Simon McVittie authored Apr 16, 2019 and Florian Müllner committed Apr 23, 2019

A window being unmanaged can cause the ShellApp to be removed from
the ShellAppSystem, which if we are unlucky is the app's last
reference, causing it to be disposed and freed. It would be bad if this
happened before we finished handling the signal.

Use g_signal_connect_object to ensure that a reference is held to
the ShellApp for the duration of the signal handler, delaying its
last-unref.

In particular, when a signal handler calls _shell_app_remove_window(),
there is a brief period for which ShellApp breaks the intended
invariant (see !497) that app->running_state is non-NULL if and only if
app->running_state->windows is also non-NULL (non-empty). Freeing the
ShellApp at this point would cause a crash. This seems likely to be the
root cause of <#750>,
<#822> and
<https://bugs.debian.org/926212

>.

Signed-off-by: Simon McVittie <smcv@debian.org>