Remove code which updates application_services_state.last_txn

This column is unused as of #12209, so let's stop writing to it.

changelog.d/12680.misc

@@ -0,0 +1 @@
+Remove code which updates unused database column `application_services_state.last_txn`.

synapse/storage/databases/main/appservice.py

@@ -296,14 +296,6 @@ async def complete_appservice_txn(
         """
 
         def _complete_appservice_txn(txn: LoggingTransaction) -> None:
-            # Set current txn_id for AS to 'txn_id'
-            self.db_pool.simple_upsert_txn(
-                txn,
-                "application_services_state",
-                {"as_id": service.id},
-                {"last_txn": txn_id},
-            )
-
             # Delete txn
             self.db_pool.simple_delete_txn(
                 txn,

synapse/storage/schema/__init__.py

@@ -61,13 +61,16 @@
 
 Changes in SCHEMA_VERSION = 69:
     - We now write to `device_lists_changes_in_room` table.
-    - Use sequence to generate future `application_services_txns.txn_id`s
+    - We now use a PostgreSQL sequence to generate future txn_ids for
+      `application_services_txns`. `application_services_state.last_txn` is no longer
+      updated.
 """
 
 
 SCHEMA_COMPAT_VERSION = (
     # We now assume that `device_lists_changes_in_room` has been filled out for
     # recent device_list_updates.
+    # ... and that `application_services_state.last_txn` is not used.
     69
 )
 """Limit on how far the synapse codebase can be rolled back without breaking db compat

tests/storage/test_appservice.py

@@ -14,7 +14,7 @@
 import json
 import os
 import tempfile
-from typing import List, Optional, cast
+from typing import List, cast
 from unittest.mock import Mock
 
 import yaml
@@ -149,15 +149,12 @@ def _add_service(self, url, as_token, id) -> None:
             outfile.write(yaml.dump(as_yaml))
             self.as_yaml_files.append(as_token)
 
-    def _set_state(
-        self, id: str, state: ApplicationServiceState, txn: Optional[int] = None
-    ):
+    def _set_state(self, id: str, state: ApplicationServiceState):
         return self.db_pool.runOperation(
             self.engine.convert_param_style(
-                "INSERT INTO application_services_state(as_id, state, last_txn) "
-                "VALUES(?,?,?)"
+                "INSERT INTO application_services_state(as_id, state) VALUES(?,?)"
             ),
-            (id, state.value, txn),
+            (id, state.value),
         )
 
     def _insert_txn(self, as_id, txn_id, events):
@@ -280,17 +277,6 @@ def test_complete_appservice_txn_first_txn(
             self.store.complete_appservice_txn(txn_id=txn_id, service=service)
         )
 
-        res = self.get_success(
-            self.db_pool.runQuery(
-                self.engine.convert_param_style(
-                    "SELECT last_txn FROM application_services_state WHERE as_id=?"
-                ),
-                (service.id,),
-            )
-        )
-        self.assertEqual(1, len(res))
-        self.assertEqual(txn_id, res[0][0])
-
         res = self.get_success(
             self.db_pool.runQuery(
                 self.engine.convert_param_style(
@@ -316,14 +302,13 @@ def test_complete_appservice_txn_updates_last_txn_state(
         res = self.get_success(
             self.db_pool.runQuery(
                 self.engine.convert_param_style(
-                    "SELECT last_txn, state FROM application_services_state WHERE as_id=?"
+                    "SELECT state FROM application_services_state WHERE as_id=?"
                 ),
                 (service.id,),
             )
         )
         self.assertEqual(1, len(res))
-        self.assertEqual(txn_id, res[0][0])
-        self.assertEqual(ApplicationServiceState.UP.value, res[0][1])
+        self.assertEqual(ApplicationServiceState.UP.value, res[0][0])
 
         res = self.get_success(
             self.db_pool.runQuery(