Set start method explicitly to "forkserver" so it works on MacOS (and if the

defaults change elsewhere in the future)

Author: duaneg

Lib/test/_test_multiprocessing.py

@@ -6815,7 +6815,7 @@ def test_std_streams_flushed_after_preload(self):
             f.write('''if 1:
                     import sys
                     print('stdout', file=sys.stdout)
-                    print('stderr', file=sys.stderr)''')
+                    print('stderr', file=sys.stderr)\n''')
 
         name = os.path.join(os.path.dirname(__file__), 'mp_preload_flush.py')
         env = {'PYTHONPATH': self._temp_dir}

Lib/test/mp_preload_flush.py

@@ -3,6 +3,7 @@
 
 if __name__ == '__main__':
     assert 'a' not in sys.modules
+    multiprocessing.set_start_method('forkserver')
     multiprocessing.set_forkserver_preload(['a'])
     for _ in range(2):
         p = multiprocessing.Process()