Add a test case for the SendJoinParser (#11441)

This would have caught the bug #11438 introduced in #11217 and fixed in #11439.

tests/federation/transport/test_client.py

@@ -0,0 +1,50 @@
+import json
+
+from synapse.api.room_versions import RoomVersions
+from synapse.federation.transport.client import SendJoinParser
+
+from tests.unittest import TestCase
+
+
+class SendJoinParserTestCase(TestCase):
+    def test_two_writes(self) -> None:
+        """Test that the parser can sensibly deserialise an input given in two slices."""
+        parser = SendJoinParser(RoomVersions.V1, True)
+        parent_event = {
+            "content": {
+                "see_room_version_spec": "The event format changes depending on the room version."
+            },
+            "event_id": "$authparent",
+            "room_id": "!somewhere:example.org",
+            "type": "m.room.minimal_pdu",
+        }
+        state = {
+            "content": {
+                "see_room_version_spec": "The event format changes depending on the room version."
+            },
+            "event_id": "$DoNotThinkAboutTheEvent",
+            "room_id": "!somewhere:example.org",
+            "type": "m.room.minimal_pdu",
+        }
+        response = [
+            200,
+            {
+                "auth_chain": [parent_event],
+                "origin": "matrix.org",
+                "state": [state],
+            },
+        ]
+        serialised_response = json.dumps(response).encode()
+
+        # Send data to the parser
+        parser.write(serialised_response[:100])
+        parser.write(serialised_response[100:])
+
+        # Retrieve the parsed SendJoinResponse
+        parsed_response = parser.finish()
+
+        # Sanity check the parsing gave us sensible data.
+        self.assertEqual(len(parsed_response.auth_events), 1, parsed_response)
+        self.assertEqual(len(parsed_response.state), 1, parsed_response)
+        self.assertEqual(parsed_response.event_dict, {}, parsed_response)
+        self.assertIsNone(parsed_response.event, parsed_response)