bug#71929: 30.0.60; crash in mark_image_cache

[Eli Zaretskii]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Cc: 71929@debbugs.gnu.org
> Date: Fri, 05 Jul 2024 08:13:14 +0800
> 
> Hello,
> 
> On Thu 04 Jul 2024 at 09:03am +03, Eli Zaretskii wrote:
> 
> >> What is the value of c->images?  IOW, why did this line segfault?
> >
> > Also, what is the value of c->refcount?
> 
> (gdb) p c
> $1 = (struct image_cache *) 0x555557c89e20
> (gdb) xpr
> There is no member named i.
> 
> (gdb) p c->images
> $2 = (struct image **) 0x35
> (gdb) xpr
> Cannot access memory at address 0x35
> 
> (gdb) p c->refcount
> $4 = 93823560581177

So it's garbled.

Po Lu, how do we handle the "shared" image cache when a frame is
deleted?  Where's the code which frees the cache if the cache's
refcount is one when the frame is deleted?