Accept join events from all servers

Make sure that we accept join events from any server, rather than just the
origin server, to make the federation join dance work correctly.

(Fixes #1893).

1 files changed, 13 insertions, 3 deletions

diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 888dd01240..2ecc0087b8 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1004,9 +1004,19 @@ class FederationHandler(BaseHandler):
         )
 
         event.internal_metadata.outlier = False
-        # Send this event on behalf of the origin server since they may not
-        # have an up to data view of the state of the room at this event so
-        # will not know which servers to send the event to.
+        # Send this event on behalf of the origin server.
+        #
+        # The reasons we have the destination server rather than the origin
+        # server send it are slightly mysterious: the origin server should have
+        # all the neccessary state once it gets the response to the send_join,
+        # so it could send the event itself if it wanted to. It may be that
+        # doing it this way reduces failure modes, or avoids certain attacks
+        # where a new server selectively tells a subset of the federation that
+        # it has joined.
+        #
+        # The fact is that, as of the current writing, Synapse doesn't send out
+        # the join event over federation after joining, and changing it now
+        # would introduce the danger of backwards-compatibility problems.
         event.internal_metadata.send_on_behalf_of = origin
 
         context, event_stream_id, max_stream_id = yield self._handle_new_event(