Xcode autocomplete popups causing crashes

[OberstKrueger]

While working in Xcode, popover autocompletion hints are causing excessive crashing in yabai. Nothing else in Xcode seems to cause it, only when typing in code and seeing those popovers appear and disappear quickly.

It started happening with yabai version 3.1.2.
Xcode version is 11.5, on macOS 10.15.5.

Output from yabai --verbose after switching to the window and typing up code that causes the popup:
thread: 70410240 | EVENT_HANDLER_WINDOW_TITLE_CHANGED: Xcode 20259
thread: 70410240 | window_manager_create_and_add_window: Xcode 20261
thread: 70410240 | EVENT_HANDLER_WINDOW_DESTROYED: Xcode 20261
Segmentation fault: 11

I can get more detailed debug logs later if necessary.

[dominiklohmann]

I managed to reproduce this while running yabai in lldb. It appears to be a timing issue.

thread: 266219520 | window_manager_create_and_add_window: Xcode 13373
thread: 266219520 | EVENT_HANDLER_WINDOW_DESTROYED: Xcode 13373
thread: 266219520 | event_signal_transmit: transmitting window_destroyed to 1 subscriber(s)
thread: 266219520 | EVENT_HANDLER_DAEMON_MESSAGE: query --windows --space
thread: 266219520 | window_manager_create_and_add_window: Xcode 13374
thread: 266219520 | EVENT_HANDLER_WINDOW_DESTROYED: Xcode 13374
thread: 266219520 | event_signal_transmit: transmitting window_destroyed to 1 subscriber(s)
Process 63811 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x000000010001574f yabai`application_notification_handler(observer=0x000000010036e360, element=0x0000000100615530, notification=@"AXUIElementDestroyed", context=0x00000001006109e0) at application.c:14:30
   11  	        uint32_t *window_id_ptr = *(uint32_t **) context;
   12  	        if (!window_id_ptr) return;
   13
-> 14  	        uint32_t window_id = *window_id_ptr;
   15  	        while (!__sync_bool_compare_and_swap((uint32_t **) context, window_id_ptr, NULL));
   16
   17  	        struct event *event = event_create(&g_event_loop, WINDOW_DESTROYED, (void *)(uintptr_t) window_id);
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
  * frame #0: 0x000000010001574f yabai`application_notification_handler(observer=0x000000010036e360, element=0x0000000100615530, notification=@"AXUIElementDestroyed", context=0x00000001006109e0) at application.c:14:30
    frame #1: 0x00007fff2dd7cf7e HIServices`_XXMIGPostNotification + 717
    frame #2: 0x00007fff2dda2b03 HIServices`_XPostNotification + 340
    frame #3: 0x00007fff2dd5ff64 HIServices`mshMIGPerform + 174
    frame #4: 0x00007fff2faa9394 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
    frame #5: 0x00007fff2faa92e0 CoreFoundation`__CFRunLoopDoSource1 + 541
    frame #6: 0x00007fff2faa7e09 CoreFoundation`__CFRunLoopRun + 2270
    frame #7: 0x00007fff2faa6ece CoreFoundation`CFRunLoopRunSpecific + 462
    frame #8: 0x00007fff2fb2f519 CoreFoundation`CFRunLoopRun + 40
    frame #9: 0x00000001000203d9 yabai`main(argc=2, argv=0x00007ffeefbff068) at yabai.c:328:5
    frame #10: 0x00007fff69aa3cc9 libdyld.dylib`start + 1
    frame #11: 0x00007fff69aa3cc9 libdyld.dylib`start + 1

[koekeishiya]

macOS APIs are such garbage, I quit.

# notification from macOS, UIElement destroyed
# CAS operation
context = 0x7f8931530fb0, window_id_ptr = 0x7f8931530d44, NEW window_id_ref = 0x0 
window_id = 13493

# we verify here that the above CAS did succeed and perform cleanup
EVENT_HANDLER_WINDOW_DESTROYED: Xcode 13493 0x7f8931530fb0 0x0

# somehow randomly for fun we get another window destroyed event 
# for the same window after  we have told macOS that we are not 
# interested in notifications AND destroyed all memory.
# Our previous atomic operations are thus rendered useless when we get here.. 
# Why do we even get here..
context = 0x7f8931530fb0, window_id_ptr = 0x8000000000000000, NEW window_id_ref = 0x0
(These values, with the exception of context, are all garbage because we freed everything in the step above).

[koekeishiya]

I changed to some async versions of the AXNotification API because they performed better, but obviously that causes race conditions when they spin off the same notification multiple times for the same entity. Went back to using the old versions.