cursor-sprite/xcursor: Add a cache for the XCursor images

The cache lives on the MetaCursorTracker and thus is shared between all
CursorSpriteXcursors and avoids constantly having to hit the disk when
the cursor shape changes.

We still do a lot of work to get the sprite from the theme into a KMS
plane or drawn into the onscreen, but avoiding disk IO is a first, good
step.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4359>

src/backends/meta-cursor-sprite-xcursor.c

@@ -30,6 +30,12 @@
 #include "meta/prefs.h"
 #include "meta/util.h"
 
+typedef struct _MetaCursorSpriteXcursorKey
+{
+  MetaCursor cursor;
+  int theme_scale;
+} MetaCursorSpriteXcursorKey;
+
 struct _MetaCursorSpriteXcursor
 {
   MetaCursorSprite parent;
@@ -40,13 +46,69 @@ struct _MetaCursorSpriteXcursor
   XcursorImages *xcursor_images;
 
   int theme_scale;
-  gboolean theme_dirty;
   gboolean invalidated;
 };
 
 G_DEFINE_TYPE (MetaCursorSpriteXcursor, meta_cursor_sprite_xcursor,
                META_TYPE_CURSOR_SPRITE)
 
+static unsigned int
+sprite_key_hash (gconstpointer data)
+{
+  const MetaCursorSpriteXcursorKey *key = data;
+
+  return key->cursor << 0  &
+         key->theme_scale << 8;
+}
+
+static gboolean
+sprite_key_equal (gconstpointer data1,
+                  gconstpointer data2)
+{
+  const MetaCursorSpriteXcursorKey *key1 = data1;
+  const MetaCursorSpriteXcursorKey *key2 = data2;
+
+  return (key1->cursor == key2->cursor &&
+          key1->theme_scale == key2->theme_scale);
+}
+
+static GHashTable *
+ensure_cache (MetaCursorSpriteXcursor *sprite_xcursor)
+{
+  MetaCursorSprite *sprite = META_CURSOR_SPRITE (sprite_xcursor);
+  MetaCursorTracker *cursor_tracker =
+    meta_cursor_sprite_get_cursor_tracker (sprite);
+  GHashTable *cache;
+  static GOnce quark_once = G_ONCE_INIT;
+
+  g_once (&quark_once, (GThreadFunc) g_quark_from_static_string,
+          (gpointer) "-meta-cursor-sprite-xcursor-cache");
+
+  cache = g_object_get_qdata (G_OBJECT (cursor_tracker),
+                              GPOINTER_TO_INT (quark_once.retval));
+  if (!cache)
+    {
+      cache = g_hash_table_new_full (sprite_key_hash, sprite_key_equal,
+                                     g_free,
+                                     (GDestroyNotify) xcursor_images_destroy);
+
+      g_object_set_qdata_full (G_OBJECT (cursor_tracker),
+                               GPOINTER_TO_INT (quark_once.retval),
+                               cache,
+                               (GDestroyNotify) g_hash_table_unref);
+    }
+
+  return cache;
+}
+
+static void
+drop_cache (MetaCursorSpriteXcursor *sprite_xcursor)
+{
+  GHashTable *cache = ensure_cache (sprite_xcursor);
+
+  g_hash_table_remove_all (cache);
+}
+
 const char *
 meta_cursor_get_name (MetaCursor cursor)
 {
@@ -293,7 +355,7 @@ load_from_current_xcursor_image (MetaCursorSpriteXcursor *sprite_xcursor)
   GError *error = NULL;
   int hotspot_x, hotspot_y;
 
-  g_assert (!meta_cursor_sprite_get_cogl_texture (sprite));
+  meta_cursor_sprite_clear_texture (sprite);
 
   xc_image = meta_cursor_sprite_xcursor_get_current_image (sprite_xcursor);
   width = (int) xc_image->width;
@@ -345,9 +407,11 @@ void
 meta_cursor_sprite_xcursor_set_theme_scale (MetaCursorSpriteXcursor *sprite_xcursor,
                                             int                      theme_scale)
 {
-  if (sprite_xcursor->theme_scale != theme_scale)
+  if (sprite_xcursor->theme_scale == theme_scale)
-    sprite_xcursor->theme_dirty = TRUE;
+    return;
+
   sprite_xcursor->theme_scale = theme_scale;
+  sprite_xcursor->xcursor_images = NULL;
 }
 
 void
@@ -397,7 +461,6 @@ meta_cursor_sprite_xcursor_tick_frame (MetaCursorSprite *sprite)
   if (sprite_xcursor->current_frame >= sprite_xcursor->xcursor_images->nimage)
     sprite_xcursor->current_frame = 0;
 
