fix: Call json_safe on globals in codejail remote_exec (#36542)

We need to make globals JSON-friendly before sending them across the
network.

Addresses edx/edx-arch-experiments#1016

Author: timmc-edx

xmodule/capa/safe_exec/remote_exec.py

@@ -7,7 +7,7 @@
 from importlib import import_module
 import requests
 
-from codejail.safe_exec import SafeExecException
+from codejail.safe_exec import SafeExecException, json_safe
 from django.conf import settings
 from edx_toggles.toggles import SettingToggle
 from requests.exceptions import RequestException, HTTPError
@@ -90,7 +90,21 @@ def send_safe_exec_request_v0(data):
     extra_files = data.pop("extra_files")
 
     codejail_service_endpoint = get_codejail_rest_service_endpoint()
-    payload = json.dumps(data)
+
+    # In rare cases an XBlock might introduce `bytes` objects (or other
+    # non-JSON-serializable objects) into the globals dict. The codejail service
+    # (via the codejail library) will call `json_safe` on the globals before
+    # JSON-encoding for the sandbox input, but here we need to call it earlier
+    # in the process so we can even transport the globals *to* the codejail
+    # service. Otherwise, we may get a TypeError when constructing the payload.
+    #
+    # This is a lossy operation (non-serializable objects will be dropped, and
+    # bytes converted to strings) but it is the same lossy operation that
+    # codejail will perform anyhow -- and it should be idempotent.
+    data_send = {**data}
+    data_send['globals_dict'] = json_safe(data_send['globals_dict'])
+
+    payload = json.dumps(data_send)
 
     try:
         response = requests.post(

xmodule/capa/safe_exec/tests/test_remote_exec.py

@@ -0,0 +1,32 @@
+"""
+Tests for remote codejail execution.
+"""
+
+import json
+from unittest import TestCase
+from unittest.mock import patch
+
+from django.test import override_settings
+
+from xmodule.capa.safe_exec.remote_exec import get_remote_exec
+
+
+class TestRemoteExec(TestCase):
+    """Tests for remote_exec."""
+
+    @override_settings(
+        ENABLE_CODEJAIL_REST_SERVICE=True,
+        CODE_JAIL_REST_SERVICE_HOST='http://localhost',
+    )
+    @patch('requests.post')
+    def test_json_encode(self, mock_post):
+        get_remote_exec({
+            'code': "out = 1 + 1",
+            'globals_dict': {'some_data': b'bytes', 'unusable': object()},
+            'extra_files': None,
+        })
+
+        mock_post.assert_called_once()
+        data_arg = mock_post.call_args_list[0][1]['data']
+        payload = json.loads(data_arg['payload'])
+        assert payload['globals_dict'] == {'some_data': 'bytes'}