Store rejected remote invite events as outliers

Currently they're stored as non-outliers even though the server isn't in
the room, which can be problematic in places where the code assumes it
has the state for all non outlier events.

In particular, there is an edge case where persisting the leave event
triggers a state resolution, which requires looking up the room version
from state. Since the server doesn't have the state, this causes an
exception to be thrown.

synapse/federation/federation_client.py

@@ -32,7 +32,6 @@ from synapse.api.errors import (
     HttpResponseException,
     SynapseError,
 )
-from synapse.events import builder
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
 from synapse.util import logcontext, unwrapFirstError
 from synapse.util.caches.expiringcache import ExpiringCache
@@ -66,6 +65,8 @@ class FederationClient(FederationBase):
         self.state = hs.get_state_handler()
         self.transport_layer = hs.get_federation_transport_client()
 
+        self.event_builder_factory = hs.get_event_builder_factory()
+
         self._get_pdu_cache = ExpiringCache(
             cache_name="get_pdu_cache",
             clock=self._clock,
@@ -571,7 +572,12 @@ class FederationClient(FederationBase):
             if "prev_state" not in pdu_dict:
                 pdu_dict["prev_state"] = []
 
-            ev = builder.EventBuilder(pdu_dict)
+            # Strip off the fields that we want to clobber.
+            pdu_dict.pop("origin", None)
+            pdu_dict.pop("origin_server_ts", None)
+            pdu_dict.pop("unsigned", None)
+
+            ev = self.event_builder_factory.new(pdu_dict)
 
             defer.returnValue(
                 (destination, ev)

synapse/handlers/federation.py

@@ -43,10 +43,7 @@ from synapse.api.errors import (
     StoreError,
     SynapseError,
 )
-from synapse.crypto.event_signing import (
+from synapse.crypto.event_signing import compute_event_signature
-    add_hashes_and_signatures,
-    compute_event_signature,
-)
 from synapse.events.validator import EventValidator
 from synapse.replication.http.federation import (
     ReplicationCleanRoomRestServlet,
@@ -58,7 +55,6 @@ from synapse.types import UserID, get_domain_from_id
 from synapse.util import logcontext, unwrapFirstError
 from synapse.util.async_helpers import Linearizer
 from synapse.util.distributor import user_joined_room
-from synapse.util.frozenutils import unfreeze
 from synapse.util.logutils import log_function
 from synapse.util.retryutils import NotRetryingDestination
 from synapse.visibility import filter_events_for_server
@@ -1083,7 +1079,9 @@ class FederationHandler(BaseHandler):
         handled_events = set()
 
         try:
-            event = self._sign_event(event)
+            self._sign_event(event)
+            event.internal_metadata.outlier = False
+
             # Try the host we successfully got a response to /make_join/
             # request first.
             try:
@@ -1289,13 +1287,7 @@ class FederationHandler(BaseHandler):
         event.internal_metadata.outlier = True
         event.internal_metadata.invite_from_remote = True
 
-        event.signatures.update(
+        self._sign_event(event)
-            compute_event_signature(
-                event,
-                self.hs.hostname,
-                self.hs.config.signing_key[0]
-            )
-        )
 
         context = yield self.state_handler.compute_event_context(event)
         yield self.persist_events_and_notify([(event, context)])
@@ -1313,7 +1305,7 @@ class FederationHandler(BaseHandler):
         # Mark as outlier as we don't have any state for this event; we're not
         # even in the room.
         event.internal_metadata.outlier = True
-        event = self._sign_event(event)
+        self._sign_event(event)
 
         # Try the host that we succesfully called /make_leave/ on first for
         # the /send_leave/ request.
@@ -1358,26 +1350,14 @@ class FederationHandler(BaseHandler):
         defer.returnValue((origin, event))
 
     def _sign_event(self, event):
-        event.internal_metadata.outlier = False
+        event.signatures.update(
-
+            compute_event_signature(
-        builder = self.event_builder_factory.new(
+                event,
-            unfreeze(event.get_pdu_json())
+                self.hs.hostname,
+                self.hs.config.signing_key[0]
+            )
         )
 
-        builder.event_id = self.event_builder_factory.create_event_id()
-        builder.origin = self.hs.hostname
-
-        if not hasattr(event, "signatures"):
-            builder.signatures = {}
-
-        add_hashes_and_signatures(
-            builder,
-            self.hs.hostname,
-            self.hs.config.signing_key[0],
-        )
-
-        return builder.build()
-
     @defer.inlineCallbacks
     @log_function
     def on_make_leave_request(self, room_id, user_id):

synapse/storage/roommember.py

@@ -591,10 +591,7 @@ class RoomMemberStore(RoomMemberWorkerStore):
             # i.e., its something that has just happened.
             # The only current event that can also be an outlier is if its an
             # invite that has come in across federation.
-            is_new_state = not backfilled and (
+            is_new_state = not backfilled
-                not event.internal_metadata.is_outlier()
-                or event.internal_metadata.is_invite_from_remote()
-            )
             is_mine = self.hs.is_mine_id(event.state_key)
             if is_new_state and is_mine:
                 if event.membership == Membership.INVITE: