fix FTS

synapse/storage/database.py

@@ -607,6 +607,8 @@ class DatabasePool:
             # Create a new in-memory DB and copy schema+data from the template
             fresh_conn = sqlite3.connect(":memory:", check_same_thread=False)
             source_conn.backup(fresh_conn)
+            # Re-register custom functions that don't survive backup()
+            engine.register_custom_functions(fresh_conn)
             initial_conn = fresh_conn
 
         self._db_pool = NativeConnectionPool(

synapse/storage/engines/_base.py

@@ -84,6 +84,12 @@ class BaseDatabaseEngine(Generic[ConnectionType, CursorType], metaclass=abc.ABCM
     @abc.abstractmethod
     def on_new_connection(self, db_conn: "LoggingDatabaseConnection") -> None: ...
 
+    def register_custom_functions(self, raw_conn: "Any") -> None:
+        """Register custom database functions on a raw connection.
+
+        Override in subclasses that need custom functions (e.g. SQLite rank()).
+        """
+
     @abc.abstractmethod
     def is_deadlock(self, error: Exception) -> bool: ...
 

synapse/storage/engines/sqlite.py

@@ -86,6 +86,14 @@ class Sqlite3Engine(BaseDatabaseEngine[sqlite3.Connection, sqlite3.Cursor]):
     def convert_param_style(self, sql: str) -> str:
         return sql
 
+    def register_custom_functions(self, raw_conn: sqlite3.Connection) -> None:
+        """Register custom SQLite functions on a raw connection.
+
+        This must be called on any connection created outside the normal
+        on_new_connection path (e.g. connections created via backup()).
+        """
+        raw_conn.create_function("rank", 1, _rank)
+
     def on_new_connection(self, db_conn: "LoggingDatabaseConnection") -> None:
         # We need to import here to avoid an import loop.
         from synapse.storage.prepare_database import prepare_database