Include event when resolving state for missing prevs

If we have a forward extremity for a room as `E`, and you receive `A`, `B`,
s.t. `A -> B -> E`, and `B` also points to an unknown event `X`, then we need
to do state res between `X` and `E`.

When that happens, we need to make sure we include `X` in the state that goes
into the state res alg.

Fixes #3934.

synapse/handlers/federation.py

@@ -323,8 +323,8 @@ class FederationHandler(BaseHandler):
                         affected=pdu.event_id,
                     )
 
-                # Calculate the state of the previous events, and
+                # Calculate the state after each of the previous events, and
-                # de-conflict them to find the current state.
+                # resolve them to find the correct state at the current event.
                 auth_chains = set()
                 event_map = {
                     event_id: pdu,
@@ -358,6 +358,20 @@ class FederationHandler(BaseHandler):
                                 )
                             )
 
+                            # we want the state *after* p; get_state_for_room returns the
+                            # state *before* p.
+                            remote_event = yield self.federation_client.get_pdu(
+                                [origin], p, outlier=True,
+                            )
+
+                            if remote_event is None:
+                                raise Exception(
+                                    "Unable to get missing prev_event %s" % (p, )
+                                )
+
+                            if remote_event.is_state():
+                                remote_state.append(remote_event)
+
                             # XXX hrm I'm not convinced that duplicate events will compare
                             # for equality, so I'm not sure this does what the author
                             # hoped.