keys (Debian instructions). May as well be future proof, so I've revoked the key from this afternoon and generated another one. Thanks to Sjoerd for the tip.

I've pushed the keys to the keyserver, but for reference:

pub   4096R/4EE4E30F 2009-09-22 [expires: 2011-09-22]
      Key fingerprint = 5ED7 A3C0 F83A D937 300B  DA54 82EC F846 4EE4 E30F

uid                  Danielle Madeley <danielle@madeley.id.au>
uid                  Danielle Madeley <danielle.madeley@collabora.co.uk>

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Please don't sign a key from someone you aren't sure is the real person, yadda yadda.

quick and dirty icon theme viewer

So very often I find myself trying to find the name of an icon in the icon theme, and I end up having to search through my icon theme with a combination of find and eog. Usually I miss the icon on my first pass through.

Today I cracked, and decided to hammer together a quick and dirty viewer and searcher for the icon theme which looks something like this:

icon theme viewer
It's written in PyGTK and so far only has the single feature of 'Search' (this is my 90% case). It could be a lot more advanced if people wanted to hack on it: it could select themes, show whether an icon is in this theme or a parent theme, what sizes are available, etc. I will certainly accept patches!

Hopefully other people find this useful too. Source code is here.

GTK+ client-side-windows and threading

Some people (including me) had noticed that recent Empathy's video calling was breaking with recent GTK+ builds. Some people (not including me) were smart enough to work out the bug was related to client-side-windows.

In CSW-enabled GTK+, calls to GDK_WINDOW_XID() — the function with returns the XID of a GdkWindow — implicitly makes a call to gdk_window_ensure_native() before retrieving the XID. It does this so that there is a window on the X server to return the XID for, and thus ideally no one's application will break under CSW. The problem in Empathy was that the first call to GDK_WINDOW_XID() happened in a thread, which in the olden days would have been harmless, but now it can magically result in your window being allocated server side.

The fix for Empathy (and maybe your program to) is to call gdk_window_ensure_native() as soon as the widget is realized. In Empathy we're calling GDK_WINDOW_XID(), because we don't want to depend on GTK+ 2.18, which looks a lot like this.

Creative Commons Attribution-ShareAlike 2.5 Australia
This work by Danielle Madeley is licensed under a Creative Commons Attribution-ShareAlike 2.5 Australia.