Extensions: add BLENDER_USER_EXTENSIONS user directory

Replace: `{BLENDER_RESOURCE_PATH_USER}/extensions`
With:    `{BLENDER_USER_EXTENSIONS}`

This follows BLENDER_USER_CONFIG & BLENDER_USER_SCRIPTS conventions.
Reading the environment variable and accessible via
`bpy.utils.user_resource('SCRIPTS')`

scripts/modules/bpy/utils/__init__.py

@@ -739,7 +739,7 @@ def user_resource(resource_type, *, path="", create=False):
     """
     Return a user resource path (normally from the users home directory).
 
-    :arg type: Resource type in ['DATAFILES', 'CONFIG', 'SCRIPTS'].
+    :arg type: Resource type in ['DATAFILES', 'CONFIG', 'SCRIPTS', 'EXTENSIONS'].
     :type type: string
     :arg path: Optional subdirectory.
     :type path: string

scripts/modules/sys_info.py

@@ -96,6 +96,7 @@ def write_sysinfo(filepath):
             output.write("datafiles: %r\n" % (bpy.utils.user_resource('DATAFILES')))
             output.write("config: %r\n" % (bpy.utils.user_resource('CONFIG')))
             output.write("scripts: %r\n" % (bpy.utils.user_resource('SCRIPTS')))
+            output.write("extensions: %r\n" % (bpy.utils.user_resource('EXTENSIONS')))
             output.write("tempdir: %r\n" % (bpy.app.tempdir))
 
             output.write(title("FFmpeg"))

source/blender/blenkernel/BKE_appdir.hh

@@ -168,6 +168,7 @@ enum {
   BLENDER_USER_CONFIG = 31,
   BLENDER_USER_DATAFILES = 32,
   BLENDER_USER_SCRIPTS = 33,
+  BLENDER_USER_EXTENSIONS = 34,
 
   /* system */
   BLENDER_SYSTEM_DATAFILES = 52,

source/blender/blenkernel/intern/appdir.cc

@@ -656,6 +656,15 @@ bool BKE_appdir_folder_id_ex(const int folder_id,
       }
       return false;
 
+    case BLENDER_USER_EXTENSIONS:
+      if (get_path_environment(path, path_maxncpy, subfolder, "BLENDER_USER_EXTENSIONS")) {
+        break;
+      }
+      if (get_path_user(path, path_maxncpy, "extensions", subfolder)) {
+        break;
+      }
+      return false;
+
     case BLENDER_SYSTEM_PYTHON:
       if (get_path_environment(path, path_maxncpy, subfolder, "BLENDER_SYSTEM_PYTHON")) {
         break;
@@ -717,6 +726,14 @@ std::optional<std::string> BKE_appdir_folder_id_user_notest(const int folder_id,
       }
       get_path_user_ex(path, sizeof(path), "scripts", subfolder, version, check_is_dir);
       break;
+    case BLENDER_USER_EXTENSIONS:
+      if (get_path_environment_ex(
+              path, sizeof(path), subfolder, "BLENDER_USER_EXTENSIONS", check_is_dir))
+      {
+        break;
+      }
+      get_path_user_ex(path, sizeof(path), "extensions", subfolder, version, check_is_dir);
+      break;
     default:
       BLI_assert_unreachable();
       break;
@@ -731,7 +748,12 @@ std::optional<std::string> BKE_appdir_folder_id_user_notest(const int folder_id,
 std::optional<std::string> BKE_appdir_folder_id_create(const int folder_id, const char *subfolder)
 {
   /* Only for user folders. */
-  if (!ELEM(folder_id, BLENDER_USER_DATAFILES, BLENDER_USER_CONFIG, BLENDER_USER_SCRIPTS)) {
+  if (!ELEM(folder_id,
+            BLENDER_USER_DATAFILES,
+            BLENDER_USER_CONFIG,
+            BLENDER_USER_SCRIPTS,
+            BLENDER_USER_EXTENSIONS))
+  {
     BLI_assert_unreachable();
     return std::nullopt;
   }

source/blender/blenkernel/intern/preferences.cc

@@ -255,14 +255,14 @@ size_t BKE_preferences_extension_repo_dirpath_get(const bUserExtensionRepo *repo
     return BLI_strncpy_rlen(dirpath, repo->custom_dirpath, dirpath_maxncpy);
   }
 
-  /* TODO: support `BLENDER_USER_EXTENSIONS`, until then add to user resource. */
-  std::optional<std::string> path = BKE_appdir_resource_path_id(BLENDER_RESOURCE_PATH_USER, false);
+  std::optional<std::string> path = BKE_appdir_folder_id_user_notest(BLENDER_USER_EXTENSIONS,
+                                                                     nullptr);
   /* Highly unlikely to fail as the directory doesn't have to exist. */
   if (!path) {
     dirpath[0] = '\0';
     return 0;
   }
-  return BLI_path_join(dirpath, dirpath_maxncpy, path.value().c_str(), "extensions", repo->module);
+  return BLI_path_join(dirpath, dirpath_maxncpy, path.value().c_str(), repo->module);
 }
 
 bUserExtensionRepo *BKE_preferences_extension_repo_find_index(const UserDef *userdef, int index)

source/blender/makesdna/DNA_userdef_types.h

@@ -625,7 +625,7 @@ typedef struct bUserExtensionRepo {
 
   /**
    * The "local" directory where extensions are stored.
-   * When unset, use `{BLENDER_RESOURCE_PATH_USER}/extensions/{bUserExtensionRepo::module}`.
+   * When unset, use `{BLENDER_USER_EXTENSIONS}/{bUserExtensionRepo::module}`.
    */
   char custom_dirpath[1024]; /* FILE_MAX */
   char remote_path[1024];    /* FILE_MAX */

source/blender/python/intern/bpy.cc

@@ -223,6 +223,7 @@ static PyObject *bpy_user_resource(PyObject * /*self*/, PyObject *args, PyObject
       {BLENDER_USER_DATAFILES, "DATAFILES"},
       {BLENDER_USER_CONFIG, "CONFIG"},
       {BLENDER_USER_SCRIPTS, "SCRIPTS"},
+      {BLENDER_USER_EXTENSIONS, "EXTENSIONS"},
       {0, nullptr},
   };
   PyC_StringEnum type = {type_items};

source/creator/creator_args.cc

@@ -755,6 +755,7 @@ static void print_help(bArgs *ba, bool all)
   PRINT("                           (other 'BLENDER_USER_*' variables override when set).\n");
   PRINT("  $BLENDER_USER_CONFIG     Directory for user configuration files.\n");
   PRINT("  $BLENDER_USER_SCRIPTS    Directory for user scripts.\n");
+  PRINT("  $BLENDER_USER_EXTENSIONS Directory for user extensions.\n");
   PRINT("  $BLENDER_USER_DATAFILES  Directory for user data files (icons, translations, ..).\n");
   PRINT("\n");
   PRINT("  $BLENDER_SYSTEM_RESOURCES  Top level directory for system files.\n");