Use a sequence to generate AS transaction IDs, drop last_txn AS state (#12209)

Switching to a sequence means there's no need to track `last_txn` on the
AS state table to generate new TXN IDs. This also means that there is
no longer contention between the AS scheduler and AS handler on updates
to the `application_services_state` table, which will prevent serialization
errors during the complete AS txn transaction.

synapse/storage/databases/main/appservice.py

@@ -29,6 +29,8 @@ from synapse.storage._base import db_to_json
 from synapse.storage.database import DatabasePool, LoggingDatabaseConnection
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.roommember import RoomMemberWorkerStore
+from synapse.storage.types import Cursor
+from synapse.storage.util.sequence import build_sequence_generator
 from synapse.types import DeviceListUpdates, JsonDict
 from synapse.util import json_encoder
 from synapse.util.caches.descriptors import _CacheContext, cached
@@ -72,6 +74,22 @@ class ApplicationServiceWorkerStore(RoomMemberWorkerStore):
         )
         self.exclusive_user_regex = _make_exclusive_regex(self.services_cache)
 
+        def get_max_as_txn_id(txn: Cursor) -> int:
+            logger.warning("Falling back to slow query, you should port to postgres")
+            txn.execute(
+                "SELECT COALESCE(max(txn_id), 0) FROM application_services_txns"
+            )
+            return txn.fetchone()[0]  # type: ignore
+
+        self._as_txn_seq_gen = build_sequence_generator(
+            db_conn,
+            database.engine,
+            get_max_as_txn_id,
+            "application_services_txn_id_seq",
+            table="application_services_txns",
+            id_column="txn_id",
+        )
+
         super().__init__(database, db_conn, hs)
 
     def get_app_services(self):
@@ -239,21 +257,7 @@ class ApplicationServiceTransactionWorkerStore(
         """
 
         def _create_appservice_txn(txn):
-            # work out new txn id (highest txn id for this service += 1)
-            # The highest id may be the last one sent (in which case it is last_txn)
-            # or it may be the highest in the txns list (which are waiting to be/are
-            # being sent)
-            last_txn_id = self._get_last_txn(txn, service.id)
-
-            txn.execute(
-                "SELECT MAX(txn_id) FROM application_services_txns WHERE as_id=?",
-                (service.id,),
-            )
-            highest_txn_id = txn.fetchone()[0]
-            if highest_txn_id is None:
-                highest_txn_id = 0
-
-            new_txn_id = max(highest_txn_id, last_txn_id) + 1
+            new_txn_id = self._as_txn_seq_gen.get_next_id_txn(txn)
 
             # Insert new txn into txn table
             event_ids = json_encoder.encode([e.event_id for e in events])
@@ -286,25 +290,8 @@ class ApplicationServiceTransactionWorkerStore(
             txn_id: The transaction ID being completed.
             service: The application service which was sent this transaction.
         """
-        txn_id = int(txn_id)
 
         def _complete_appservice_txn(txn):
-            # Debugging query: Make sure the txn being completed is EXACTLY +1 from
-            # what was there before. If it isn't, we've got problems (e.g. the AS
-            # has probably missed some events), so whine loudly but still continue,
-            # since it shouldn't fail completion of the transaction.
-            last_txn_id = self._get_last_txn(txn, service.id)
-            if (last_txn_id + 1) != txn_id:
-                logger.error(
-                    "appservice: Completing a transaction which has an ID > 1 from "
-                    "the last ID sent to this AS. We've either dropped events or "
-                    "sent it to the AS out of order. FIX ME. last_txn=%s "
-                    "completing_txn=%s service_id=%s",
-                    last_txn_id,
-                    txn_id,
-                    service.id,
-                )
-
             # Set current txn_id for AS to 'txn_id'
             self.db_pool.simple_upsert_txn(
                 txn,
@@ -376,17 +363,6 @@ class ApplicationServiceTransactionWorkerStore(
             device_list_summary=DeviceListUpdates(),
         )
 
-    def _get_last_txn(self, txn, service_id: Optional[str]) -> int:
-        txn.execute(
-            "SELECT last_txn FROM application_services_state WHERE as_id=?",
-            (service_id,),
-        )
-        last_txn_id = txn.fetchone()
-        if last_txn_id is None or last_txn_id[0] is None:  # no row exists
-            return 0
-        else:
-            return int(last_txn_id[0])  # select 'last_txn' col
-
     async def set_appservice_last_pos(self, pos: int) -> None:
         def set_appservice_last_pos_txn(txn):
             txn.execute(