[font] Add serial API

New API:
+ hb_font_get_serial()
+ hb_font_changed()

Fixes https://github.com/harfbuzz/harfbuzz/issues/2426

Unused internally as of now.

docs/harfbuzz-sections.txt

@@ -373,6 +373,8 @@ hb_font_glyph_from_string
 hb_font_glyph_to_string
 hb_font_is_immutable
 hb_font_make_immutable
+hb_font_get_serial
+hb_font_changed
 hb_font_reference
 hb_font_set_face
 hb_font_set_funcs

src/hb-font.cc

@@ -1623,6 +1623,8 @@ DEFINE_NULL_INSTANCE (hb_font_t) =
 {
   HB_OBJECT_HEADER_STATIC,
 
+  0, /* serial */
+
   nullptr, /* parent */
   const_cast<hb_face_t *> (&_hb_Null_hb_face_t),
 
@@ -1852,6 +1854,11 @@ hb_font_set_user_data (hb_font_t          *font,
 		       hb_destroy_func_t   destroy /* May be NULL. */,
 		       hb_bool_t           replace)
 {
+  if (hb_object_is_immutable (font))
+    return false;
+
+  font->serial++;
+
   return hb_object_set_user_data (font, key, data, destroy, replace);
 }
 
@@ -1910,6 +1917,45 @@ hb_font_is_immutable (hb_font_t *font)
   return hb_object_is_immutable (font);
 }
 
+/**
+ * hb_font_get_serial:
+ * @font: #hb_font_t to work upon
+ *
+ * Returns the internal serial number of the font. The serial
+ * number is increased every time a setting on the font is
+ * changed, using a setter function.
+ *
+ * Return value: serial number
+ *
+ * Since: REPLACEME.
+ **/
+unsigned long
+hb_font_get_serial (hb_font_t *font)
+{
+  return font->serial;
+}
+
+/**
+ * hb_font_changed:
+ * @font: #hb_font_t to work upon
+ *
+ * Notifies the @font that underlying font data has changed.
+ * This has the effect of increasing the serial as returned
+ * by hb_font_get_serial(), which invalidates internal caches.
+ *
+ * Since: REPLACEME.
+ **/
+void
+hb_font_changed (hb_font_t *font)
+{
+  if (hb_object_is_immutable (font))
+    return;
+
+  font->serial++;
+
+  font->mults_changed ();
+}
+
 /**
  * hb_font_set_parent:
  * @font: #hb_font_t to work upon
@@ -1926,6 +1972,11 @@ hb_font_set_parent (hb_font_t *font,
   if (hb_object_is_immutable (font))
     return;
 
+  if (parent == font->parent)
+    return;
+
+  font->serial++;
+
   if (!parent)
     parent = hb_font_get_empty ();
 
@@ -1968,6 +2019,11 @@ hb_font_set_face (hb_font_t *font,
   if (hb_object_is_immutable (font))
     return;
 
+  if (face == font->face)
+    return;
+
+  font->serial++;
+
   if (unlikely (!face))
     face = hb_face_get_empty ();
 
@@ -2022,6 +2078,8 @@ hb_font_set_funcs (hb_font_t         *font,
     return;
   }
 
+  font->serial++;
+
   if (font->destroy)
     font->destroy (font->user_data);
 
@@ -2059,6 +2117,8 @@ hb_font_set_funcs_data (hb_font_t         *font,
     return;
   }
 
+  font->serial++;
+
   if (font->destroy)
     font->destroy (font->user_data);
 
@@ -2085,6 +2145,11 @@ hb_font_set_scale (hb_font_t *font,
   if (hb_object_is_immutable (font))
     return;
 
+  if (font->x_scale == x_scale && font->y_scale == y_scale)
+    return;
+
+  font->serial++;
+
   font->x_scale = x_scale;
   font->y_scale = y_scale;
   font->mults_changed ();
@@ -2127,6 +2192,11 @@ hb_font_set_ppem (hb_font_t    *font,
   if (hb_object_is_immutable (font))
     return;
 
+  if (font->x_ppem == x_ppem && font->y_ppem == y_ppem)
+    return;
+
+  font->serial++;
+
   font->x_ppem = x_ppem;
   font->y_ppem = y_ppem;
 }
@@ -2169,6 +2239,11 @@ hb_font_set_ptem (hb_font_t *font,
   if (hb_object_is_immutable (font))
     return;
 
+  if (font->ptem == ptem)
+    return;
+
+  font->serial++;
+
   font->ptem = ptem;
 }
 
@@ -2216,6 +2291,11 @@ hb_font_set_synthetic_slant (hb_font_t *font, float slant)
   if (hb_object_is_immutable (font))
     return;
 
+  if (font->slant == slant)
+    return;
+
+  font->serial++;
+
   font->slant = slant;
   font->mults_changed ();
 }
@@ -2263,6 +2343,8 @@ hb_font_set_variations (hb_font_t            *font,
   if (hb_object_is_immutable (font))
     return;
 
+  font->serial++;
+
   if (!variations_length)
   {
     hb_font_set_var_coords_normalized (font, nullptr, 0);
@@ -2322,6 +2404,8 @@ hb_font_set_var_coords_design (hb_font_t    *font,
   if (hb_object_is_immutable (font))
     return;
 
+  font->serial++;
+
   int *normalized = coords_length ? (int *) hb_calloc (coords_length, sizeof (int)) : nullptr;
   float *design_coords = coords_length ? (float *) hb_calloc (coords_length, sizeof (float)) : nullptr;
 
@@ -2355,6 +2439,8 @@ hb_font_set_var_named_instance (hb_font_t *font,
   if (hb_object_is_immutable (font))
     return;
 
+  font->serial++;
+
   unsigned int coords_length = hb_ot_var_named_instance_get_design_coords (font->face, instance_index, nullptr, nullptr);
 
   float *coords = coords_length ? (float *) hb_calloc (coords_length, sizeof (float)) : nullptr;
@@ -2391,6 +2477,8 @@ hb_font_set_var_coords_normalized (hb_font_t    *font,
   if (hb_object_is_immutable (font))
     return;
 
+  font->serial++;
+
   int *copy = coords_length ? (int *) hb_calloc (coords_length, sizeof (coords[0])) : nullptr;
   int *unmapped = coords_length ? (int *) hb_calloc (coords_length, sizeof (coords[0])) : nullptr;
   float *design_coords = coords_length ? (float *) hb_calloc (coords_length, sizeof (design_coords[0])) : nullptr;

src/hb-font.h

@@ -1002,6 +1002,12 @@ hb_font_make_immutable (hb_font_t *font);
 HB_EXTERN hb_bool_t
 hb_font_is_immutable (hb_font_t *font);
 
+HB_EXTERN unsigned long
+hb_font_get_serial (hb_font_t *font);
+
+HB_EXTERN void
+hb_font_changed (hb_font_t *font);
+
 HB_EXTERN void
 hb_font_set_parent (hb_font_t *font,
 		    hb_font_t *parent);

src/hb-font.hh

@@ -104,6 +104,7 @@ DECLARE_NULL_INSTANCE (hb_font_funcs_t);
 struct hb_font_t
 {
   hb_object_header_t header;
+  unsigned long serial;
 
   hb_font_t *parent;
   hb_face_t *face;