Generate real events when we reject invites (#7804)

Fixes #2181. 

The basic premise is that, when we
fail to reject an invite via the remote server, we can generate our own
out-of-band leave event and persist it as an outlier, so that we have something
to send to the client.

synapse/handlers/message.py

@@ -15,7 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Optional, Tuple
+from typing import TYPE_CHECKING, Optional, Tuple
 
 from canonicaljson import encode_canonical_json, json
 
@@ -55,6 +55,9 @@ from synapse.visibility import filter_events_for_client
 
 from ._base import BaseHandler
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
@@ -349,7 +352,7 @@ _DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY = 7 * 24 * 60 * 60 * 1000
 
 
 class EventCreationHandler(object):
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         self.hs = hs
         self.auth = hs.get_auth()
         self.store = hs.get_datastore()
@@ -814,11 +817,17 @@ class EventCreationHandler(object):
                 403, "This event is not allowed in this context", Codes.FORBIDDEN
             )
 
-        try:
-            await self.auth.check_from_context(room_version, event, context)
-        except AuthError as err:
-            logger.warning("Denying new event %r because %s", event, err)
-            raise err
+        if event.internal_metadata.is_out_of_band_membership():
+            # the only sort of out-of-band-membership events we expect to see here
+            # are invite rejections we have generated ourselves.
+            assert event.type == EventTypes.Member
+            assert event.content["membership"] == Membership.LEAVE
+        else:
+            try:
+                await self.auth.check_from_context(room_version, event, context)
+            except AuthError as err:
+                logger.warning("Denying new event %r because %s", event, err)
+                raise err
 
         # Ensure that we can round trip before trying to persist in db
         try: