#12098: Make multiprocessing's child processes inherit sys.flags on Windows

Initial patch by Sergey Mezentsev.

Lib/multiprocessing/forking.py

@@ -324,7 +324,8 @@ else:
             return [sys.executable, '--multiprocessing-fork']
         else:
             prog = 'from multiprocessing.forking import main; main()'
-            return [_python_exe, '-c', prog, '--multiprocessing-fork']
+            opts = util._args_from_interpreter_flags()
+            return [_python_exe] + opts + ['-c', prog, '--multiprocessing-fork']
 
 
     def main():

Lib/multiprocessing/util.py

@@ -7,6 +7,7 @@
 # Licensed to PSF under a Contributor Agreement.
 #
 
+import sys
 import functools
 import itertools
 import weakref
@@ -295,3 +296,33 @@ class ForkAwareLocal(threading.local):
         register_after_fork(self, lambda obj : obj.__dict__.clear())
     def __reduce__(self):
         return type(self), ()
+
+#
+# Get options for python to produce the same sys.flags
+#
+
+def _args_from_interpreter_flags():
+    """Return a list of command-line arguments reproducing the current
+    settings in sys.flags and sys.warnoptions."""
+    flag_opt_map = {
+        'debug': 'd',
+        # 'inspect': 'i',
+        # 'interactive': 'i',
+        'optimize': 'O',
+        'dont_write_bytecode': 'B',
+        'no_user_site': 's',
+        'no_site': 'S',
+        'ignore_environment': 'E',
+        'verbose': 'v',
+        'bytes_warning': 'b',
+        'quiet': 'q',
+        'hash_randomization': 'R',
+    }
+    args = []
+    for flag, opt in flag_opt_map.items():
+        v = getattr(sys.flags, flag)
+        if v > 0:
+            args.append('-' + opt * v)
+    for opt in sys.warnoptions:
+        args.append('-W' + opt)
+    return args

Lib/test/support.py

@@ -1596,24 +1596,8 @@ def strip_python_stderr(stderr):
 def args_from_interpreter_flags():
     """Return a list of command-line arguments reproducing the current
     settings in sys.flags and sys.warnoptions."""
-    flag_opt_map = {
-        'bytes_warning': 'b',
-        'dont_write_bytecode': 'B',
-        'hash_randomization': 'R',
-        'ignore_environment': 'E',
-        'no_user_site': 's',
-        'no_site': 'S',
-        'optimize': 'O',
-        'verbose': 'v',
-    }
-    args = []
-    for flag, opt in flag_opt_map.items():
-        v = getattr(sys.flags, flag)
-        if v > 0:
-            args.append('-' + opt * v)
-    for opt in sys.warnoptions:
-        args.append('-W' + opt)
-    return args
+    from multiprocessing.util import _args_from_interpreter_flags
+    return _args_from_interpreter_flags()
 
 #============================================================
 # Support for assertions about logging.

Lib/test/test_multiprocessing.py

@@ -2814,8 +2814,41 @@ class TestInvalidFamily(unittest.TestCase):
         with self.assertRaises(ValueError):
             multiprocessing.connection.Listener('/var/test.pipe')
 
+#
+# Issue 12098: check sys.flags of child matches that for parent
+#
+
+class TestFlags(unittest.TestCase):
+    @classmethod
+    def run_in_grandchild(cls, conn):
+        conn.send(tuple(sys.flags))
+
+    @classmethod
+    def run_in_child(cls):
+        import json
+        r, w = multiprocessing.Pipe(duplex=False)
+        p = multiprocessing.Process(target=cls.run_in_grandchild, args=(w,))
+        p.start()
+        grandchild_flags = r.recv()
+        p.join()
+        r.close()
+        w.close()
+        flags = (tuple(sys.flags), grandchild_flags)
+        print(json.dumps(flags))
+
+    def test_flags(self):
+        import json, subprocess
+        # start child process using unusual flags
+        prog = ('from test.test_multiprocessing import TestFlags; ' +
+                'TestFlags.run_in_child()')
+        data = subprocess.check_output(
+            [sys.executable, '-E', '-S', '-O', '-c', prog])
+        child_flags, grandchild_flags = json.loads(data.decode('ascii'))
+        self.assertEqual(child_flags, grandchild_flags)
+
 testcases_other = [OtherTest, TestInvalidHandle, TestInitializers,
-                   TestStdinBadfiledescriptor, TestWait, TestInvalidFamily]
+                   TestStdinBadfiledescriptor, TestWait, TestInvalidFamily,
+                   TestFlags]
 
 #
 #

Misc/NEWS

@@ -38,6 +38,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #12098: multiprocessing on Windows now starts child processes
+  using the same sys.flags as the current process.  Initial patch by
+  Sergey Mezentsev.
+
 - Issue #13031: Small speed-up for tarfile when unzipping tarfiles.
   Patch by Justin Peel.