diff --git a/tests/storage/test_events.py b/tests/storage/test_events.py
new file mode 100644
index 0000000000..71210ce606
--- /dev/null
+++ b/tests/storage/test_events.py
@@ -0,0 +1,334 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 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 synapse.api.constants import EventTypes, Membership
+from synapse.api.room_versions import RoomVersions
+from synapse.federation.federation_base import event_from_pdu_json
+from synapse.rest import admin
+from synapse.rest.client.v1 import login, room
+
+from tests.unittest import HomeserverTestCase
+
+
+class ExtremPruneTestCase(HomeserverTestCase):
+ servlets = [
+ admin.register_servlets,
+ room.register_servlets,
+ login.register_servlets,
+ ]
+
+ def prepare(self, reactor, clock, homeserver):
+ self.state = self.hs.get_state_handler()
+ self.persistence = self.hs.get_storage().persistence
+ self.store = self.hs.get_datastore()
+
+ self.register_user("user", "pass")
+ self.token = self.login("user", "pass")
+
+ self.room_id = self.helper.create_room_as(
+ "user", room_version=RoomVersions.V6.identifier, tok=self.token
+ )
+
+ body = self.helper.send(self.room_id, body="Test", tok=self.token)
+ local_message_event_id = body["event_id"]
+
+ # Fudge a remote event and persist it. This will be the extremity before
+ # the gap.
+ self.remote_event_1 = event_from_pdu_json(
+ {
+ "type": EventTypes.Message,
+ "state_key": "@user:other",
+ "content": {},
+ "room_id": self.room_id,
+ "sender": "@user:other",
+ "depth": 5,
+ "prev_events": [local_message_event_id],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ self.persist_event(self.remote_event_1)
+
+ # Check that the current extremities is the remote event.
+ self.assert_extremities([self.remote_event_1.event_id])
+
+ def persist_event(self, event, state=None):
+ """Persist the event, with optional state
+ """
+ context = self.get_success(
+ self.state.compute_event_context(event, old_state=state)
+ )
+ self.get_success(self.persistence.persist_event(event, context))
+
+ def assert_extremities(self, expected_extremities):
+ """Assert the current extremities for the room
+ """
+ extremities = self.get_success(
+ self.store.get_prev_events_for_room(self.room_id)
+ )
+ self.assertCountEqual(extremities, expected_extremities)
+
+ def test_prune_gap(self):
+ """Test that we drop extremities after a gap when we see an event from
+ the same domain.
+ """
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other",
+ "depth": 50,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([remote_event_2.event_id])
+
+ def test_do_not_prune_gap_if_state_different(self):
+ """Test that we don't prune extremities after a gap if the resolved
+ state is different.
+ """
+
+ # Fudge a second event which points to an event we don't have.
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Message,
+ "state_key": "@user:other",
+ "content": {},
+ "room_id": self.room_id,
+ "sender": "@user:other",
+ "depth": 10,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ # Now we persist it with state with a dropped history visibility
+ # setting. The state resolution across the old and new event will then
+ # include it, and so the resolved state won't match the new state.
+ state_before_gap = dict(
+ self.get_success(self.state.get_current_state(self.room_id))
+ )
+ state_before_gap.pop(("m.room.history_visibility", ""))
+
+ context = self.get_success(
+ self.state.compute_event_context(
+ remote_event_2, old_state=state_before_gap.values()
+ )
+ )
+
+ self.get_success(self.persistence.persist_event(remote_event_2, context))
+
+ # Check that we haven't dropped the old extremity.
+ self.assert_extremities([self.remote_event_1.event_id, remote_event_2.event_id])
+
+ def test_prune_gap_if_old(self):
+ """Test that we drop extremities after a gap when the previous extremity
+ is "old"
+ """
+
+ # Advance the clock for many days to make the old extremity "old". We
+ # also set the depth to "lots".
+ self.reactor.advance(7 * 24 * 60 * 60)
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other2",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other2",
+ "depth": 10000,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([remote_event_2.event_id])
+
+ def test_do_not_prune_gap_if_other_server(self):
+ """Test that we do not drop extremities after a gap when we see an event
+ from a different domain.
+ """
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other2",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other2",
+ "depth": 10,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([self.remote_event_1.event_id, remote_event_2.event_id])
+
+ def test_prune_gap_if_dummy_remote(self):
+ """Test that we drop extremities after a gap when the previous extremity
+ is a local dummy event and only points to remote events.
+ """
+
+ body = self.helper.send_event(
+ self.room_id, type=EventTypes.Dummy, content={}, tok=self.token
+ )
+ local_message_event_id = body["event_id"]
+ self.assert_extremities([local_message_event_id])
+
+ # Advance the clock for many days to make the old extremity "old". We
+ # also set the depth to "lots".
+ self.reactor.advance(7 * 24 * 60 * 60)
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other2",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other2",
+ "depth": 10000,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([remote_event_2.event_id])
+
+ def test_prune_gap_if_dummy_local(self):
+ """Test that we don't drop extremities after a gap when the previous
+ extremity is a local dummy event and points to local events.
+ """
+
+ body = self.helper.send(self.room_id, body="Test", tok=self.token)
+
+ body = self.helper.send_event(
+ self.room_id, type=EventTypes.Dummy, content={}, tok=self.token
+ )
+ local_message_event_id = body["event_id"]
+ self.assert_extremities([local_message_event_id])
+
+ # Advance the clock for many days to make the old extremity "old". We
+ # also set the depth to "lots".
+ self.reactor.advance(7 * 24 * 60 * 60)
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other2",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other2",
+ "depth": 10000,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([remote_event_2.event_id, local_message_event_id])
+
+ def test_do_not_prune_gap_if_not_dummy(self):
+ """Test that we do not drop extremities after a gap when the previous extremity
+ is not a dummy event.
+ """
+
+ body = self.helper.send(self.room_id, body="test", tok=self.token)
+ local_message_event_id = body["event_id"]
+ self.assert_extremities([local_message_event_id])
+
+ # Fudge a second event which points to an event we don't have. This is a
+ # state event so that the state changes (otherwise we won't prune the
+ # extremity as they'll have the same state group).
+ remote_event_2 = event_from_pdu_json(
+ {
+ "type": EventTypes.Member,
+ "state_key": "@user:other2",
+ "content": {"membership": Membership.JOIN},
+ "room_id": self.room_id,
+ "sender": "@user:other2",
+ "depth": 10000,
+ "prev_events": ["$some_unknown_message"],
+ "auth_events": [],
+ "origin_server_ts": self.clock.time_msec(),
+ },
+ RoomVersions.V6,
+ )
+
+ state_before_gap = self.get_success(self.state.get_current_state(self.room_id))
+
+ self.persist_event(remote_event_2, state=state_before_gap.values())
+
+ # Check the new extremity is just the new remote event.
+ self.assert_extremities([local_message_event_id, remote_event_2.event_id])
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