Fix SQL delta file taking a long time to run

changelog.d/9516.bugfix

@@ -0,0 +1 @@
+Fix a bug where users' pushers were not all deleted when they deactivated their account.

synapse/storage/databases/main/pusher.py

@@ -39,6 +39,11 @@ def __init__(self, database: DatabasePool, db_conn: Connection, hs: "HomeServer"
             db_conn, "pushers", "id", extra_tables=[("deleted_pushers", "stream_id")]
         )
 
+        self.db_pool.updates.register_background_update_handler(
+            "remove_deactivated_pushers",
+            self._remove_deactivated_pushers,
+        )
+
     def _decode_pushers_rows(self, rows: Iterable[dict]) -> Iterator[PusherConfig]:
         """JSON-decode the data in the rows returned from the `pushers` table
 
@@ -284,6 +289,54 @@ async def set_throttle_params(
             lock=False,
         )
 
+    async def _remove_deactivated_pushers(self, progress: dict, batch_size: int) -> int:
+        """A background update that deletes all pushers for deactivated users.
+
+        Note that we don't proacively tell the pusherpool that we've deleted
+        these (just because its a bit off a faff to do from here), but they will
+        get cleaned up at the next restart
+        """
+
+        last_user = progress.get("last_user", "")
+
+        def _delete_pushers(txn) -> int:
+
+            sql = """
+                SELECT name FROM users
+                WHERE deactivated = ? and name > ?
+                ORDER BY name ASC
+                LIMIT ?
+            """
+
+            txn.execute(sql, (True, last_user, batch_size))
+            users = [row[0] for row in txn]
+
+            self.db_pool.simple_delete_many_txn(
+                txn,
+                table="pushers",
+                column="user_name",
+                iterable=users,
+                keyvalues={},
+            )
+
+            if users:
+                self.db_pool.updates._background_update_progress_txn(
+                    txn, "remove_deactivated_pushers", {"last_user": users[-1]}
+                )
+
+            return len(users)
+
+        number_deleted = await self.db_pool.runInteraction(
+            "_remove_deactivated_pushers", _delete_pushers
+        )
+
+        if number_deleted < batch_size:
+            await self.db_pool.updates._end_background_update(
+                "remove_deactivated_pushers"
+            )
+
+        return number_deleted
+
 
 class PusherStore(PusherWorkerStore):
     def get_pushers_stream_token(self) -> int:

synapse/storage/databases/main/schema/delta/59/08delete_pushers_for_deactivated_accounts.sql

@@ -14,8 +14,7 @@
  */
 
 
--- We may not have deleted all pushers for deactivated accounts. Do so now.
---
--- Note: We don't bother updating the `deleted_pushers` table as it's just use
--- to stop pushers on workers, and that will happen when they get next restarted.
-DELETE FROM pushers WHERE user_name IN (SELECT name FROM users WHERE deactivated = 1);
+-- We may not have deleted all pushers for deactivated accounts, so we set up a
+-- background job to delete them.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (5908, 'remove_deactivated_pushers', '{}');