Signals in Python without glib

[Justin Mazzola Paluska]

On Mon, Jul 24, 2006 at 12:56:28AM -0400, Havoc Pennington wrote:
> If you don't have an existing main loop or don't need to block on 
> anything other than dbus, you don't need to do the main loop, you can 
> just use the blocking API on DBusConnection instead.

Does this apply for signals emitted too?  I ask because there are
things I only seem to be able to do by catching signals emitted by
objects.

> You could also use threads, though depending on your other code and your 
> experience with threads, that could be either harder or easier.

For our framework, using threads is par for the course, so using them
is probably easiest -- the attached code uses them.  The best solution
(in Python, the language of our app) that I could come up with is
attached [0].

Essentially, what I do is start a thread that sits in a loop calling
dispatch() on the connection, then sleep a little if there's nothing
to do [1].  This seems to work OK, but has raised a few questions for
me:

1.  What do I have to worry about for thread safety?  Can I still make
    blocking calls from other threads?

2.  I don't get signals after a bit of time.  If I run the glib
    eventloop, I can leave the program running for a while and still
    get signals from Avahi when new things come up and when things go
    down.  After a few seconds, my solution seems to stop working.

3.  I sniffed the message queue by putting the following code in my
    while loop:

        if dispatch_status == 0:
            print "dispatching"         # -
            msg = con.borrow_message()	# |
            print msg.get_type()	# + The added code is here
            print msg.get_sender()	# |
            con.return_message(msg)     # _
            dispatch_status = con.dispatch()

    Most of the time msg.get_type() would give me message type 3,
    signal messages.  However, sometimes I would get a message type 2
    (method return I think) and the code would segfault.  It's likely
    that it's segfaulting because I haven't locked something when I
    should have, but it was learning code, so I chucked it.

    However, I still want to know how I should handle non-signal
    messages.

    I've read through
    http://dbus.freedesktop.org/doc/api/html/dbus-mainloop_8c-source.html
    for the dbus-mainloop.c source and it does far more than simple,
    brain-dead loop.  However, I don't know how much more I need to
    implement.

I'm worried most about problems 1 and 2, but they are probably related
to problem 3.

Thanks,
	--Justin

[0] If you have Avahi's Python bindings installed, run it with:

        python2.4 service_type domain

    For example, 

        python2.4 _workstation._tcp local

    should return the names of all of the workstations on your subnet.

[1] I'd love to block on something rather than polling, but the
    set_dispatch_status_function function isn't available in the
    Python bindings in Debian Testing yet.
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