bug#39962: 27.0.90; Crash in Emacs 27.0.90

[Stefan Monnier]

> But what does "valid" mean in this case?  The part that looks at the
> stack uses the stack-marking routines, and thus inherits the
> "conservative" nature of stack marking.  The code also makes it quite
> clear that it only considers "live" objects as valid, and a killed
> buffer is not "live".

[ I understand I'm late to this game, but just want to clarify
  a point here.  ]

There are two uses of the word "live" at play here:
- The use coming from Emacs for whether a buffer has been killed or not.
- The use coming from the realm of memory management where it means that
  the object can't be reclaimed because some pointers still point to it
  (and may still be dereferenced or compared).

The GC's marking is only concerned about liveness in the
memory-management sense.


        Stefan