UI Unresponsive Again on macOS

[Bob Halley]

I build from the master branch about once a week.  I had no problem with last 
week's build made early in the morning California time on Sunday the 6th, but 
the build I just made this morning becomes completely unresponsive after a 
short time.  I don't have an exact recipe to reproduce -- I was just typing, 
and then stopped for a bit, and then it became completely unresponsive.  This 
happens every time I try to use it.

I suspect one or both of these changes:

a65236214d9202fb69a6ba5169d4ac1a4bcb0b0d
3bd2e9e975ed29daaf03ca7559e4664aade0674f

as they are new since my last working build and seem to be relevant.

I obtained backtraces of all the threads.  I did two snapshots of thread 1 as 
it seemed to be spinning a bit.

(lldb) thread list
Process 85104 stopped
* thread #1: tid = 0x408ea9, 0x00007fff8d86790a 
libsystem_platform.dylib`_platform_memcmp + 298, queue = 
'com.apple.main-thread', stop reason = signal SIGSTOP
  thread #2: tid = 0x408ec9, 0x00007fff8d789f4a libsystem_kernel.dylib`__select 
+ 10
  thread #3: tid = 0x408ed6, 0x00007fff8d78238a 
libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.NSEventThread'
  thread #4: tid = 0x409120, 0x00007fff8d78a4e2 
libsystem_kernel.dylib`__workq_kernreturn + 10

