When an activation times out, we'll be signalled two signals on the
startup sequence object: "timeout", and "complete".
Normally, the "complete" signal is emitted when a startup sequence is
completed succesfully by it being used for activation, and in this case,
the xdg_activation implementation should remove the sequence from the
startup notification machinery.
However, in the timeout case, we should not remove it, as the startup
notification machinery itself will deal with this. If we would, we'd end
up with use-after-free issues, as the sequence would be finalized when
removed the first time.
To avoid this, just clean up the Wayland side in the "timeout" signal
handler, leaving the "complete" signal handler early out if it was
already handled by it.
This avoids crashes like:
0) g_type_check_instance (type_instance=type_instance@entry=0xdd6740)
1) g_signal_handlers_disconnect_matched (instance=0xdd6740, ...)
2) meta_startup_notification_remove_sequence (sn=0x4cc890,
seq=0xdd6740) at
../src/core/startup-notification.c:544
3) startup_sequence_timeout (data=0x4cc890, ...) at
../src/core/startup-notification.c:504
4) g_timeout_dispatch (...) at ../glib/gmain.c:4933
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2081>
