Extensions: support using a default local user directory

This is needed so extensions repositories can reference
user-script directories without them having hard-coded paths
which will be invalid when upgrading a Blender version.

scripts/modules/addon_utils.py

@@ -68,7 +68,7 @@ def _paths_with_extension_repos():
         for repo in _preferences.filepaths.extension_repos:
             if not repo.enabled:
                 continue
-            dirpath = repo.directory
+            dirpath = repo.directory_or_default
             if not os.path.isdir(dirpath):
                 continue
             addon_paths.append((dirpath, "%s.%s" % (_ext_base_pkg_idname, repo.module)))
@@ -652,7 +652,7 @@ def _extension_dirpath_from_preferences():
         for repo in _preferences.filepaths.extension_repos:
             if not repo.enabled:
                 continue
-            repos_dict[repo.module] = repo.directory
+            repos_dict[repo.module] = repo.directory_or_default
     return repos_dict
 
 

scripts/modules/bpy_types.py

@@ -1307,6 +1307,18 @@ class RenderEngine(StructRNA, metaclass=RNAMeta):
     __slots__ = ()
 
 
+class UserExtensionRepo(StructRNA):
+    __slots__ = ()
+
+    @property
+    def directory_or_default(self):
+        """Return ``directory`` or a default path derived from the users scripts path."""
+        if directory := self.directory:
+            return directory
+        import os
+        return os.path.join(_bpy.utils.user_resource('SCRIPTS', path="extensions"), self.module)
+
+
 class HydraRenderEngine(RenderEngine):
     __slots__ = ()
 

source/blender/makesdna/DNA_userdef_types.h

@@ -623,6 +623,10 @@ typedef struct bUserExtensionRepo {
    */
   char module[48];
 
+  /**
+   * The "local" directory where extensions are stored.
+   * When unset, use `{BLENDER_USER_SCRIPTS}/extensions/{bUserExtensionRepo::module}`.
+   */
   char dirpath[1024];     /* FILE_MAX */
   char remote_path[1024]; /* FILE_MAX */
 

source/blender/makesrna/intern/rna_userdef.cc

@@ -6612,7 +6612,10 @@ static void rna_def_userdef_filepaths_extension_repo(BlenderRNA *brna)
 
   prop = RNA_def_property(srna, "directory", PROP_STRING, PROP_DIRPATH);
   RNA_def_property_string_sdna(prop, nullptr, "dirpath");
-  RNA_def_property_ui_text(prop, "Local Directory", "The local directory containing extensions");
+  RNA_def_property_ui_text(prop,
+                           "Local Directory",
+                           "The local directory containing extensions. "
+                           "When unset, a path is used based on the user scripts path");
   RNA_def_property_translation_context(prop, BLT_I18NCONTEXT_EDITOR_FILEBROWSER);
   RNA_def_property_string_funcs(
       prop, nullptr, nullptr, "rna_userdef_extension_repo_directory_set");