test_runner: Monkey-patch to allow using multiprocessing in tests.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/test_runner.py

@@ -1,9 +1,10 @@
+import multiprocessing
 import os
 import random
 import shutil
 from functools import partial
 from multiprocessing.sharedctypes import Synchronized
-from typing import Any, Callable, Dict, List, Optional, Set, Tuple, Type, Union
+from typing import Any, Callable, Dict, List, Optional, Set, Tuple, Type, Union, cast
 from unittest import TestLoader, TestSuite, runner
 from unittest.result import TestResult
 
@@ -122,6 +123,15 @@ def run_subsuite(args: SubsuiteArgs) -> Tuple[int, Any]:
     process_instrumented_calls(partial(result.addInstrumentation, None))
     return subsuite_index, result.events
 
+# Monkey-patch django.test.runner to allow using multiprocessing
+# inside tests without a “daemonic processes are not allowed to have
+# children” error.
+class NoDaemonContext(multiprocessing.context.ForkContext):
+    class Process(multiprocessing.context.ForkProcess):
+        daemon = cast(bool, property(lambda self: False, lambda self, value: None))
+
+django_runner.multiprocessing = NoDaemonContext()
+
 def destroy_test_databases(worker_id: Optional[int]=None) -> None:
     for alias in connections:
         connection = connections[alias]