[buffer] In hb_buffer_get_positions(), return NULL if inside message callback

As discussed in https://github.com/harfbuzz/harfbuzz/issues/2468#issuecomment-645666066

Part of fixing https://github.com/harfbuzz/harfbuzz/issues/2468

src/hb-buffer.cc

@@ -1363,6 +1363,11 @@ hb_buffer_get_glyph_infos (hb_buffer_t  *buffer,
  * Returns @buffer glyph position array.  Returned pointer
  * is valid as long as @buffer contents are not modified.
  *
+ * If buffer did not have positions before, the positions will be
+ * initialized to zeros, unless this function is called from
+ * within a buffer message callback (see hb_buffer_set_message_func()),
+ * in which case %NULL is returned.
+ *
  * Return value: (transfer none) (array length=length):
  * The @buffer glyph position array.
  * The value valid as long as buffer has not been modified.
@@ -1373,12 +1378,17 @@ hb_glyph_position_t *
 hb_buffer_get_glyph_positions (hb_buffer_t  *buffer,
 			       unsigned int *length)
 {
-  if (!buffer->have_positions)
-    buffer->clear_positions ();
-
   if (length)
     *length = buffer->len;
 
+  if (!buffer->have_positions)
+  {
+    if (unlikely (buffer->message_depth))
+      return nullptr;
+
+    buffer->clear_positions ();
+  }
+
   return (hb_glyph_position_t *) buffer->pos;
 }
 

src/hb-buffer.hh

@@ -128,6 +128,9 @@ struct hb_buffer_t
   hb_buffer_message_func_t message_func;
   void *message_data;
   hb_destroy_func_t message_destroy;
+  unsigned message_depth; /* How deeply are we inside a message callback? */
+#else
+  static constexpr unsigned message_depth = 0u;
 #endif
 
   /* Internal debugging. */
@@ -400,10 +403,16 @@ struct hb_buffer_t
 #else
     if (!messaging ())
       return true;
+
+    message_depth++;
+
     va_list ap;
     va_start (ap, fmt);
     bool ret = message_impl (font, fmt, ap);
     va_end (ap);
+
+    message_depth--;
+
     return ret;
 #endif
   }