-  meta_cursor_sprite_clear_texture (sprite);
   load_from_current_xcursor_image (sprite_xcursor);
 }
 
@@ -413,28 +476,38 @@ meta_cursor_sprite_xcursor_get_current_frame_time (MetaCursorSprite *sprite)
   return xcursor_images->images[sprite_xcursor->current_frame]->delay;
 }
 
-static void
+static gboolean
 load_cursor_from_theme (MetaCursorSprite *sprite)
 {
   MetaCursorSpriteXcursor *sprite_xcursor = META_CURSOR_SPRITE_XCURSOR (sprite);
+  GHashTable *cache = ensure_cache (sprite_xcursor);
+  XcursorImages *xcursor_images;
+  MetaCursorSpriteXcursorKey key = {
+    .cursor = sprite_xcursor->cursor,
+    .theme_scale = sprite_xcursor->theme_scale,
+  };
 
   g_assert (sprite_xcursor->cursor != META_CURSOR_INVALID);
 
-  sprite_xcursor->theme_dirty = FALSE;
+  xcursor_images = g_hash_table_lookup (cache, &key);
-
+  if (!xcursor_images)
-  /* We might be reloading with a different scale. If so clear the old data. */
-  if (sprite_xcursor->xcursor_images)
     {
-      meta_cursor_sprite_clear_texture (sprite);
+      xcursor_images = load_cursor_on_client (sprite_xcursor->cursor,
-      xcursor_images_destroy (sprite_xcursor->xcursor_images);
+                                              sprite_xcursor->theme_scale);
+
+      g_hash_table_insert (cache,
+                           g_memdup2 (&key, sizeof (key)),
+                           xcursor_images);
     }
 
   sprite_xcursor->current_frame = 0;
-  sprite_xcursor->xcursor_images =
-    load_cursor_on_client (sprite_xcursor->cursor,
-                           sprite_xcursor->theme_scale);
 
+  if (sprite_xcursor->xcursor_images == xcursor_images)
+    return FALSE;
+
+  sprite_xcursor->xcursor_images = xcursor_images;
   load_from_current_xcursor_image (sprite_xcursor);
+  return TRUE;
 }
 
 static gboolean
@@ -443,11 +516,8 @@ meta_cursor_sprite_xcursor_realize_texture (MetaCursorSprite *sprite)
   MetaCursorSpriteXcursor *sprite_xcursor = META_CURSOR_SPRITE_XCURSOR (sprite);
   gboolean retval = sprite_xcursor->invalidated;
 
-  if (sprite_xcursor->theme_dirty)
+  if (load_cursor_from_theme (sprite))
-    {
+    retval = TRUE;
-      load_cursor_from_theme (sprite);
-      retval = TRUE;
-    }
 
   sprite_xcursor->invalidated = FALSE;
 
@@ -554,7 +624,8 @@ on_prefs_changed (MetaCursorSprite *cursor,
   MetaCursorSpriteXcursor *sprite_xcursor =
     META_CURSOR_SPRITE_XCURSOR (user_data);
 
-  sprite_xcursor->theme_dirty = TRUE;
+  drop_cache (sprite_xcursor);
+  sprite_xcursor->xcursor_images = NULL;
 }
 
 MetaCursorSpriteXcursor *
@@ -580,32 +651,17 @@ meta_cursor_sprite_xcursor_new (MetaCursor         cursor,
   return sprite_xcursor;
 }
 
-static void
-meta_cursor_sprite_xcursor_finalize (GObject *object)
-{
-  MetaCursorSpriteXcursor *sprite_xcursor = META_CURSOR_SPRITE_XCURSOR (object);
-
-  g_clear_pointer (&sprite_xcursor->xcursor_images,
-                   xcursor_images_destroy);
-
-  G_OBJECT_CLASS (meta_cursor_sprite_xcursor_parent_class)->finalize (object);
-}
-
 static void
 meta_cursor_sprite_xcursor_init (MetaCursorSpriteXcursor *sprite_xcursor)
 {
   sprite_xcursor->theme_scale = 1;
-  sprite_xcursor->theme_dirty = TRUE;
 }
 
 static void
 meta_cursor_sprite_xcursor_class_init (MetaCursorSpriteXcursorClass *klass)
 {
-  GObjectClass *object_class = G_OBJECT_CLASS (klass);
   MetaCursorSpriteClass *cursor_sprite_class = META_CURSOR_SPRITE_CLASS (klass);
 
-  object_class->finalize = meta_cursor_sprite_xcursor_finalize;
-
   cursor_sprite_class->realize_texture =
     meta_cursor_sprite_xcursor_realize_texture;
   cursor_sprite_class->invalidate =