Handle half-created indices in receipts index background update (#14650)

When Synapse is terminated while running the background update to create
the `receipts_graph` or `receipts_linearized` indexes, the indexes may
be successfully created (or marked as invalid on postgres) while the
background update remains unfinished. When Synapse next starts up, the
background update will fail because the index already exists, or exists
but is invalid on postgres.

Use the existing code to create indices in background updates, since it
handles these edge cases.

Signed-off-by: Sean Quah <seanq@matrix.org>

synapse/storage/background_updates.py

@@ -544,6 +544,48 @@ class BackgroundUpdater:
                 The named index will be dropped upon completion of the new index.
         """
 
+        async def updater(progress: JsonDict, batch_size: int) -> int:
+            await self.create_index_in_background(
+                index_name=index_name,
+                table=table,
+                columns=columns,
+                where_clause=where_clause,
+                unique=unique,
+                psql_only=psql_only,
+                replaces_index=replaces_index,
+            )
+            await self._end_background_update(update_name)
+            return 1
+
+        self._background_update_handlers[update_name] = _BackgroundUpdateHandler(
+            updater, oneshot=True
+        )
+
+    async def create_index_in_background(
+        self,
+        index_name: str,
+        table: str,
+        columns: Iterable[str],
+        where_clause: Optional[str] = None,
+        unique: bool = False,
+        psql_only: bool = False,
+        replaces_index: Optional[str] = None,
+    ) -> None:
+        """Add an index in the background.
+
+        Args:
+            update_name: update_name to register for
+            index_name: name of index to add
+            table: table to add index to
+            columns: columns/expressions to include in index
+            where_clause: A WHERE clause to specify a partial unique index.
+            unique: true to make a UNIQUE index
+            psql_only: true to only create this index on psql databases (useful
+                for virtual sqlite tables)
+            replaces_index: The name of an index that this index replaces.
+                The named index will be dropped upon completion of the new index.
+        """
+
         def create_index_psql(conn: Connection) -> None:
             conn.rollback()
             # postgres insists on autocommit for the index
@@ -618,16 +660,11 @@ class BackgroundUpdater:
         else:
             runner = create_index_sqlite
 
-        async def updater(progress: JsonDict, batch_size: int) -> int:
-            if runner is not None:
-                logger.info("Adding index %s to %s", index_name, table)
-                await self.db_pool.runWithConnection(runner)
-            await self._end_background_update(update_name)
-            return 1
+        if runner is None:
+            return
 
-        self._background_update_handlers[update_name] = _BackgroundUpdateHandler(
-            updater, oneshot=True
-        )
+        logger.info("Adding index %s to %s", index_name, table)
+        await self.db_pool.runWithConnection(runner)
 
     async def _end_background_update(self, update_name: str) -> None:
         """Removes a completed background update task from the queue.