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-rw-r--r--synapse/handlers/federation.py82
1 files changed, 60 insertions, 22 deletions
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 4e77bfa55e..2944deaa00 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -49,6 +49,7 @@ from synapse.api.errors import (
     FederationPullAttemptBackoffError,
     HttpResponseException,
     NotFoundError,
+    PartialStateConflictError,
     RequestSendFailed,
     SynapseError,
 )
@@ -56,7 +57,7 @@ from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.crypto.event_signing import compute_event_signature
 from synapse.event_auth import validate_event_for_room_version
 from synapse.events import EventBase
-from synapse.events.snapshot import EventContext
+from synapse.events.snapshot import EventContext, UnpersistedEventContextBase
 from synapse.events.validator import EventValidator
 from synapse.federation.federation_client import InvalidResponseError
 from synapse.http.servlet import assert_params_in_dict
@@ -68,7 +69,6 @@ from synapse.replication.http.federation import (
     ReplicationCleanRoomRestServlet,
     ReplicationStoreRoomOnOutlierMembershipRestServlet,
 )
-from synapse.storage.databases.main.events import PartialStateConflictError
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
 from synapse.types import JsonDict, StrCollection, get_domain_from_id
 from synapse.types.state import StateFilter
@@ -952,7 +952,20 @@ class FederationHandler:
         #
         # Note that this requires the /send_join request to come back to the
         # same server.
+        prev_event_ids = None
         if room_version.msc3083_join_rules:
+            # Note that the room's state can change out from under us and render our
+            # nice join rules-conformant event non-conformant by the time we build the
+            # event. When this happens, our validation at the end fails and we respond
+            # to the requesting server with a 403, which is misleading — it indicates
+            # that the user is not allowed to join the room and the joining server
+            # should not bother retrying via this homeserver or any others, when
+            # in fact we've just messed up with building the event.
+            #
+            # To reduce the likelihood of this race, we capture the forward extremities
+            # of the room (prev_event_ids) just before fetching the current state, and
+            # hope that the state we fetch corresponds to the prev events we chose.
+            prev_event_ids = await self.store.get_prev_events_for_room(room_id)
             state_ids = await self._state_storage_controller.get_current_state_ids(
                 room_id
             )
@@ -990,15 +1003,21 @@ class FederationHandler:
         )
 
         try:
-            event, context = await self.event_creation_handler.create_new_client_event(
-                builder=builder
+            (
+                event,
+                unpersisted_context,
+            ) = await self.event_creation_handler.create_new_client_event(
+                builder=builder,
+                prev_event_ids=prev_event_ids,
             )
         except SynapseError as e:
             logger.warning("Failed to create join to %s because %s", room_id, e)
             raise
 
         # Ensure the user can even join the room.
-        await self._federation_event_handler.check_join_restrictions(context, event)
+        await self._federation_event_handler.check_join_restrictions(
+            unpersisted_context, event
+        )
 
         # The remote hasn't signed it yet, obviously. We'll do the full checks
         # when we get the event back in `on_send_join_request`
@@ -1178,7 +1197,7 @@ class FederationHandler:
             },
         )
 
-        event, context = await self.event_creation_handler.create_new_client_event(
+        event, _ = await self.event_creation_handler.create_new_client_event(
             builder=builder
         )
 
@@ -1228,12 +1247,13 @@ class FederationHandler:
             },
         )
 
-        event, context = await self.event_creation_handler.create_new_client_event(
-            builder=builder
-        )
+        (
+            event,
+            unpersisted_context,
+        ) = await self.event_creation_handler.create_new_client_event(builder=builder)
 
         event_allowed, _ = await self.third_party_event_rules.check_event_allowed(
-            event, context
+            event, unpersisted_context
         )
         if not event_allowed:
             logger.warning("Creation of knock %s forbidden by third-party rules", event)
@@ -1406,15 +1426,20 @@ class FederationHandler:
                 try:
                     (
                         event,
-                        context,
+                        unpersisted_context,
                     ) = await self.event_creation_handler.create_new_client_event(
                         builder=builder
                     )
 
-                    event, context = await self.add_display_name_to_third_party_invite(
-                        room_version_obj, event_dict, event, context
+                    (
+                        event,
+                        unpersisted_context,
+                    ) = await self.add_display_name_to_third_party_invite(
+                        room_version_obj, event_dict, event, unpersisted_context
                     )
 
+                    context = await unpersisted_context.persist(event)
+
                     EventValidator().validate_new(event, self.config)
 
                     # We need to tell the transaction queue to send this out, even
@@ -1483,14 +1508,19 @@ class FederationHandler:
             try:
                 (
                     event,
-                    context,
+                    unpersisted_context,
                 ) = await self.event_creation_handler.create_new_client_event(
                     builder=builder
                 )
-                event, context = await self.add_display_name_to_third_party_invite(
-                    room_version_obj, event_dict, event, context
+                (
+                    event,
+                    unpersisted_context,
+                ) = await self.add_display_name_to_third_party_invite(
+                    room_version_obj, event_dict, event, unpersisted_context
                 )
 
+                context = await unpersisted_context.persist(event)
+
                 try:
                     validate_event_for_room_version(event)
                     await self._event_auth_handler.check_auth_rules_from_context(event)
@@ -1522,8 +1552,8 @@ class FederationHandler:
         room_version_obj: RoomVersion,
         event_dict: JsonDict,
         event: EventBase,
-        context: EventContext,
-    ) -> Tuple[EventBase, EventContext]:
+        context: UnpersistedEventContextBase,
+    ) -> Tuple[EventBase, UnpersistedEventContextBase]:
         key = (
             EventTypes.ThirdPartyInvite,
             event.content["third_party_invite"]["signed"]["token"],
@@ -1557,11 +1587,14 @@ class FederationHandler:
             room_version_obj, event_dict
         )
         EventValidator().validate_builder(builder)
-        event, context = await self.event_creation_handler.create_new_client_event(
-            builder=builder
-        )
+
+        (
+            event,
+            unpersisted_context,
+        ) = await self.event_creation_handler.create_new_client_event(builder=builder)
+
         EventValidator().validate_new(event, self.config)
-        return event, context
+        return event, unpersisted_context
 
     async def _check_signature(self, event: EventBase, context: EventContext) -> None:
         """
@@ -1861,6 +1894,11 @@ class FederationHandler:
                 logger.info("Updating current state for %s", room_id)
                 # TODO(faster_joins): notify workers in notify_room_un_partial_stated
                 #   https://github.com/matrix-org/synapse/issues/12994
+                #
+                # NB: there's a potential race here. If room is purged just before we
+                # call this, we _might_ end up inserting rows into current_state_events.
+                # (The logic is hard to chase through.) We think this is fine, but if
+                # not the HS admin should purge the room again.
                 await self.state_handler.update_current_state(room_id)
 
                 logger.info("Handling any pending device list updates")