Update KeyUploadServlet to handle case where client sends device_keys: null (#19023)

Author: anoadragon453

changelog.d/19023.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in 1.139.1 where a client could receive an Internal Server Error if they set `device_keys: null` in the request to [`POST /_matrix/client/v3/keys/upload`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3keysupload).

synapse/rest/client/keys.py

@@ -270,21 +270,21 @@ async def on_POST(
                 400, "To upload keys, you must pass device_id when authenticating"
             )
 
-        if "device_keys" in body:
+        if "device_keys" in body and isinstance(body["device_keys"], dict):
             # Validate the provided `user_id` and `device_id` fields in
             # `device_keys` match that of the requesting user. We can't do
             # this directly in the pydantic model as we don't have access
             # to the requester yet.
             #
             # TODO: We could use ValidationInfo when we switch to Pydantic v2.
             # https://docs.pydantic.dev/latest/concepts/validators/#validation-info
-            if body["device_keys"]["user_id"] != user_id:
+            if body["device_keys"].get("user_id") != user_id:
                 raise SynapseError(
                     code=HTTPStatus.BAD_REQUEST,
                     errcode=Codes.BAD_JSON,
                     msg="Provided `user_id` in `device_keys` does not match that of the authenticated user",
                 )
-            if body["device_keys"]["device_id"] != device_id:
+            if body["device_keys"].get("device_id") != device_id:
                 raise SynapseError(
                     code=HTTPStatus.BAD_REQUEST,
                     errcode=Codes.BAD_JSON,

tests/rest/client/test_keys.py

@@ -160,6 +160,26 @@ def test_upload_keys_fails_on_invalid_user_id_or_device_id(self) -> None:
             channel.result,
         )
 
+    def test_upload_keys_succeeds_when_fields_are_explicitly_set_to_null(self) -> None:
+        """
+        This is a regression test for https://github.com/element-hq/synapse/pull/19023.
+        """
+        device_id = "DEVICE_ID"
+        self.register_user("alice", "wonderland")
+        alice_token = self.login("alice", "wonderland", device_id=device_id)
+
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/upload",
+            {
+                "device_keys": None,
+                "one_time_keys": None,
+                "fallback_keys": None,
+            },
+            alice_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
 
 class KeyQueryTestCase(unittest.HomeserverTestCase):
     servlets = [