Re: GDBusProxy missing bus signals during construction

[Andrejs Hanins <andrejs hanins ubnt com>]

Hi,

On 07/23/2015 05:01 PM, Roger James wrote:
> I am trying to implement a Zeroconf service browser using a PyGobject Gio.DBusProxy object to talk to 
> avahi. It all works fine but the browser randomly misses the initial one or more ItemNew signals from the 
> avahi server. It sometimes even misses the AllForNow signal. Subsequent ItemNew signals are seen OK as 
> services come and go.
>
> It appears that the problem is that the GLIb objects subscribes to the dbus signals during the construction 
> process (the DO_NOT_CONNECT_SIGNALS is not set). It then notifies(GDbusProxy.new) or 
> returns(GDBusProxy.new_sync) to the calling object. Once I get the new proxy object back I can use connect 
> on the parent gobject to hook the incoming dbus signal to my applications signal handling callback. However 
> any dbus signals that arrive from server (avahi) before that connection is made are discarded. If set 
> DO_NOT_CONNECT_SIGNALS I would have mimic the whole of DBusProxy/DBUsConection's handling of dbus side 
> signal mapping. I don't want to have to do that. What am I missing?
>
> Some fundamental questions are.
>
> 1. How can I stop this happening.
> 2. Is avahi breaking some sort of dbus protocol or convention in sending these signals immediately a new 
> bus connection is made?
> 3. Is the a flaw in the GDBusProxy design/documenation? (sorry guys, but have spent a while looing at this.)
I had a very similar problem using native GDbus/Glibmm and the only solution which helped me was to subscribe 
to signals *in advance*, i.e. before the proxy object is created. I would also be happy to know a better way.
> Thanks,
>
> Roger
>
> Here is my python code
>
> #!/usr/bin/env python
> from gi.repository import Gio, GLib, GObject, Gtk
> import avahi
> import signal
> import time
>
> class avahibrowser(Gio.Application):
>     def service_resolved(self, *args):
>         print 'service resolved'
>         print 'name:', args[2]
>         print 'address:', args[7]
>         print 'port:', args[8]
>
>     def print_error(self, *args):
>         print 'error_handler'
>         print args[0]
>
>     def browserCallback(self, proxy, sender, signal, args):
>         if signal == 'ItemNew':
>             # Arguments are [0] i interface, [1] i protocol [2] s name [3] s type  [4] s domain [5] u flags
>             print "Found service '%s' type '%s' domain '%s' " % (args[2], args[3], args[4])
>             #self.avahiserver.ResolveService('iisssiu',
>             #    args[0], # Interface
>             #    args[1], # Protocol
>             #    args[2], # Name
>             #    args[3], # Service Type
>             #    args[4], # Domain
>             #    avahi.PROTO_UNSPEC, dbus.UInt32(0),
>             #    reply_handler=self.service_resolved, error_handler=self.print_error)
>         else:
>             print 'signal', signal, 'arguments', args
>             def do_activate(self):  # Define this to suppress glib warning
>         pass
>             def new_browser_proxy_callback(self, source_object, res, user_data):
>         #source_object.connect('g-signal', self.browserCallback)
>         self.avahibrowser = Gio.DBusProxy.new_finish(res)
>         self.avahibrowser.connect('g-signal', self.browserCallback)
>             def new_server_proxy_callback(self, source_object, res, user_data):
>         self.avahiserver = Gio.DBusProxy.new_finish(res)
>         avahibrowserpath = self.avahiserver.ServiceBrowserNew('(iissu)',
>                                 avahi.IF_UNSPEC,
>                                 avahi.PROTO_INET,
>                                 '_scratch._tcp',
>                                 'local',
>                                 0)
>
>                 Gio.DBusProxy.new(self.systemDBusConnection, 0, None,
>                                             avahi.DBUS_NAME,
>                                             avahibrowserpath,
>                                             avahi.DBUS_INTERFACE_SERVICE_BROWSER, None,
>                                             self.new_browser_proxy_callback, None)
>             def bus_get_callback(self, source_object, res, user_data):
>         self.systemDBusConnection = Gio.bus_get_finish(res)
>                 Gio.DBusProxy.new(self.systemDBusConnection, 0, None,
>                                             avahi.DBUS_NAME,
>                                             avahi.DBUS_PATH_SERVER,
>                                             avahi.DBUS_INTERFACE_SERVER, None,
>                                             self.new_server_proxy_callback, None)
>                                                 # Gnome application initialization routine
>     def __init__(self, application_id, flags):
>         Gio.Application.__init__(self, application_id=application_id, flags=flags)
>                 Gio.bus_get(Gio.BusType.SYSTEM, None, self.bus_get_callback, None)
>         def InitSignal(app):
>     def signal_action(signal):
>         if signal is 1:
>             print("Caught signal SIGHUP(1)")
>         elif signal is 2:
>             print("Caught signal SIGINT(2)")
>         elif signal is 15:
>             print("Caught signal SIGTERM(15)")
>         app.release()
>
>     def idle_handler(*args):
>         print("Python signal handler activated.")
>         GLib.idle_add(signal_action, priority=GLib.PRIORITY_HIGH)
>
>     def handler(*args):
>         print("GLib signal handler activated.")
>         signal_action(args[0])
>
>     def install_glib_handler(sig):
>         unix_signal_add = None
>
>         if hasattr(GLib, "unix_signal_add"):
>             unix_signal_add = GLib.unix_signal_add
>         elif hasattr(GLib, "unix_signal_add_full"):
>             unix_signal_add = GLib.unix_signal_add_full
>
>         if unix_signal_add:
>             print("Register GLib signal handler: %r" % sig)
>             unix_signal_add(GLib.PRIORITY_HIGH, sig, handler, sig)
>         else:
>             print("Can't install GLib signal handler, too old gi.")
>
>     SIGS = [getattr(signal, s, None) for s in "SIGINT SIGTERM SIGHUP".split()]
>     for sig in filter(None, SIGS):
>         print("Register Python signal handler: %r" % sig)
>         signal.signal(sig, idle_handler)
>         GLib.idle_add(install_glib_handler, sig, priority=GLib.PRIORITY_HIGH)
>
> if __name__ == "__main__":
>     Application = avahibrowser("uk.co.beardandsandals.avahibrowser", Gio.ApplicationFlags.FLAGS_NONE)
>         # Set up python and Gio signal handling
>     InitSignal(Application)
>         Application.hold()
>
>     Application.run(None)
>         print 'Exiting'
>
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