2 files changed, 77 insertions, 0 deletions
diff --git a/changelog.d/8258.misc b/changelog.d/8258.misc
new file mode 100644
index 0000000000..3c27803be4
--- /dev/null
+++ b/changelog.d/8258.misc
@@ -0,0 +1 @@
+Track the `stream_ordering` of the last successfully-sent event to every destination, so we can use this information to 'catch up' a remote server after an outage.
diff --git a/tests/federation/test_federation_catch_up.py b/tests/federation/test_federation_catch_up.py
index 73c51c9d6c..6cdcc378f0 100644
--- a/tests/federation/test_federation_catch_up.py
+++ b/tests/federation/test_federation_catch_up.py
@@ -28,6 +28,24 @@ class FederationCatchUpTestCases(FederatingHomeserverTestCase):
             return_value=make_awaitable(["test", "host2"])
         )
 
+        # whenever send_transaction is called, record the pdu data
+        self.pdus = []
+        self.failed_pdus = []
+        self.is_online = True
+        self.hs.get_federation_transport_client().send_transaction.side_effect = (
+            self.record_transaction
+        )
+
+    async def record_transaction(self, txn, json_cb):
+        if self.is_online:
+            data = json_cb()
+            self.pdus.extend(data["pdus"])
+            return {}
+        else:
+            data = json_cb()
+            self.failed_pdus.extend(data["pdus"])
+            raise IOError("Failed to connect because this is a test!")
+
     def get_destination_room(self, room: str, destination: str = "host2") -> dict:
         """
         Gets the destination_rooms entry for a (destination, room_id) pair.
@@ -80,3 +98,61 @@ class FederationCatchUpTestCases(FederatingHomeserverTestCase):
         self.assertEqual(row_1["event_id"], event_id_1)
         self.assertEqual(row_2["event_id"], event_id_2)
         self.assertEqual(row_1["stream_ordering"], row_2["stream_ordering"] - 1)
+
+    @override_config({"send_federation": True})
+    def test_catch_up_last_successful_stream_ordering_tracking(self):
+        """
+        Tests that we populate the `destination_rooms` table as needed.
+        """
+        self.register_user("u1", "you the one")
+        u1_token = self.login("u1", "you the one")
+        room = self.helper.create_room_as("u1", tok=u1_token)
+
+        # take the remote offline
+        self.is_online = False
+
+        self.get_success(
+            event_injection.inject_member_event(self.hs, room, "@user:host2", "join")
+        )
+
+        self.helper.send(room, "wombats!", tok=u1_token)
+        self.pump()
+
+        lsso_1 = self.get_success(
+            self.hs.get_datastore().get_destination_last_successful_stream_ordering(
+                "host2"
+            )
+        )
+
+        self.assertIsNone(
+            lsso_1,
+            "There should be no last successful stream ordering for an always-offline destination",
+        )
+
+        # bring the remote online
+        self.is_online = True
+
+        event_id_2 = self.helper.send(room, "rabbits!", tok=u1_token)["event_id"]
+
+        lsso_2 = self.get_success(
+            self.hs.get_datastore().get_destination_last_successful_stream_ordering(
+                "host2"
+            )
+        )
+        row_2 = self.get_destination_room(room)
+
+        self.assertEqual(
+            self.pdus[0]["content"]["body"],
+            "rabbits!",
+            "Test fault: didn't receive the right PDU",
+        )
+        self.assertEqual(
+            row_2["event_id"],
+            event_id_2,
+            "Test fault: destination_rooms not updated correctly",
+        )
+        self.assertEqual(
+            lsso_2,
+            row_2["stream_ordering"],
+            "Send succeeded but not marked as last_successful_stream_ordering",
+        )