(lldb) bt
* thread #1: tid = 0x408ea9, 0x00007fff8d86790a 
libsystem_platform.dylib`_platform_memcmp + 298, queue = 
'com.apple.main-thread', stop reason = signal SIGSTOP
  * frame #0: 0x00007fff8d86790a libsystem_platform.dylib`_platform_memcmp + 298
    frame #1: 0x00007fff780a6b0b CoreFoundation`__CFStringEqual + 235
    frame #2: 0x00007fff78098f70 CoreFoundation`CFEqual + 432
    frame #3: 0x00007fff7810382a CoreFoundation`CFRunLoopRunSpecific + 90
    frame #4: 0x00007fff7768fa5c HIToolbox`RunCurrentEventLoopInMode + 240
    frame #5: 0x00007fff7768f799 HIToolbox`ReceiveNextEventCommon + 184
    frame #6: 0x00007fff7768f6c6 
HIToolbox`_BlockUntilNextEventMatchingListInModeWithFilter + 71
    frame #7: 0x00007fff75c355b4 AppKit`_DPSNextEvent + 1120
    frame #8: 0x00007fff763afd6b AppKit`-[NSApplication(NSEvent) 
_nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 2789
    frame #9: 0x00000001001b3fcb Emacs`ns_select(nfds=6, readfds=<unavailable>, 
writefds=<unavailable>, exceptfds=0x0000000000000000, 
timeout=0x00007fff92e0be40, sigmask=0x0000000000000000) + 811 at nsterm.m:4186 
[opt]
    frame #10: 0x00000001001a0b88 
Emacs`really_call_select(arg=0x00007fff5fbfebb0) + 88 at thread.c:566 [opt]
    frame #11: 0x000000010011e64b 
Emacs`flush_stack_call_func(func=<unavailable>, arg=<unavailable>) + 43 at 
alloc.c:5111 [opt]
    frame #12: 0x00000001001a0b27 Emacs`thread_select(func=<unavailable>, 
max_fds=<unavailable>, rfds=<unavailable>, wfds=<unavailable>, 
efds=<unavailable>, timeout=<unavailable>, sigmask=0x0000000000000000) + 55 at 
thread.c:589 [opt]
    frame #13: 0x0000000100184620 
Emacs`wait_reading_process_output(time_limit=<unavailable>, 
nsecs=<unavailable>, read_kbd=<unavailable>, do_display=<unavailable>, 
wait_for_cell=0, wait_proc=<unavailable>, just_wait_proc=<unavailable>) + 4096 
at process.c:5342 [opt]
    frame #14: 0x0000000100008185 Emacs`sit_for(timeout=<unavailable>, 
reading=<unavailable>, display_option=1) + 261 at dispnew.c:5763 [opt]
    frame #15: 0x00000001000c4237 Emacs`read_char(commandflag=1, 
map=4363902419, prev_event=0, used_mouse_menu=0x00007fff5fbff7ef, 
end_time=0x0000000000000000) + 5511 at keyboard.c:2722 [opt]
    frame #16: 0x00000001000c0d8e Emacs`read_key_sequence(keybuf=<unavailable>, 
bufsize=30, prompt=<unavailable>, dont_downcase_last=<unavailable>, 
can_return_switch_frame=<unavailable>, fix_current_buffer=<unavailable>, 
prevent_redisplay=<unavailable>) + 1854 at keyboard.c:9109 [opt]
    frame #17: 0x00000001000bf4ed Emacs`command_loop_1 + 1229 at 
keyboard.c:1370 [opt]
    frame #18: 0x000000010013cc07 
Emacs`internal_condition_case(bfun=(Emacs`command_loop_1 at keyboard.c:1261), 
handlers=<unavailable>, hfun=(Emacs`cmd_error at keyboard.c:940)) + 87 at 
eval.c:1324 [opt]
    frame #19: 0x00000001000ce5e0 Emacs`command_loop_2(ignore=<unavailable>) + 
48 at keyboard.c:1112 [opt]
    frame #20: 0x000000010013c4ce Emacs`internal_catch(tag=<unavailable>, 
func=(Emacs`command_loop_2 at keyboard.c:1108), arg=0) + 78 at eval.c:1091 [opt]
    frame #21: 0x00000001000be71e Emacs`command_loop + 158 at keyboard.c:1091 
[opt]
    frame #22: 0x00000001000be62f Emacs`recursive_edit_1 + 111 at 
keyboard.c:697 [opt]
    frame #23: 0x00000001000be863 Emacs`Frecursive_edit + 227 at keyboard.c:768 
[opt]
    frame #24: 0x00000001000bd43d Emacs`main(argc=0, argv=<unavailable>) + 6077 
at emacs.c:1683 [opt]
    frame #25: 0x00007fff8d65b255 libdyld.dylib`start + 1
(lldb) c
Process 85104 resuming
Process 85104 stopped
(lldb) bt
* thread #1: tid = 0x408ea9, 0x00007fff780d14a2 
CoreFoundation`__CFRunLoopFindMode + 82, stop reason = signal SIGSTOP
  * frame #0: 0x00007fff780d14a2 CoreFoundation`__CFRunLoopFindMode + 82
    frame #1: 0x00007fff781038aa CoreFoundation`CFRunLoopRunSpecific + 218
    frame #2: 0x00007fff7768fa5c HIToolbox`RunCurrentEventLoopInMode + 240
    frame #3: 0x00007fff7768f799 HIToolbox`ReceiveNextEventCommon + 184
    frame #4: 0x00007fff7768f6c6 
HIToolbox`_BlockUntilNextEventMatchingListInModeWithFilter + 71
    frame #5: 0x00007fff75c355b4 AppKit`_DPSNextEvent + 1120
    frame #6: 0x00007fff763afd6b AppKit`-[NSApplication(NSEvent) 
_nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 2789
    frame #7: 0x00000001001b3fcb Emacs`ns_select(nfds=6, readfds=<unavailable>, 
writefds=<unavailable>, exceptfds=0x0000000000000000, 
timeout=0x00007fff92e0be40, sigmask=0x0000000000000000) + 811 at nsterm.m:4186 
[opt]
    frame #8: 0x00000001001a0b88 
Emacs`really_call_select(arg=0x00007fff5fbfebb0) + 88 at thread.c:566 [opt]
    frame #9: 0x000000010011e64b 
Emacs`flush_stack_call_func(func=<unavailable>, arg=<unavailable>) + 43 at 
alloc.c:5111 [opt]
    frame #10: 0x00000001001a0b27 Emacs`thread_select(func=<unavailable>, 
max_fds=<unavailable>, rfds=<unavailable>, wfds=<unavailable>, 
efds=<unavailable>, timeout=<unavailable>, sigmask=0x0000000000000000) + 55 at 
thread.c:589 [opt]
    frame #11: 0x0000000100184620 
Emacs`wait_reading_process_output(time_limit=<unavailable>, 
nsecs=<unavailable>, read_kbd=<unavailable>, do_display=<unavailable>, 
wait_for_cell=0, wait_proc=<unavailable>, just_wait_proc=<unavailable>) + 4096 
at process.c:5342 [opt]
    frame #12: 0x0000000100008185 Emacs`sit_for(timeout=<unavailable>, 
reading=<unavailable>, display_option=1) + 261 at dispnew.c:5763 [opt]
    frame #13: 0x00000001000c4237 Emacs`read_char(commandflag=1, 
map=4363902419, prev_event=0, used_mouse_menu=0x00007fff5fbff7ef, 
end_time=0x0000000000000000) + 5511 at keyboard.c:2722 [opt]
    frame #14: 0x00000001000c0d8e Emacs`read_key_sequence(keybuf=<unavailable>, 
bufsize=30, prompt=<unavailable>, dont_downcase_last=<unavailable>, 
can_return_switch_frame=<unavailable>, fix_current_buffer=<unavailable>, 
prevent_redisplay=<unavailable>) + 1854 at keyboard.c:9109 [opt]
    frame #15: 0x00000001000bf4ed Emacs`command_loop_1 + 1229 at 
keyboard.c:1370 [opt]
    frame #16: 0x000000010013cc07 
Emacs`internal_condition_case(bfun=(Emacs`command_loop_1 at keyboard.c:1261), 
handlers=<unavailable>, hfun=(Emacs`cmd_error at keyboard.c:940)) + 87 at 
eval.c:1324 [opt]
    frame #17: 0x00000001000ce5e0 Emacs`command_loop_2(ignore=<unavailable>) + 
48 at keyboard.c:1112 [opt]
    frame #18: 0x000000010013c4ce Emacs`internal_catch(tag=<unavailable>, 
func=(Emacs`command_loop_2 at keyboard.c:1108), arg=0) + 78 at eval.c:1091 [opt]
    frame #19: 0x00000001000be71e Emacs`command_loop + 158 at keyboard.c:1091 
[opt]
    frame #20: 0x00000001000be62f Emacs`recursive_edit_1 + 111 at 
keyboard.c:697 [opt]
    frame #21: 0x00000001000be863 Emacs`Frecursive_edit + 227 at keyboard.c:768 
[opt]
    frame #22: 0x00000001000bd43d Emacs`main(argc=0, argv=<unavailable>) + 6077 
at emacs.c:1683 [opt]
    frame #23: 0x00007fff8d65b255 libdyld.dylib`start + 1

