Sort child events according to MSC1772 for the spaces summary API. (#9954)

This should help ensure that equivalent results are achieved between
homeservers querying for the summary of a space.

This implements modified MSC1772 rules, according to MSC2946.
The different is that the origin_server_ts of the m.room.create event
is not used as a tie-breaker since this might not be known if the
homeserver is not part of the room.

tests/handlers/test_space_summary.py

@@ -0,0 +1,81 @@
+#  Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+from typing import Any, Optional
+from unittest import mock
+
+from synapse.handlers.space_summary import _child_events_comparison_key
+
+from tests import unittest
+
+
+def _create_event(room_id: str, order: Optional[Any] = None):
+    result = mock.Mock()
+    result.room_id = room_id
+    result.content = {}
+    if order is not None:
+        result.content["order"] = order
+    return result
+
+
+def _order(*events):
+    return sorted(events, key=_child_events_comparison_key)
+
+
+class TestSpaceSummarySort(unittest.TestCase):
+    def test_no_order_last(self):
+        """An event with no ordering is placed behind those with an ordering."""
+        ev1 = _create_event("!abc:test")
+        ev2 = _create_event("!xyz:test", "xyz")
+
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+    def test_order(self):
+        """The ordering should be used."""
+        ev1 = _create_event("!abc:test", "xyz")
+        ev2 = _create_event("!xyz:test", "abc")
+
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+    def test_order_room_id(self):
+        """Room ID is a tie-breaker for ordering."""
+        ev1 = _create_event("!abc:test", "abc")
+        ev2 = _create_event("!xyz:test", "abc")
+
+        self.assertEqual([ev1, ev2], _order(ev1, ev2))
+
+    def test_invalid_ordering_type(self):
+        """Invalid orderings are considered the same as missing."""
+        ev1 = _create_event("!abc:test", 1)
+        ev2 = _create_event("!xyz:test", "xyz")
+
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+        ev1 = _create_event("!abc:test", {})
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+        ev1 = _create_event("!abc:test", [])
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+        ev1 = _create_event("!abc:test", True)
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+    def test_invalid_ordering_value(self):
+        """Invalid orderings are considered the same as missing."""
+        ev1 = _create_event("!abc:test", "foo\n")
+        ev2 = _create_event("!xyz:test", "xyz")
+
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))
+
+        ev1 = _create_event("!abc:test", "a" * 51)
+        self.assertEqual([ev2, ev1], _order(ev1, ev2))