bug#21999: 25.0.50; Binary with --enable-checking immediately aborts with '0<=size'

[Dima Kogan <dima@secretsauce.net>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: David Engster <deng@randomsample.de>
>> Date: Mon, 23 Nov 2015 20:52:42 +0100
>> 
>> I've compiled from the emacs-25 branch (f146ea73a9ca6) with
>> 
>>   CFLAGS="-O0 -g3" ./configure --enable-checking
>> 
>> Running 'emacs -Q' directly from the src directory immediately aborts:
>> 
>>    lisp.h:1543: Emacs fatal error: assertion failed: 0 <= size
>>    Fatal error 6: Aborted
>
> Dima, can you look into this?

Sure. This comes from recent changes that created gc_asize() for use in
the GC, and changed ASIZE() to eassume() if we're not in the GC:

  https://github.com/emacs-mirror/emacs/commit/8afaa1321f808#diff-0e5d67da0ba3fb5c2886841cb3d0ccecR1547

This is a very recent change, so we're now seeing some of the effects.
In this particular case FONT_ENTITY_P() was called from the GC; it
called ASIZE(), which saw a marked object so we barfed in the eassume().
This eassume() is only fatal if --enable-checking, which is why that is
significant.

I don't know what the plan is here, so no patch is attached. Cc-ing
Paul, since he authored the patch in question.