[gegl] buffer: always increase revision of duplicate tiles, even empty

[Ell <ell src gnome org>]

commit 5a8c84e7711636099efedce70bd4c34c459cafdf
Author: Ell <ell_se yahoo com>
Date:   Tue Aug 14 03:42:03 2018 -0400

    buffer: always increase revision of duplicate tiles, even empty
    
    In gegl_tile_dup(), increate the revision of the newly-created
    duplicate tile unconditionally, even if it's empty, so that it gets
    stored.
    
    Instead of special-casing empty tiles in the above function, mark
    newly-craeted empty tiles as stored in the empty-tile handler.
    This way, newly-created empty tiles are not stored, but copied
    empty tiles are, since we might need them to replace outdated data
    in the tile backend.  Tile backends may, in turn, test for empty
    tiles upon a SET command, and drop the existing tile instead of
    storing the empty tile.
    
    Remove the now-unnecessary revision bump in gegl_buffer_copy().

 gegl/buffer/gegl-buffer-access.c      | 1 -
 gegl/buffer/gegl-tile-handler-empty.c | 9 ++++++++-
 gegl/buffer/gegl-tile.c               | 8 +++-----
 3 files changed, 11 insertions(+), 7 deletions(-)
---
diff --git a/gegl/buffer/gegl-buffer-access.c b/gegl/buffer/gegl-buffer-access.c
index a95d6df4d..80e9128e3 100644
--- a/gegl/buffer/gegl-buffer-access.c
+++ b/gegl/buffer/gegl-buffer-access.c
@@ -2432,7 +2432,6 @@ gegl_buffer_copy (GeglBuffer          *src,
                 dst_tile->x = dtx;
                 dst_tile->y = dty;
                 dst_tile->z = 0;
-                dst_tile->rev++;
 
                 gegl_tile_handler_cache_insert (cache, dst_tile, dtx, dty, 0);
 
diff --git a/gegl/buffer/gegl-tile-handler-empty.c b/gegl/buffer/gegl-tile-handler-empty.c
index b8655d890..d4d481ed2 100644
--- a/gegl/buffer/gegl-tile-handler-empty.c
+++ b/gegl/buffer/gegl-tile-handler-empty.c
@@ -107,8 +107,15 @@ get_tile (GeglTileSource *gegl_tile_source,
       empty->tile    = _new_empty_tile (tile_size);
     }
 
-  return gegl_tile_handler_dup_tile (GEGL_TILE_HANDLER (empty),
+  tile = gegl_tile_handler_dup_tile (GEGL_TILE_HANDLER (empty),
                                      empty->tile, x, y, z);
+
+  /* no need to store the tile, since we'll just create another empty tile on-
+   * demand if it's dropped.
+   */
+  gegl_tile_mark_as_stored (tile);
+
+  return tile;
 }
 
 static gpointer
diff --git a/gegl/buffer/gegl-tile.c b/gegl/buffer/gegl-tile.c
index 57404acb8..0694ee801 100644
--- a/gegl/buffer/gegl-tile.c
+++ b/gegl/buffer/gegl-tile.c
@@ -143,12 +143,10 @@ gegl_tile_dup (GeglTile *src)
   tile->clone_state    = CLONE_STATE_CLONED;
   tile->n_clones       = src->n_clones;
 
-  /* if the tile is not empty, mark it as dirty, since, even though the in-
-   * memory tile data is shared with the source tile, the stored tile data is
-   * separate.
+  /* mark the tile as dirty, since, even though the in-memory tile data is
+   * shared with the source tile, the stored tile data is separate.
    */
-  if (! tile->is_zero_tile)
-    tile->rev++;
+  tile->rev++;
 
   tile->destroy_notify      = src->destroy_notify;
   tile->destroy_notify_data = src->destroy_notify_data;