Merge back search indexer

sphinx/builders/html/__init__.py

@@ -181,7 +181,7 @@ class StandaloneHTMLBuilder(Builder):
 
     imgpath: str = ''
     domain_indices: list[DOMAIN_INDEX_TYPE] = []
-    post_transform_merge_attr = ['images']
+    post_transform_merge_attr = ['images', 'indexer']
 
     def __init__(self, app: Sphinx, env: BuildEnvironment) -> None:
         super().__init__(app, env)
@@ -235,12 +235,20 @@ def init(self) -> None:
         self.use_index = self.get_builder_config('use_index', 'html')
 
     def merge_env_post_transform(self, new_attrs: dict[str, Any]) -> None:
-        """Merge images back to the main builder after parallel
+        """Merge images and search indexer back to the main builder after parallel
         post-transformation.
 
         param new_attrs: the attributes from the parallel subprocess to be
                          udpated in the main builder (self)
         """
+        # handle indexer
+        self.indexer._all_titles.update(new_attrs['indexer']._all_titles)
+        self.indexer._filenames.update(new_attrs['indexer']._filenames)
+        self.indexer._index_entries.update(new_attrs['indexer']._index_entries)
+        self.indexer._mapping.update(new_attrs['indexer']._mapping)
+        self.indexer._title_mapping.update(new_attrs['indexer']._title_mapping)
+        self.indexer._titles.update(new_attrs['indexer']._titles)
+        # handle images
         for filepath, filename in new_attrs['images'].items():
             if filepath not in self.images:
                 self.images[filepath] = filename