Fix T91197: marking assets from Python may crash

When using `asset_mark` function from a Python script and afterwards
updating the preview image, a crash might happen. The preview image is
generated by the `asset_mark` function. This may happen on a background
thread, introducing potential synchronization issues.

This patch fixes this by separating the preview generation
`ID.asset_generate_preview` from the mark as asset `ID.asset_mark`.

Note: this separation of "mark as asset" and "generate preview" also
applies to the `ED_asset_mark_id()` C function; if it is desired to have
previews rendered after marking as asset, a call to
`ED_asset_generate_preview()` is now also required.

Reviewed By: sybren

Maniphest Tasks: T91197

Differential Revision: https://developer.blender.org/D12922

source/blender/editors/asset/ED_asset_mark_clear.h

@@ -34,7 +34,14 @@ struct bContext;
  *
  * \return whether the datablock was marked as asset; false when it is not capable of becoming an
  * asset, or when it already was an asset. */
-bool ED_asset_mark_id(const struct bContext *C, struct ID *id);
+bool ED_asset_mark_id(struct ID *id);
+
+/**
+ * Generate preview image for the given datablock.
+ *
+ * The preview image might be generated using a background thread.
+ */
+void ED_asset_generate_preview(const struct bContext *C, struct ID *id);
 
 /**
  * Remove the asset metadata, turning the ID into a "normal" ID.

source/blender/editors/asset/intern/asset_mark_clear.cc

@@ -40,7 +40,7 @@
 #include "ED_asset_list.h"
 #include "ED_asset_mark_clear.h"
 
-bool ED_asset_mark_id(const bContext *C, ID *id)
+bool ED_asset_mark_id(ID *id)
 {
   if (id->asset_data) {
     return false;
@@ -53,14 +53,17 @@ bool ED_asset_mark_id(const bContext *C, ID *id)
 
   id->asset_data = BKE_asset_metadata_create();
 
-  UI_icon_render_id(C, nullptr, id, ICON_SIZE_PREVIEW, true);
-
   /* Important for asset storage to update properly! */
   ED_assetlist_storage_tag_main_data_dirty();
 
   return true;
 }
 
+void ED_asset_generate_preview(const bContext *C, ID *id)
+{
+  UI_icon_render_id(C, nullptr, id, ICON_SIZE_PREVIEW, true);
+}
+
 bool ED_asset_clear_id(ID *id)
 {
   if (!id->asset_data) {

source/blender/editors/asset/intern/asset_ops.cc

@@ -145,7 +145,9 @@ void AssetMarkHelper::operator()(const bContext &C, PointerRNAVec &ids)
       continue;
     }
 
-    if (ED_asset_mark_id(&C, id)) {
+    if (ED_asset_mark_id(id)) {
+      ED_asset_generate_preview(&C, id);
+
       stats.last_id = id;
       stats.tot_created++;
     }

source/blender/makesrna/intern/rna_ID.c

@@ -669,14 +669,22 @@ static ID *rna_ID_copy(ID *id, Main *bmain)
   return newid;
 }
 
-static void rna_ID_asset_mark(ID *id, bContext *C)
+static void rna_ID_asset_mark(ID *id)
 {
-  if (ED_asset_mark_id(C, id)) {
+  if (ED_asset_mark_id(id)) {
     WM_main_add_notifier(NC_ID | NA_EDITED, NULL);
     WM_main_add_notifier(NC_ASSET | NA_ADDED, NULL);
   }
 }
 
+static void rna_ID_asset_generate_preview(ID *id, bContext *C)
+{
+  ED_asset_generate_preview(C, id);
+
+  WM_main_add_notifier(NC_ID | NA_EDITED, NULL);
+  WM_main_add_notifier(NC_ASSET | NA_EDITED, NULL);
+}
+
 static void rna_ID_asset_clear(ID *id)
 {
   if (ED_asset_clear_id(id)) {
@@ -1986,13 +1994,17 @@ static void rna_def_ID(BlenderRNA *brna)
       func,
       "Enable easier reuse of the data-block through the Asset Browser, with the help of "
       "customizable metadata (like previews, descriptions and tags)");
-  RNA_def_function_flag(func, FUNC_USE_CONTEXT);
 
   func = RNA_def_function(srna, "asset_clear", "rna_ID_asset_clear");
   RNA_def_function_ui_description(
       func,
       "Delete all asset metadata and turn the asset data-block back into a normal data-block");
 
+  func = RNA_def_function(srna, "asset_generate_preview", "rna_ID_asset_generate_preview");
+  RNA_def_function_ui_description(
+      func, "Generate preview image (might be scheduled in a background thread)");
+  RNA_def_function_flag(func, FUNC_USE_CONTEXT);
+
   func = RNA_def_function(srna, "override_create", "rna_ID_override_create");
   RNA_def_function_ui_description(func,
                                   "Create an overridden local copy of this linked data-block (not "