Move registrations off the main worker (#18552)

This is mainly moving a few store methods around. Note that this doesn't
yet remove the replication servlet to avoid breaking during rollout.

Author: sandhose

changelog.d/18552.misc

@@ -0,0 +1 @@
+Allow user registrations to be done on workers.

docs/upgrade.md

@@ -117,6 +117,14 @@ each upgrade are complete before moving on to the next upgrade, to avoid
 stacking them up. You can monitor the currently running background updates with
 [the Admin API](usage/administration/admin_api/background_updates.html#status).
 
+# Upgrading to v1.135.0
+
+## `on_user_registration` module API callback may now run on any worker
+
+Previously, the `on_user_registration` callback would only run on the main
+process. Modules relying on this callback must assume that they may now be
+called from any worker, not just the main process.
+
 # Upgrading to v1.134.0
 
 ## ICU bundled with Synapse

synapse/app/generic_worker.py

@@ -118,7 +118,6 @@ class GenericWorkerStore(
     # FIXME(https://github.com/matrix-org/synapse/issues/3714): We need to add
     # UserDirectoryStore as we write directly rather than going via the correct worker.
     UserDirectoryStore,
-    StatsStore,
     UIAuthWorkerStore,
     EndToEndRoomKeyStore,
     PresenceStore,
@@ -154,6 +153,7 @@ class GenericWorkerStore(
     StreamWorkerStore,
     EventsWorkerStore,
     RegistrationWorkerStore,
+    StatsStore,
     SearchStore,
     TransactionWorkerStore,
     LockStore,

synapse/handlers/register.py

@@ -49,7 +49,6 @@
 from synapse.replication.http.login import RegisterDeviceReplicationServlet
 from synapse.replication.http.register import (
     ReplicationPostRegisterActionsServlet,
-    ReplicationRegisterServlet,
 )
 from synapse.spam_checker_api import RegistrationBehaviour
 from synapse.types import GUEST_USER_ID_PATTERN, RoomAlias, UserID, create_requester
@@ -120,7 +119,6 @@ def __init__(self, hs: "HomeServer"):
         self._spam_checker_module_callbacks = hs.get_module_api_callbacks().spam_checker
 
         if hs.config.worker.worker_app:
-            self._register_client = ReplicationRegisterServlet.make_client(hs)
             self._register_device_client = RegisterDeviceReplicationServlet.make_client(
                 hs
             )
@@ -738,37 +736,20 @@ async def register_with_store(
             shadow_banned: Whether to shadow-ban the user
             approved: Whether to mark the user as approved by an administrator
         """
-        if self.hs.config.worker.worker_app:
-            await self._register_client(
-                user_id=user_id,
-                password_hash=password_hash,
-                was_guest=was_guest,
-                make_guest=make_guest,
-                appservice_id=appservice_id,
-                create_profile_with_displayname=create_profile_with_displayname,
-                admin=admin,
-                user_type=user_type,
-                address=address,
-                shadow_banned=shadow_banned,
-                approved=approved,
-            )
-        else:
-            await self.store.register_user(
-                user_id=user_id,
-                password_hash=password_hash,
-                was_guest=was_guest,
-                make_guest=make_guest,
-                appservice_id=appservice_id,
-                create_profile_with_displayname=create_profile_with_displayname,
-                admin=admin,
-                user_type=user_type,
-                shadow_banned=shadow_banned,
-                approved=approved,
-            )
+        await self.store.register_user(
+            user_id=user_id,
+            password_hash=password_hash,
+            was_guest=was_guest,
+            make_guest=make_guest,
+            appservice_id=appservice_id,
+            create_profile_with_displayname=create_profile_with_displayname,
+            admin=admin,
+            user_type=user_type,
+            shadow_banned=shadow_banned,
+            approved=approved,
+        )
 
-            # Only call the account validity module(s) on the main process, to avoid
-            # repeating e.g. database writes on all of the workers.
-            await self._account_validity_handler.on_user_registration(user_id)
+        await self._account_validity_handler.on_user_registration(user_id)
 
     async def register_device(
         self,

synapse/replication/http/register.py

@@ -33,6 +33,8 @@
 logger = logging.getLogger(__name__)
 
 
+# FIXME(2025-07-22): Remove this on the next release, this may only be used
+# during rollout to Synapse 1.134 and can be removed after that release.
 class ReplicationRegisterServlet(ReplicationEndpoint):
     """Register a new user"""