Fix backfilled events being rejected for no state_groups

changelog.d/10439.bugfix

@@ -0,0 +1 @@
+Fix events with floating outlier state being rejected over federation.

synapse/handlers/federation.py

@@ -2711,9 +2711,11 @@ async def _update_auth_events_and_context_for_auth(
                             event.event_id,
                             e.event_id,
                         )
-                        context = await self.state_handler.compute_event_context(e)
+                        missing_auth_event_context = (
+                            await self.state_handler.compute_event_context(e)
+                        )
                         await self._auth_and_persist_event(
-                            origin, e, context, auth_events=auth
+                            origin, e, missing_auth_event_context, auth_events=auth
                         )
 
                         if e.event_id in event_auth_events:

tests/handlers/test_federation.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+from typing import List
 from unittest import TestCase
 
 from synapse.api.constants import EventTypes
@@ -22,6 +23,7 @@
 from synapse.logging.context import LoggingContext, run_in_background
 from synapse.rest import admin
 from synapse.rest.client.v1 import login, room
+from synapse.util.stringutils import random_string
 
 from tests import unittest
 
@@ -39,6 +41,8 @@ def make_homeserver(self, reactor, clock):
         hs = self.setup_test_homeserver(federation_http_client=None)
         self.handler = hs.get_federation_handler()
         self.store = hs.get_datastore()
+        self.state_store = hs.get_storage().state
+        self._event_auth_handler = hs.get_event_auth_handler()
         return hs
 
     def test_exchange_revoked_invite(self):
@@ -190,6 +194,134 @@ def test_rejected_state_event_state(self):
 
         self.assertEqual(sg, sg2)
 
+    def test_backfill_floating_outlier_membership_auth(self):
+        """
+        As the remote federated homeserver, check that we can properly process
+        an event with auth_events that include a floating membership event from
+        the OTHER_SERVER.
+
+        Regression test, see #10439.
+        """
+
+        def filter_auth_events_on_event(event: EventBase, auth_events: List[EventBase]):
+            # Create a StateMap[str]
+            auth_event_state_map = {
+                (e.type, e.state_key): e.event_id for e in auth_events
+            }
+            # Actually strip down and use the necessary auth events
+            auth_event_ids = self._event_auth_handler.compute_auth_events(
+                event=event,
+                current_state_ids=auth_event_state_map,
+                for_verification=False,
+            )
+
+            # Replace the auth_events with the stripped down ones
+            event.auth_events = auth_event_ids
+
+        OTHER_SERVER = "otherserver"
+        OTHER_USER = "@otheruser:" + OTHER_SERVER
+
+        # create the room
+        user_id = self.register_user("kermit", "test")
+        tok = self.login("kermit", "test")
+        room_id = self.helper.create_room_as(
+            room_creator=user_id,
+            is_public=True,
+            tok=tok,
+            extra_content={
+                "preset": "public_chat",
+            },
+        )
+        room_version = self.get_success(self.store.get_room_version(room_id))
+
+        prev_event_ids = self.get_success(self.store.get_prev_events_for_room(room_id))
+        (
+            most_recent_prev_event_id,
+            most_recent_prev_event_depth,
+        ) = self.get_success(self.store.get_max_depth_of(prev_event_ids))
+        # mapping from (type, state_key) -> state_event_id
+        prev_state_map = self.get_success(
+            self.state_store.get_state_ids_for_event(most_recent_prev_event_id)
+        )
+        # List of state event ID's
+        prev_state_ids = list(prev_state_map.values())
+        auth_event_ids = prev_state_ids
+        auth_events = list(
+            self.get_success(self.store.get_events(auth_event_ids)).values()
+        )
+
+        # build a floating outlier member state event
+        fake_prev_event_id = "$" + random_string(43)
+        member_event = event_from_pdu_json(
+            {
+                "type": EventTypes.Member,
+                "content": {
+                    "membership": "join",
+                },
+                "state_key": OTHER_USER,
+                "room_id": room_id,
+                "sender": OTHER_USER,
+                "depth": most_recent_prev_event_depth,
+                "prev_events": [fake_prev_event_id],
+                "auth_events": auth_event_ids.copy(),
+                "origin_server_ts": self.clock.time_msec(),
+                "signatures": {
+                    OTHER_SERVER: {"ed25519:key_version": "SomeSignatureHere"}
+                },
+            },
+            room_version,
+            outlier=True,
+        )
+        filter_auth_events_on_event(member_event, auth_events)
+
+        # build and send an event authed based on the member event
+        raw_auth_events = auth_events + [member_event]
+        message_event = event_from_pdu_json(
+            {
+                "type": EventTypes.Message,
+                "content": {},
+                "room_id": room_id,
+                "sender": OTHER_USER,
+                "depth": most_recent_prev_event_depth,
+                "prev_events": prev_event_ids.copy(),
+                "auth_events": [ev.event_id for ev in raw_auth_events],
+                "origin_server_ts": self.clock.time_msec(),
+                "signatures": {
+                    OTHER_SERVER: {"ed25519:key_version": "SomeSignatureHere"}
+                },
+            },
+            room_version,
+        )
+        filter_auth_events_on_event(message_event, raw_auth_events.copy())
+
+        # Stub the /event_auth response from the OTHER_SERVER
+        async def get_event_auth(destination, room_id, event_id):
+            return [
+                auth_event
+                for auth_event in raw_auth_events
+                if auth_event.type is EventTypes.Create
+                or auth_event.type is EventTypes.PowerLevels
+                or auth_event.type is EventTypes.JoinRules
+                or (
+                    auth_event.type is EventTypes.Member
+                    and auth_event.state_key == message_event.sender
+                )
+            ]
+
+        self.handler.federation_client.get_event_auth = get_event_auth
+
+        with LoggingContext("receive_pdu"):
+            d = run_in_background(
+                self.handler.on_receive_pdu, OTHER_SERVER, message_event
+            )
+        self.get_success(d)
+
+        # Try and get the events
+        stored_event = self.get_success(
+            self.store.get_event(message_event.event_id, allow_none=True)
+        )
+        self.assertTrue(stored_event is not None)
+
     @unittest.override_config(
         {"rc_invites": {"per_user": {"per_second": 0.5, "burst_count": 3}}}
     )