Inhibit null-string composition component (Bug#6988).

lisp/ChangeLog

+2012-02-01  Kenichi Handa  <handa@m17n.org>
+
+	* composite.el (compose-region, compose-string): Signal error for
+	a null string component (Bug#6988).
+
 2012-01-31  Chong Yidong  <cyd@gnu.org>
 
 	* frame.el (set-frame-font): New arg ALL-FRAMES.

lisp/composite.el

@@ -211,7 +211,7 @@ or a vector or list of integers and rules.
 If it is a character, it is an alternate character to display instead
 of the text in the region.
 
-If it is a string, the elements are alternate characters.  In
+If it is a string, the elements are one or more alternate characters.  In
 this case, TAB element has a special meaning.  If the first
 character is TAB, the glyphs are displayed with left padding space
 so that no pixel overlaps with the previous column.  If the last
@@ -234,7 +234,9 @@ text in the composition."
   (let ((modified-p (buffer-modified-p))
 	(inhibit-read-only t))
     (if (or (vectorp components) (listp components))
-	(setq components (encode-composition-components components)))
+	(setq components (encode-composition-components components))
+      (if (= (length components) 0)
+	  (error "Invalid composition component: %s" components)))
     (compose-region-internal start end components modification-func)
     (restore-buffer-modified-p modified-p)))
 
@@ -267,7 +269,9 @@ Optional 5th argument MODIFICATION-FUNC is a function to call to
 adjust the composition when it gets invalid because of a change of
 text in the composition."
   (if (or (vectorp components) (listp components))
-      (setq components (encode-composition-components components)))
+      (setq components (encode-composition-components components))
+    (if (= (length components) 0)
+	(error "Invalid composition component: %s" components)))
   (or start (setq start 0))
   (or end (setq end (length string)))
   (compose-string-internal string start end components modification-func)

src/ChangeLog

+2012-02-01  Kenichi Handa  <handa@m17n.org>
+
+	* xdisp.c (BUILD_COMPOSITE_GLYPH_STRING): Initialize first_s to
+	NULL (Bug#6988).
+	(x_produce_glyphs): If the component of a composition is a null
+	string, set it->pixel_width to 1 to avoid zero-width glyph.
+
 2012-01-31  Glenn Morris  <rgm@gnu.org>
 
 	* nsterm.m (syms_of_nsterm) <x-toolkit-scroll-bars>:

src/xdisp.c

@@ -22738,7 +22738,7 @@ compute_overhangs_and_x (struct glyph_string *s, int x, int backward_p)
     ptrdiff_t cmp_id = (row)->glyphs[area][START].u.cmp.id;		    \
     struct composition *cmp = composition_table[cmp_id];		    \
     XChar2b *char2b;							    \
-    struct glyph_string *first_s IF_LINT (= NULL);			    \
+    struct glyph_string *first_s = NULL;				    \
     int n;								    \
     									    \
     char2b = (XChar2b *) alloca ((sizeof *char2b) * cmp->glyph_len);	    \
@@ -24400,7 +24400,7 @@ x_produce_glyphs (struct it *it)
 	  /* Initialize the bounding box.  */
 	  if (pcm)
 	    {
-	      width = pcm->width;
+	      width = cmp->glyph_len > 0 ? pcm->width : 0;
 	      ascent = pcm->ascent;
 	      descent = pcm->descent;
 	      lbearing = pcm->lbearing;
@@ -24408,7 +24408,7 @@ x_produce_glyphs (struct it *it)
 	    }
 	  else
 	    {
-	      width = font->space_width;
+	      width = cmp->glyph_len > 0 ? font->space_width : 0;
 	      ascent = FONT_BASE (font);
 	      descent = FONT_DESCENT (font);
 	      lbearing = 0;
@@ -24595,6 +24595,10 @@ x_produce_glyphs (struct it *it)
 	it->glyph_row->contains_overlapping_glyphs_p = 1;
       it->pixel_width = cmp->pixel_width;
+      if (it->pixel_width == 0)
+	/* We assure that all visible glyphs have at least 1-pixel
+	   width.  */
+	it->pixel_width = 1;
       it->ascent = it->phys_ascent = cmp->ascent;
       it->descent = it->phys_descent = cmp->descent;
       if (face->box != FACE_NO_BOX)