Assets: Remove appended asset when dropping operation fails

When dropping an asset somewhere, it is appended and then a drop operation is
called to actually add it to the scene based on current context. If this drop
operation fails, the appended data-block is now still in the .blend. The user
may not notice and not expect this.
Instead idea is to rollback any changes done by dropping code if the operation
fails, namely removing the appended data-block again.

Adds a new `cancel()` callback which is called if the drop operator returns
`OPERATOR_CANCELLED` to drop-boxes and a generic function to deal with assets
on drop failure.

Also removes the `free_id_on_error` property of the `NODE_OT_add_group`
operator, which was used as ad-hoc solution to get this same behavior.

1 files changed, 6 insertions, 1 deletions

diff --git a/source/blender/windowmanager/WM_api.h b/source/blender/windowmanager/WM_api.h
index 69177f39c04..d5de45c74fd 100644
--- a/source/blender/windowmanager/WM_api.h
+++ b/source/blender/windowmanager/WM_api.h
@@ -678,7 +678,8 @@ struct wmDropBox *WM_dropbox_add(
     ListBase *lb,
     const char *idname,
     bool (*poll)(struct bContext *, struct wmDrag *, const struct wmEvent *event, const char **),
-    void (*copy)(struct wmDrag *, struct wmDropBox *));
+    void (*copy)(struct wmDrag *, struct wmDropBox *),
+    void (*cancel)(struct Main *, struct wmDrag *, struct wmDropBox *));
 ListBase *WM_dropboxmap_find(const char *idname, int spaceid, int regionid);
 
 /* ID drag and drop */
@@ -690,6 +691,10 @@ bool WM_drag_is_ID_type(const struct wmDrag *drag, int idcode);
 struct wmDragAsset *WM_drag_get_asset_data(const struct wmDrag *drag, int idcode);
 struct ID *WM_drag_get_local_ID_or_import_from_asset(const struct wmDrag *drag, int idcode);
 
+void WM_drag_free_imported_drag_ID(struct Main *bmain,
+                                   struct wmDrag *drag,
+                                   struct wmDropBox *drop);
+
 /* Set OpenGL viewport and scissor */
 void wmViewport(const struct rcti *winrct);
 void wmPartialViewport(rcti *drawrct, const rcti *winrct, const rcti *partialrct);