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.

4 files changed, 59 insertions, 3 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);
diff --git a/source/blender/windowmanager/WM_types.h b/source/blender/windowmanager/WM_types.h
index 24aa5bc4869..d40c0715a09 100644
--- a/source/blender/windowmanager/WM_types.h
+++ b/source/blender/windowmanager/WM_types.h
@@ -904,6 +904,12 @@ typedef struct wmDropBox {
   void (*copy)(struct wmDrag *, struct wmDropBox *);
 
   /**
+   * If the operator is cancelled (returns `OPERATOR_CANCELLED`), this can be used for cleanup of
+   * `copy()` resources.
+   */
+  void (*cancel)(struct Main *, struct wmDrag *, struct wmDropBox *);
+
+  /**
    * If poll succeeds, operator is called.
    * Not saved in file, so can be pointer.
    */
diff --git a/source/blender/windowmanager/intern/wm_dragdrop.c b/source/blender/windowmanager/intern/wm_dragdrop.c
index 6fdcbab889c..9684c21605a 100644
--- a/source/blender/windowmanager/intern/wm_dragdrop.c
+++ b/source/blender/windowmanager/intern/wm_dragdrop.c
@@ -39,6 +39,7 @@
 #include "BKE_context.h"
 #include "BKE_global.h"
 #include "BKE_idtype.h"
+#include "BKE_lib_id.h"
 
 #include "GPU_shader.h"
 #include "GPU_state.h"
@@ -95,11 +96,13 @@ ListBase *WM_dropboxmap_find(const char *idname, int spaceid, int regionid)
 wmDropBox *WM_dropbox_add(ListBase *lb,
                           const char *idname,
                           bool (*poll)(bContext *, wmDrag *, const wmEvent *, const char **),
-                          void (*copy)(wmDrag *, wmDropBox *))
+                          void (*copy)(wmDrag *, wmDropBox *),
+                          void (*cancel)(struct Main *, wmDrag *, wmDropBox *))
 {
   wmDropBox *drop = MEM_callocN(sizeof(wmDropBox), "wmDropBox");
   drop->poll = poll;
   drop->copy = copy;
+  drop->cancel = cancel;
   drop->ot = WM_operatortype_find(idname, 0);
   drop->opcontext = WM_OP_INVOKE_DEFAULT;
 
@@ -382,6 +385,9 @@ static ID *wm_drag_asset_id_import(wmDragAsset *asset_drag)
 /**
  * When dragging a local ID, return that. Otherwise, if dragging an asset-handle, link or append
  * that depending on what was chosen by the drag-box (currently append only in fact).
+ *
+ * Use #WM_drag_free_imported_drag_ID() as cancel callback of the drop-box, so that the asset
+ * import is rolled back if the drop operator fails.
  */
 ID *WM_drag_get_local_ID_or_import_from_asset(const wmDrag *drag, int idcode)
 {
@@ -402,6 +408,39 @@ ID *WM_drag_get_local_ID_or_import_from_asset(const wmDrag *drag, int idcode)
   return wm_drag_asset_id_import(asset_drag);
 }
 
+/**
+ * \brief Free asset ID imported for cancelled drop.
+ *
+ * If the asset was imported (linked/appended) using #WM_drag_get_local_ID_or_import_from_asset()`
+ * (typically via a #wmDropBox.copy() callback), we want the ID to be removed again if the drop
+ * operator cancels.
+ * This is for use as #wmDropBox.cancel() callback.
+ */
+void WM_drag_free_imported_drag_ID(struct Main *bmain, wmDrag *drag, wmDropBox *drop)
+{
+  if (drag->type != WM_DRAG_ASSET) {
+    return;
+  }
+
+  wmDragAsset *asset_drag = WM_drag_get_asset_data(drag, 0);
+  if (!asset_drag) {
+    return;
+  }
+
+  /* Get name from property, not asset data - it may have changed after importing to ensure
+   * uniqueness (name is assumed to be set from the imported ID name). */
+  char name[MAX_ID_NAME - 2];
+  RNA_string_get(drop->ptr, "name", name);
+  if (!name[0]) {
+    return;
+  }
+
+  ID *id = BKE_libblock_find_name(bmain, asset_drag->id_type, name);
+  if (id) {
+    BKE_id_delete(bmain, id);
+  }
+}
+
 /* ************** draw ***************** */
 
 static void wm_drop_operator_draw(const char *name, int x, int y)
diff --git a/source/blender/windowmanager/intern/wm_event_system.c b/source/blender/windowmanager/intern/wm_event_system.c
index e132caede0d..7b047e68aee 100644
--- a/source/blender/windowmanager/intern/wm_event_system.c
+++ b/source/blender/windowmanager/intern/wm_event_system.c
@@ -2820,8 +2820,14 @@ static int wm_handlers_do_intern(bContext *C, wmEvent *event, ListBase *handlers
                   ListBase single_lb = {drag, drag};
                   event->customdata = &single_lb;
 
-                  wm_operator_call_internal(
+                  int op_retval = wm_operator_call_internal(
                       C, drop->ot, drop->ptr, NULL, drop->opcontext, false, event);
+                  OPERATOR_RETVAL_CHECK(op_retval);
+
+                  if ((op_retval & OPERATOR_CANCELLED) && drop->cancel) {
+                    drop->cancel(CTX_data_main(C), drag, drop);
+                  }
+
                   action |= WM_HANDLER_BREAK;
 
                   /* Free the drags. */