bug#26932: 25.1; Crash triggered a few times a day with network process

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 19 Jun 2017 01:57:54 +0100 (BST)
> From: Vivek Dasmohapatra <vivek@etla.org>
> cc: 26932@debbugs.gnu.org
> 
> so maybe last_marked[9] is involved?
> 
> idx 9 seems to be a list with every car being:
> 
> (gdb) p last_marked[9]
> $120 = 48953379
> (gdb) p XTYPE(last_marked[9])
> $121 = Lisp_Cons
> (gdb) p *XCONS(last_marked[9])
> $122 = {car = 8760836, u = {cdr = 48953363, chain = 0x2eaf813}}
> (gdb) p XTYPE(8760836)
> $123 = Lisp_String
> (gdb) p *XSTRING(8760836)
> $124 = {size = 4, size_byte = -1, intervals = 0x0,
>    data = 0xb374bb <pure+2999995> "DEAD"}
> 
> So... a reaped list? Not helpful anyway. Nothing identifiable here.

I think you should go back along last_marked[] and look for some valid
identifiable object.

> Going up the backtrace all I find is that we're in the modeline display
> code and we're _about_ to eval the mode-line-frame-identification
> value:
> 
> (:eval (mode-line-frame-control))
> 
> But GC happens before we actually call it.

Indeed.  The exact evaluation is not really relevant because the
problem is with an object Emacs is trying to mark, which is most
probably unrelated to what's involved in the :eval form.