Re: text-property/intervals question

[Stefan Monnier]

> Is it legal for a `struct interval' to have a total_length field of 0?
> 
> I was debugging Alex Schroeder's problem with overlays in comint
> buffers, and when I did:
> 
>    M-x global-font-lock-mode
>    M-x shell
>    M-x ansi-color-for-comint-mode-on
> 
> and then in the shell buffer:
> 
>    ls --color /tmp
> 
> Emacs hung.
> 
> Upon investigation, it turns out that Fprevious_single_char_property_change
> was looping infinitely because Fprevious_property_change was returning
> a position the same as its argument (which it's not supposed to do).
> 
> The reason is because in this code from Fprevious_property_change:
> 
>   /* Start with the interval containing the char before point.  */
>   if (i->position == XFASTINT (position))
>     i = previous_interval (i);
> 
> it turns out that the previous interval had a total_length field of 0,
> and so the previous interval was at the same position as `i'!
> 
> If zero-length intervals are legal (I didn't think they were), then I
> suppose this could be fixed by changing the `if' to `while', but if they
> aren't legal, there's something funny going on.  However, I don't
> really understand this code, so I'd appreciate any advice or help.

It seems that this bug is fixed by the patch below,


        Stefan

--- intervals.c.~1.109.~        Mon Nov 12 20:06:41 2001
+++ intervals.c Sun Nov 25 21:15:13 2001
@@ -1690,7 +1690,7 @@
   if (NULL_INTERVAL_P (source))
     {
       Lisp_Object buf;
-      if (!inherit && ! NULL_INTERVAL_P (tree))
+      if (!inherit && !NULL_INTERVAL_P (tree) && length > 0)
        {
          XSETBUFFER (buf, buffer);
          set_text_properties_1 (make_number (position),