bug#50319: 27.2; crash

[Eli Zaretskii <eliz@gnu.org>]

> From: Devon Sean McCullough <Emacs-Hacker2020@jovi.net>
> Date: Wed, 1 Sep 2021 09:30:57 -0400
> 
> Launch Emacs, attach a debugger, use it routinely for many weeks,
> switching between various apps, until it dereferences a bad pointer
> upon switching from some other app:
> 
> (lldb) process attach --pid 24105
> ⋮
> Executable module set to "/Applications/Emacs.app/Contents/MacOS/Emacs-x86_64-10_14".
> Architecture set to: x86_64h-apple-macosx-.
> ⋮
> (lldb) Process 24105 stopped
> * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x172505fec)
>      frame #0: 0x000000010cdcb104 Emacs-x86_64-10_14`redisplay_window + 4244
> Emacs-x86_64-10_14`redisplay_window:
> ->  0x10cdcb104 <+4244>: movq   -0x14(%rbx), %rcx
>      0x10cdcb108 <+4248>: addq   $-0x100, %rbx
>      0x10cdcb10f <+4255>: testl  $0x100000, %ecx           ; imm = 0x100000
>      0x10cdcb115 <+4261>: je     0x10cdcb0a0               ; <+4144>
> Target 0: (Emacs-x86_64-10_14) stopped.
> (lldb) bt all
> * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x172505fec)
>    * frame #0: 0x000000010cdcb104 Emacs-x86_64-10_14`redisplay_window + 4244

Can you show where is this place in xdisp.c?  I'm afraid I cannot
figure that out from the disassembly alone.

Also, what did you do when it segfaulted? just clicked on an Emacs
frame with a mouse?  The backtrace doesn't look like a frame redraw,
it looks like a routine redisplay when Emacs becomes idle.

Thanks.