When using g_network_monitor_get_default() from another thread, it’s
possible for network-changed events to be processed after an instance of
GNetworkMonitor has been disposed, causing use-after-free problems.
Fix that by moving some of the initialisation into the GInitable.init()
chain, rather than in a main context idle callback.
This includes a unit test which probabilistically reproduces the bug
(but can’t do so deterministically due to it being a race condition).
Commit amended by Philip Withnall <withnall@endlessm.com> before
pushing.
https://bugzilla.gnome.org/show_bug.cgi?id=793727
