Re: Crash using text property 'composite on w32

[Chong Yidong <cyd@stupidchicken.com>]

"Juanma Barranquero" <lekktu@gmail.com> writes:

> Program received signal SIGSEGV, Segmentation fault.
> 0x010c798a in run_composition_function (from=267, to=269, prop=520)
>    at composite.c:456
> 456       func = COMPOSITION_MODIFICATION_FUNC (prop);
> (gdb) bt
> #0  0x010c798a in run_composition_function (from=267, to=269, prop=520)
>    at composite.c:456
> #1  0x010c7d1c in update_compositions (from=269, to=271, check_mask=3)
>    at composite.c:514

The trouble here is that the code expects the property `prop' obtained
from find_composition to be a valid composition, which is a cons.  So
probably the way to fix this is to stick in a couple of
COMPOSITION_VALID_P checks into update_compositions, around
composite.c:514.

The strange thing, though, is that the elisp manual describes the
composition property as follows (documented by Handa on 2007-04-19):

  `composition'
     This text property is used to display a sequence of characters as a
     single glyph composed from components.  For instance, in Thai a
     base consonant is composed with the following combining vowel as a
     single glyph.  The value should be a character or a sequence
     (vector, list, or string) of integers.

        * If it is a character, it means to display that character
          instead of the text in the region.

        * If it is a string, it means to display that string's contents
          instead of the text in the region.

        * If it is a vector or list, the elements are characters
          interleaved with internal codes specifying how to compose the
          following character with the previous one.

Is this a mistake?  The assumption made in the Emacs code is that the
only valid form for a `composition' property is a cons cell.  For
example, the definition of COMPOSITION_VALID_P in composite.h:96 is

#define COMPOSITION_VALID_P(start, end, prop)  \
  (CONSP (prop)                                \
   && .....

So it looks like the documentation needs to be fixed too.