GNU bug report logs - #75521
scratch/igc: Delete unused macro DEFVAR_LISP_NOPROX

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Message #61 received at 75521 <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> protonmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 75521 <at> debbugs.gnu.org, stefankangas <at> gmail.com
Subject: Re: bug#75521: scratch/igc: Delete unused macro DEFVAR_LISP_NOPROX
Date: Mon, 13 Jan 2025 17:18:58 +0000

"Eli Zaretskii" <eliz <at> gnu.org> writes:

>> Date: Mon, 13 Jan 2025 16:46:39 +0000
>> From: Pip Cet <pipcet <at> protonmail.com>
>> Cc: 75521 <at> debbugs.gnu.org, stefankangas <at> gmail.com
>>
>> "Eli Zaretskii" <eliz <at> gnu.org> writes:
>>
>> >> Date: Mon, 13 Jan 2025 14:16:04 +0000
>> >> From: Pip Cet <pipcet <at> protonmail.com>
>> >> Cc: 75521 <at> debbugs.gnu.org, stefankangas <at> gmail.com
>> >>
>> >> "Eli Zaretskii" <eliz <at> gnu.org> writes:
>> >>
>> >> >> But then font_style_table is sometimes modified so it points to
>> >> >> (i.e. contains as its element) a new vector, and I don't see how
>> >> >> Vfont_weight_table is protected then.
>> >> >
>> >> > Modified where?
>> >>
>> >> font_style_to_value:
>> >>
>> >>       if (! noerror)
>> >> 	return -1;
>> >>       eassert (len < 255);
>> >>       elt = make_vector (2, make_fixnum (100));
>> >>       ASET (elt, 1, val);
>> >>       ASET (font_style_table, prop - FONT_WEIGHT_INDEX,
>> >> 	    CALLN (Fvconcat, table, make_vector (1, elt)));
>> >>       return (100 << 8) | (i << 4);
>> >
>> > OK, but the values gets plugged int font_style_table, right?  And my
>> > reading of mark_object_root_visitor is that it marks its argument
>> > recursively, so any object reachable from font_style_table should also
>> > be protected?  Am I wrong?
>>
>> No, you're not.  Initially, Vfont_weight_table is protected because it's
>> reached recursively from font_style_table.  If font_style_table is
>> modified no longer to contain Vfont_weight_table (by the code above),
>> Vfont_weight_table loses its protection.  Accessing Vfont_weight_table
>> from Lisp when it's no longer protected will eventually cause a crash.
>
> But the above code just uses ASET to put in one of font_style_table's
> slots a Lisp vector.  That vector is therefore reachable from
> font_style_table (which is also a Lisp vector).  No?

But the old vector, which was in the slot before, is no longer reachable
after the ASET.  That old vector can still be referenced from Lisp as
font-weight-table (or one of the other two).  That means an object is
reachable from Lisp but not protected from GC.

Pip

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