EmitSignal does not support a destination

[whot]

Note that this is an issue with dbusmock and the underlying dbus.service.signal. But since it's dbusmock that's the immediate consumer of my code, let's file a bug here :smile:

I ran into the case of libportal not receiving signals emitted by dbusmock. The reason is that libportal subscribes to signals like this:

    call->signal_id = g_dbus_connection_signal_subscribe (call->portal->bus,
                                                          PORTAL_BUS_NAME,
                                                          REQUEST_INTERFACE,
                                                          "Response",
                                                          call->request_path,
                                                          NULL,
                                                          DBUS_SIGNAL_FLAGS_NO_MATCH_RULE,  // <- the culprit                                                                                                                                                                           
                                                          response_received,
                                                          call,
                                                          NULL);

DBUS_SIGNAL_FLAGS_NO_MATCH_RULE means no AddMatch etc. is sent to the dbus server. For the normal use-case of libportal talking to xdg-desktop-portal this doesn't matter because that sends those Response signals like this:

        g_dbus_connection_emit_signal (connection,
                                       request->sender, // <- Destination argument
                                       g_dbus_interface_skeleton_get_object_path (G_DBUS_INTERFACE_SKELETON (skeleton)),
                                       "org.freedesktop.portal.Request",
                                       "Response",                                                     
                                       signal_variant,
                                       NULL);

But for dbusmock's EmitSignal(), the destination is always (null) and those signals get filtered (somewhere, not 100% sure where) and never arrive in libportal.

The libportal use-case is justified, this is a single-use object and only one destination will ever need that signal, and the object is destroyed immediately after anyway. But the problem is that I cannot test this with dbusmock.

The use of the dbus.service.signal decorator makes this dependent on python-dbus for which I've filed an MR now: https://gitlab.freedesktop.org/dbus/dbus-python/-/merge_requests/18, so we'd need this first before attempting to fix it here.

[oSoMoN]

Is this fixed with 4a191d8 ? If so, can we get a release of python-dbusmock with this addition to the API? I need EmitSignalDetailed for the tests I'm writing for https://phabricator.services.mozilla.com/D140803, and I'd rather refer to a released version on pypi than vendoring an unreleased version into the firefox source tree.

[oSoMoN]

Following up on my previous comment. In the context of the unit tests I'm writing for firefox that talk to a mocked WebExtensions portal, I pulled the latest changes from python-dbusmock into my virtualenv, making the EmitSignalDetailed available. I used it in my tests, and I verified that the destination was correctly set on the emitted signal (using dbus-monitor), but the code talking to the portal still wasn't notified of the signal. So it looks like something else is missing for this to work.

[whot]

At least for the tests I wrote in libportal, this patch was sufficient, at least for the dbusmock bit. The only other issue was convincing libportal to connect with the right address, the default g_bus_get_sync() and similar methods don't work if you keep switching the bus address. see the libportal/portal.c hunks in that MR.

[martinpitt]

I was still working on #131, but there's really no urgency to that. I released 0.28.0, it's also on PyPI. Fedora and Debian updates are coming.

[oSoMoN]

Thanks @martinpitt !

[martinpitt]

This was released months ago. I'm closing this, please yell if there's still something left here. Thanks!