bug#39962: 27.0.90; Crash in Emacs 27.0.90

[Paul Eggert]

Although I haven't been following this in detail, I'd like to suggest trying a 
GDB watchpoint to find the bug. Watchpoints have been invaluable to me when I 
debug garbage-collector and other memory management issues.

Say you have a repeatable way to make the problem happen. You discover that some 
random part of Emacs (you don't know what) has zapped the 'next' field of some 
data structure D to make it a null pointer. You want to find out who zapped it.

To do that, first compute the address of the field you're interested in:

(gdb) p &D->next

You'll get a number like 0x16082a730.

Then:

(gdb) watch *(void *)0x16082a730
(gdb) run

The last command re-runs Emacs, and should stop each time the memory location at 
0x16082a730 changes. If you're lucky, this will be just a few times and you can 
figure out who's zapping D->next. If you're unlucky you can come up with a 
fancier watchpoint that may do the trick. (I'm using a plain 'watch' rather than 
a 'watch -l' in the example above, merely to keep things simple.)