1 files changed, 16 insertions, 11 deletions
diff --git a/synapse/storage/prepare_database.py b/synapse/storage/prepare_database.py
index 1629d2a53c..e45adfcb55 100644
--- a/synapse/storage/prepare_database.py
+++ b/synapse/storage/prepare_database.py
@@ -131,17 +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
-            # workers doing it at once.
-            if (
-                config.worker.worker_app is not None
-                and version_info.current_version != SCHEMA_VERSION
-            ):
-                raise UpgradeDatabaseException(
-                    OUTDATED_SCHEMA_ON_WORKER_ERROR
-                    % (SCHEMA_VERSION, version_info.current_version)
-                )
-
+            # This should be run on all processes, master or worker. The master will
+            # apply the deltas, while workers will check if any outstanding deltas
+            # exist and raise an PrepareDatabaseException if they do.
             _upgrade_existing_database(
                 cur,
                 version_info,
@@ -149,6 +141,7 @@ def prepare_database(
                 config,
                 databases=databases,
             )
+
         else:
             logger.info("%r: Initialising new database", databases)
 
@@ -357,6 +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