Attempt to deflake Device list doesn't change if remote server is down

Hopefully closes #1136

tests/50federation/40devicelists.pl

@@ -564,9 +564,9 @@
          })
       );
 
-      # First we succesfully request the remote user's keys while the remote server is up.
+      # First we successfully request the remote user's keys while the remote server is up.
       # We do this once they share a room.
-      matrix_create_room(
+      matrix_create_room_synced(
          $local_user,
          preset => "public_chat",
       )->then( sub {
@@ -576,6 +576,14 @@
             room_id     => $room_id,
             user_id     => $outbound_client_user,
          )
+      # Wait for the user to see the join.
+      })->then( sub {
+         matrix_sync($local_user)
+      })->then( sub {
+         sync_until_user_in_device_list(
+            $local_user,
+            new_User(server_name => "", http => "", user_id => $outbound_client_user)
+         )
       })->then( sub {
          do_request_json_for( $local_user,
             method  => "POST",
@@ -589,8 +597,17 @@
       })->then( sub {
          ( $first_keys_query_body ) = @_;
          map {$_->cancel} @respond_with_keys;
+         log_if_fail "first_keys_query_body";
          log_if_fail (Dumper $first_keys_query_body);
-
+      })->then( sub {
+         # Need to ensure the local server has enough time to cache/store our response.
+         # This will definitely have happened when the user's sync result sees
+         # $outbound_client_user in the list of changes for a second time)
+         sync_until_user_in_device_list(
+            $local_user,
+            new_User(server_name => "", http => "", user_id => $outbound_client_user)
+         )
+      })->then( sub {
          # We take the remote server 'offline' and then make the same request
          # for the users keys. We expect no change in the keys. We respond with
          # 400 instead of 503 to avoid the server being marked as down.
@@ -629,6 +646,7 @@
             }
          };
 
+         log_if_fail "second_keys_query_body";
          log_if_fail (Dumper $second_keys_query_body);
          assert_deeply_eq( $second_keys_query_body->{ device_keys }, $first_keys_query_body->{ device_keys }, "Query matches while federation server is down." );
          Future->done(1)