Created attachment 323332 [details] [review]
proposed fix

Hello,

We're trying to make the debian graphical installer accessible, and stumbled on initialization issues. The symptom is that multithreaded applications are completely stuck, with this kind of backtrace:

- g_mutex_lock_slowpath
- g_mutex_lock
- gdk_event_prepare
- g_main_context_prepare(context=0x13ac1c0)
- g_main_context_iterate(context=0x13ac1c0)
- g_main_loop_run
- send_and_allow_reentry
- Accessibility_DeviceEventController_NotifyListenersSync
- spi_atk_bridge_key_listener
- gail
- gtk_invoke_key_snoopers
- IA__gtk_main_do_event
- gdk_event_dispatch
- g_main_dispatch(context=0x13ac1c0)
- g_main_context_dispatch(context=0x13ac1c0)
- g_main_context_iterate(context=0x13ac1c0)
- g_main_loop_run
...

So what happens is that send_and_allow_reentry ends up running the glib main loop with the same context as the main application. Since gdk_event_dispatch does GDK_THREADS_ENTER/LEAVE around calling the gdk_event_funcs, the gdk thread mutex is busy, and thus gtk_event_prepare can't acquire it, we have a deadlock.

Usually there is no such problem because send_and_allow_reentry switches to another context before entering the glib main loop. It happens that in or debian graphical installer case, spi_global_app_data->main_context was NULL!

This happens because spi_atk_create_socket dropped it because it couldn't establish the p2p socket (because the cache path was not available, which is another issue that I'll file separately). I don't see why spi_atk_create_socket drops that context just because it couldn't establish the p2p socket: when configured with --disable-p2p, that function would just return -1 without dropping that context, and even with p2p, if the temporary directory can't be built in the user's runtime directory or other such errors, -1 is just returned without dropping the context.

I'd thus recommend applying the attached patch, which avoids dropping the context when the p2p socket can't be established. With it we don't have stuck multithreaded applications in our testcase.

Samuel