Refactor to ensure we call check_consistency (#9470)

The idea here is to stop people forgetting to call `check_consistency`. Folks can still just pass in `None` to the new args in `build_sequence_generator`, but hopefully they won't.

synapse/storage/databases/main/events_worker.py

@@ -45,6 +45,7 @@ from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_cla
 from synapse.storage.database import DatabasePool
 from synapse.storage.engines import PostgresEngine
 from synapse.storage.util.id_generators import MultiWriterIdGenerator, StreamIdGenerator
+from synapse.storage.util.sequence import build_sequence_generator
 from synapse.types import Collection, JsonDict, get_domain_from_id
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.lrucache import LruCache
@@ -156,6 +157,21 @@ class EventsWorkerStore(SQLBaseStore):
         self._event_fetch_list = []
         self._event_fetch_ongoing = 0
 
+        # We define this sequence here so that it can be referenced from both
+        # the DataStore and PersistEventStore.
+        def get_chain_id_txn(txn):
+            txn.execute("SELECT COALESCE(max(chain_id), 0) FROM event_auth_chains")
+            return txn.fetchone()[0]
+
+        self.event_chain_id_gen = build_sequence_generator(
+            db_conn,
+            database.engine,
+            get_chain_id_txn,
+            "event_auth_chain_id",
+            table="event_auth_chains",
+            id_column="chain_id",
+        )
+
     def process_replication_rows(self, stream_name, instance_name, token, rows):
         if stream_name == EventsStream.NAME:
             self._stream_id_gen.advance(instance_name, token)