Run _upgrade_existing_database on workers if at current schema_version (#11346)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelog.d/11346.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in v1.47.0rc1 which caused worker processes to not halt startup in the presence of outstanding database migrations.

synapse/storage/prepare_database.py

@@ -131,9 +131,9 @@ def prepare_database(
                     "config==None in prepare_database, but database is not empty"
                 )
 
-            # if it's a worker app, refuse to upgrade the database, to avoid multiple
+            # This should be run on all processes, master or worker. The master will
-            # workers doing it at once.
+            # apply the deltas, while workers will check if any outstanding deltas
-            if config.worker.worker_app is None:
+            # exist and raise an PrepareDatabaseException if they do.
             _upgrade_existing_database(
                 cur,
                 version_info,
@@ -141,14 +141,6 @@ def prepare_database(
                 config,
                 databases=databases,
             )
-            elif version_info.current_version < SCHEMA_VERSION:
-                # If the DB is on an older version than we expect then we refuse
-                # to start the worker (as the main process needs to run first to
-                # update the schema).
-                raise UpgradeDatabaseException(
-                    OUTDATED_SCHEMA_ON_WORKER_ERROR
-                    % (SCHEMA_VERSION, version_info.current_version)
-                )
 
         else:
             logger.info("%r: Initialising new database", databases)
@@ -358,6 +350,18 @@ def _upgrade_existing_database(
 
     is_worker = config and config.worker.worker_app is not None
 
+    # If the schema version needs to be updated, and we are on a worker, we immediately
+    # know to bail out as workers cannot update the database schema. Only one process
+    # must update the database at the time, therefore we delegate this task to the master.
+    if is_worker and current_schema_state.current_version < SCHEMA_VERSION:
+        # If the DB is on an older version than we expect then we refuse
+        # to start the worker (as the main process needs to run first to
+        # update the schema).
+        raise UpgradeDatabaseException(
+            OUTDATED_SCHEMA_ON_WORKER_ERROR
+            % (SCHEMA_VERSION, current_schema_state.current_version)
+        )
+
     if (
         current_schema_state.compat_version is not None
         and current_schema_state.compat_version > SCHEMA_VERSION

tests/storage/test_rollback_worker.py

@@ -11,6 +11,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from typing import List
+from unittest import mock
+
 from synapse.app.generic_worker import GenericWorkerServer
 from synapse.storage.database import LoggingDatabaseConnection
 from synapse.storage.prepare_database import PrepareDatabaseException, prepare_database
@@ -19,6 +22,22 @@ from synapse.storage.schema import SCHEMA_VERSION
 from tests.unittest import HomeserverTestCase
 
 
+def fake_listdir(filepath: str) -> List[str]:
+    """
+    A fake implementation of os.listdir which we can use to mock out the filesystem.
+
+    Args:
+        filepath: The directory to list files for.
+
+    Returns:
+        A list of files and folders in the directory.
+    """
+    if filepath.endswith("full_schemas"):
+        return [SCHEMA_VERSION]
+
+    return ["99_add_unicorn_to_database.sql"]
+
+
 class WorkerSchemaTests(HomeserverTestCase):
     def make_homeserver(self, reactor, clock):
         hs = self.setup_test_homeserver(
@@ -51,7 +70,7 @@ class WorkerSchemaTests(HomeserverTestCase):
 
         prepare_database(db_conn, db_pool.engine, self.hs.config)
 
-    def test_not_upgraded(self):
+    def test_not_upgraded_old_schema_version(self):
         """Test that workers don't start if the DB has an older schema version"""
         db_pool = self.hs.get_datastore().db_pool
         db_conn = LoggingDatabaseConnection(
@@ -67,3 +86,34 @@ class WorkerSchemaTests(HomeserverTestCase):
 
         with self.assertRaises(PrepareDatabaseException):
             prepare_database(db_conn, db_pool.engine, self.hs.config)
+
+    def test_not_upgraded_current_schema_version_with_outstanding_deltas(self):
+        """
+        Test that workers don't start if the DB is on the current schema version,
+        but there are still outstanding delta migrations to run.
+        """
+        db_pool = self.hs.get_datastore().db_pool
+        db_conn = LoggingDatabaseConnection(
+            db_pool._db_pool.connect(),
+            db_pool.engine,
+            "tests",
+        )
+
+        # Set the schema version of the database to the current version
+        cur = db_conn.cursor()
+        cur.execute("UPDATE schema_version SET version = ?", (SCHEMA_VERSION,))
+
+        db_conn.commit()
+
+        # Path `os.listdir` here to make synapse think that there is a migration
+        # file ready to be run.
+        # Note that we can't patch this function for the whole method, else Synapse
+        # will try to find the file when building the database initially.
+        with mock.patch("os.listdir", mock.Mock(side_effect=fake_listdir)):
+            with self.assertRaises(PrepareDatabaseException):
+                # Synapse should think that there is an outstanding migration file due to
+                # patching 'os.listdir' in the function decorator.
+                #
+                # We expect Synapse to raise an exception to indicate the master process
+                # needs to apply this migration file.
+                prepare_database(db_conn, db_pool.engine, self.hs.config)