Prototype glyph flag safe-to-kashida

Fixes https://github.com/harfbuzz/harfbuzz/issues/3721

src/hb-buffer.h

@@ -149,10 +149,11 @@ typedef struct hb_glyph_info_t {
  * Since: 1.5.0
  */
 typedef enum { /*< flags >*/
-  HB_GLYPH_FLAG_UNSAFE_TO_BREAK		= 0x00000001,
-  HB_GLYPH_FLAG_UNSAFE_TO_CONCAT	= 0x00000002,
+  HB_GLYPH_FLAG_UNSAFE_TO_BREAK			= 0x00000001,
+  HB_GLYPH_FLAG_UNSAFE_TO_CONCAT		= 0x00000002,
+  HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA		= 0x00000004,
 
-  HB_GLYPH_FLAG_DEFINED			= 0x00000003 /* OR of all defined flags */
+  HB_GLYPH_FLAG_DEFINED				= 0x00000007 /* OR of all defined flags */
 } hb_glyph_flags_t;
 
 HB_EXTERN hb_glyph_flags_t

src/hb-buffer.hh

@@ -462,6 +462,12 @@ struct hb_buffer_t
 		      start, end,
 		      true);
   }
+  void safe_to_insert_kashida (unsigned int start = 0, unsigned int end = -1)
+  {
+    _set_glyph_flags (HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA,
+		      start, end,
+		      true);
+  }
   void unsafe_to_concat (unsigned int start = 0, unsigned int end = -1)
   {
     if (likely ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) == 0))

src/hb-ot-shape.cc

@@ -1148,9 +1148,15 @@ hb_propagate_flags (hb_buffer_t *buffer)
     unsigned int mask = 0;
     for (unsigned int i = start; i < end; i++)
       mask |= info[i].mask & HB_GLYPH_FLAG_DEFINED;
+
+    if (mask & HB_GLYPH_FLAG_UNSAFE_TO_BREAK)
+      mask &= ~HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA;
+    if (mask & HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA)
+      mask |= HB_GLYPH_FLAG_UNSAFE_TO_BREAK | HB_GLYPH_FLAG_UNSAFE_TO_CONCAT;
+
     if (mask)
       for (unsigned int i = start; i < end; i++)
-	info[i].mask |= mask;
+	info[i].mask = mask;
   }
 }
 

src/hb-ot-shaper-arabic.cc

@@ -331,7 +331,7 @@ arabic_joining (hb_buffer_t *buffer)
     if (entry->prev_action != NONE && prev != UINT_MAX)
     {
       info[prev].arabic_shaping_action() = entry->prev_action;
-      buffer->unsafe_to_break (prev, i + 1);
+      buffer->safe_to_insert_kashida (prev, i + 1);
     }
     else
     {
@@ -365,7 +365,7 @@ arabic_joining (hb_buffer_t *buffer)
     if (entry->prev_action != NONE && prev != UINT_MAX)
     {
       info[prev].arabic_shaping_action() = entry->prev_action;
-      buffer->unsafe_to_break (prev, buffer->len);
+      buffer->safe_to_insert_kashida (prev, buffer->len);
     }
     else if (2 <= state && state <= 5) /* States that have a possible prev_action. */
     {