(lldb) thread select 2
(lldb) bt
* thread #2: tid = 0x408ec9, 0x00007fff8d789f4a libsystem_kernel.dylib`__select 
+ 10
  * frame #0: 0x00007fff8d789f4a libsystem_kernel.dylib`__select + 10
    frame #1: 0x00000001001b68b7 Emacs`-[EmacsApp 
fd_handler:](self=<unavailable>, _cmd=<unavailable>, unused=<unavailable>) + 
295 at nsterm.m:5493 [opt]
    frame #2: 0x00007fff79b28c6d Foundation`__NSThread__start__ + 1243
    frame #3: 0x00007fff8d872aab libsystem_pthread.dylib`_pthread_body + 180
    frame #4: 0x00007fff8d8729f7 libsystem_pthread.dylib`_pthread_start + 286
    frame #5: 0x00007fff8d8721fd libsystem_pthread.dylib`thread_start + 13

(lldb) thread select 3
(lldb) bt
* thread #3: tid = 0x408ed6, 0x00007fff8d78238a 
libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.NSEventThread'
  * frame #0: 0x00007fff8d78238a libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x00007fff8d7817d7 libsystem_kernel.dylib`mach_msg + 55
    frame #2: 0x00007fff78104c94 CoreFoundation`__CFRunLoopServiceMachPort + 212
    frame #3: 0x00007fff78104121 CoreFoundation`__CFRunLoopRun + 1361
    frame #4: 0x00007fff78103974 CoreFoundation`CFRunLoopRunSpecific + 420
    frame #5: 0x00007fff75d82a62 AppKit`_NSEventThread + 205
    frame #6: 0x00007fff8d872aab libsystem_pthread.dylib`_pthread_body + 180
    frame #7: 0x00007fff8d8729f7 libsystem_pthread.dylib`_pthread_start + 286
    frame #8: 0x00007fff8d8721fd libsystem_pthread.dylib`thread_start + 13

(lldb) thread select 4
(lldb) bt
* thread #4: tid = 0x409120, 0x00007fff8d78a4e2 
libsystem_kernel.dylib`__workq_kernreturn + 10
  * frame #0: 0x00007fff8d78a4e2 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #1: 0x00007fff8d872791 libsystem_pthread.dylib`_pthread_wqthread + 
1426
    frame #2: 0x00007fff8d8721ed libsystem_pthread.dylib`start_wqthread + 13