Fix messages from multiple senders in historical chunk (MSC2716) (#10276)

Fix messages from multiple senders in historical chunk. This also means that an app service does not need to define `?user_id` when using this endpoint.

Follow-up to https://github.com/matrix-org/synapse/pull/9247

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

3 files changed, 76 insertions, 11 deletions

diff --git a/changelog.d/10276.bugfix b/changelog.d/10276.bugfix
new file mode 100644
index 0000000000..42adc57ad1
--- /dev/null
+++ b/changelog.d/10276.bugfix
@@ -0,0 +1 @@
+Fix historical batch send endpoint (MSC2716) rejecting batches with messages from multiple senders.
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 307f5f9a94..42476a18e5 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -240,6 +240,37 @@ class Auth:
         except KeyError:
             raise MissingClientTokenError()
 
+    async def validate_appservice_can_control_user_id(
+        self, app_service: ApplicationService, user_id: str
+    ):
+        """Validates that the app service is allowed to control
+        the given user.
+
+        Args:
+            app_service: The app service that controls the user
+            user_id: The author MXID that the app service is controlling
+
+        Raises:
+            AuthError: If the application service is not allowed to control the user
+                (user namespace regex does not match, wrong homeserver, etc)
+                or if the user has not been registered yet.
+        """
+
+        # It's ok if the app service is trying to use the sender from their registration
+        if app_service.sender == user_id:
+            pass
+        # Check to make sure the app service is allowed to control the user
+        elif not app_service.is_interested_in_user(user_id):
+            raise AuthError(
+                403,
+                "Application service cannot masquerade as this user (%s)." % user_id,
+            )
+        # Check to make sure the user is already registered on the homeserver
+        elif not (await self.store.get_user_by_id(user_id)):
+            raise AuthError(
+                403, "Application service has not registered this user (%s)" % user_id
+            )
+
     async def _get_appservice_user_id(
         self, request: Request
     ) -> Tuple[Optional[str], Optional[ApplicationService]]:
@@ -261,13 +292,11 @@ class Auth:
             return app_service.sender, app_service
 
         user_id = request.args[b"user_id"][0].decode("utf8")
+        await self.validate_appservice_can_control_user_id(app_service, user_id)
+
         if app_service.sender == user_id:
             return app_service.sender, app_service
 
-        if not app_service.is_interested_in_user(user_id):
-            raise AuthError(403, "Application service cannot masquerade as this user.")
-        if not (await self.store.get_user_by_id(user_id)):
-            raise AuthError(403, "Application service has not registered this user")
         return user_id, app_service
 
     async def get_user_by_access_token(
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 9c58e3689e..ebf4e32230 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -29,6 +29,7 @@ from synapse.api.errors import (
     SynapseError,
 )
 from synapse.api.filtering import Filter
+from synapse.appservice import ApplicationService
 from synapse.events.utils import format_event_for_client_v2
 from synapse.http.servlet import (
     RestServlet,
@@ -47,11 +48,13 @@ from synapse.storage.state import StateFilter
 from synapse.streams.config import PaginationConfig
 from synapse.types import (
     JsonDict,
+    Requester,
     RoomAlias,
     RoomID,
     StreamToken,
     ThirdPartyInstanceID,
     UserID,
+    create_requester,
 )
 from synapse.util import json_decoder
 from synapse.util.stringutils import parse_and_validate_server_name, random_string
@@ -309,7 +312,7 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
         self.room_member_handler = hs.get_room_member_handler()
         self.auth = hs.get_auth()
 
-    async def inherit_depth_from_prev_ids(self, prev_event_ids) -> int:
+    async def _inherit_depth_from_prev_ids(self, prev_event_ids) -> int:
         (
             most_recent_prev_event_id,
             most_recent_prev_event_depth,
@@ -378,6 +381,25 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
 
         return insertion_event
 
+    async def _create_requester_for_user_id_from_app_service(
+        self, user_id: str, app_service: ApplicationService
+    ) -> Requester:
+        """Creates a new requester for the given user_id
+        and validates that the app service is allowed to control
+        the given user.
+
+        Args:
+            user_id: The author MXID that the app service is controlling
+            app_service: The app service that controls the user
+
+        Returns:
+            Requester object
+        """
+
+        await self.auth.validate_appservice_can_control_user_id(app_service, user_id)
+
+        return create_requester(user_id, app_service=app_service)
+
     async def on_POST(self, request, room_id):
         requester = await self.auth.get_user_by_req(request, allow_guest=False)
 
@@ -443,7 +465,9 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
             if event_dict["type"] == EventTypes.Member:
                 membership = event_dict["content"].get("membership", None)
                 event_id, _ = await self.room_member_handler.update_membership(
-                    requester,
+                    await self._create_requester_for_user_id_from_app_service(
+                        state_event["sender"], requester.app_service
+                    ),
                     target=UserID.from_string(event_dict["state_key"]),
                     room_id=room_id,
                     action=membership,
@@ -463,7 +487,9 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
                     event,
                     _,
                 ) = await self.event_creation_handler.create_and_send_nonmember_event(
-                    requester,
+                    await self._create_requester_for_user_id_from_app_service(
+                        state_event["sender"], requester.app_service
+                    ),
                     event_dict,
                     outlier=True,
                     prev_event_ids=[fake_prev_event_id],
@@ -479,7 +505,9 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
         events_to_create = body["events"]
 
         prev_event_ids = prev_events_from_query
-        inherited_depth = await self.inherit_depth_from_prev_ids(prev_events_from_query)
+        inherited_depth = await self._inherit_depth_from_prev_ids(
+            prev_events_from_query
+        )
 
         # Figure out which chunk to connect to. If they passed in
         # chunk_id_from_query let's use it. The chunk ID passed in comes
@@ -509,7 +537,10 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
                 base_insertion_event,
                 _,
             ) = await self.event_creation_handler.create_and_send_nonmember_event(
-                requester,
+                await self._create_requester_for_user_id_from_app_service(
+                    base_insertion_event_dict["sender"],
+                    requester.app_service,
+                ),
                 base_insertion_event_dict,
                 prev_event_ids=base_insertion_event_dict.get("prev_events"),
                 auth_event_ids=auth_event_ids,
@@ -558,7 +589,9 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
             }
 
             event, context = await self.event_creation_handler.create_event(
-                requester,
+                await self._create_requester_for_user_id_from_app_service(
+                    ev["sender"], requester.app_service
+                ),
                 event_dict,
                 prev_event_ids=event_dict.get("prev_events"),
                 auth_event_ids=auth_event_ids,
@@ -588,7 +621,9 @@ class RoomBatchSendEventRestServlet(TransactionRestServlet):
         # where topological_ordering is just depth.
         for (event, context) in reversed(events_to_persist):
             ev = await self.event_creation_handler.handle_new_client_event(
-                requester=requester,
+                await self._create_requester_for_user_id_from_app_service(
+                    event["sender"], requester.app_service
+                ),
                 event=event,
                 context=context,
             )