https://github.com/python/cpython/commit/298d5440eb83f2dfd5651bac86d1592ec358d54c
commit: 298d5440eb83f2dfd5651bac86d1592ec358d54c
branch: main
author: Gregory P. Smith <[email protected]>
committer: gpshead <[email protected]>
date: 2026-01-12T15:47:21-08:00
summary:
gh-143706: Fix sys.argv not set during multiprocessing forkserver `__main__`
preload (#143717)
The forkserver was not passing sys.argv to its main() function, causing
sys.argv to be empty during `__main__` module import in child processes. This
fixes a non-obvious regression inadvertently introduced by the gh-126631 main
preloading fix.
files:
A Lib/test/mp_preload_sysargv.py
A Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst
M Lib/multiprocessing/forkserver.py
M Lib/test/_test_multiprocessing.py
diff --git a/Lib/multiprocessing/forkserver.py
b/Lib/multiprocessing/forkserver.py
index cc8947c5e04fb1..15c455a598dc27 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -152,6 +152,8 @@ def ensure_running(self):
main_kws['sys_path'] = data['sys_path']
if 'init_main_from_path' in data:
main_kws['main_path'] = data['init_main_from_path']
+ if 'sys_argv' in data:
+ main_kws['sys_argv'] = data['sys_argv']
with socket.socket(socket.AF_UNIX) as listener:
address = connection.arbitrary_address('AF_UNIX')
@@ -197,7 +199,7 @@ def ensure_running(self):
#
def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
- *, authkey_r=None):
+ *, sys_argv=None, authkey_r=None):
"""Run forkserver."""
if authkey_r is not None:
try:
@@ -209,6 +211,8 @@ def main(listener_fd, alive_r, preload, main_path=None,
sys_path=None,
authkey = b''
if preload:
+ if sys_argv is not None:
+ sys.argv[:] = sys_argv
if sys_path is not None:
sys.path[:] = sys_path
if '__main__' in preload and main_path is not None:
diff --git a/Lib/test/_test_multiprocessing.py
b/Lib/test/_test_multiprocessing.py
index 844539104e3a3e..cc07062eee6f98 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -7086,6 +7086,26 @@ def test_preload_main(self):
out = out.decode().split("\n")
self.assertEqual(out, ['__main__', '__mp_main__', 'f', 'f', ''])
+ def test_preload_main_sys_argv(self):
+ # gh-143706: Check that sys.argv is set before __main__ is pre-loaded
+ if multiprocessing.get_start_method() != "forkserver":
+ self.skipTest("forkserver specific test")
+
+ name = os.path.join(os.path.dirname(__file__), 'mp_preload_sysargv.py')
+ _, out, err = test.support.script_helper.assert_python_ok(
+ name, 'foo', 'bar')
+ self.assertEqual(err, b'')
+
+ out = out.decode().split("\n")
+ expected_argv = "['foo', 'bar']"
+ self.assertEqual(out, [
+ f"module:{expected_argv}",
+ f"fun:{expected_argv}",
+ f"module:{expected_argv}",
+ f"fun:{expected_argv}",
+ '',
+ ])
+
#
# Mixins
#
diff --git a/Lib/test/mp_preload_sysargv.py b/Lib/test/mp_preload_sysargv.py
new file mode 100644
index 00000000000000..5ad38cda889f55
--- /dev/null
+++ b/Lib/test/mp_preload_sysargv.py
@@ -0,0 +1,22 @@
+# gh-143706: Test that sys.argv is correctly set during main module import
+# when using forkserver with __main__ preloading.
+
+import multiprocessing
+import sys
+
+# This will be printed during module import - sys.argv should be correct here
+print(f"module:{sys.argv[1:]}")
+
+def fun():
+ # This will be printed when the function is called
+ print(f"fun:{sys.argv[1:]}")
+
+if __name__ == "__main__":
+ ctx = multiprocessing.get_context("forkserver")
+ ctx.set_forkserver_preload(['__main__'])
+
+ fun()
+
+ p = ctx.Process(target=fun)
+ p.start()
+ p.join()
diff --git
a/Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst
b/Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst
new file mode 100644
index 00000000000000..5bdefbb7913c14
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst
@@ -0,0 +1,5 @@
+Fix :mod:`multiprocessing` forkserver so that :data:`sys.argv` is correctly
+set before ``__main__`` is preloaded. Previously, :data:`sys.argv` was empty
+during main module import in forkserver child processes. This fixes a
+regression introduced in 3.13.8 and 3.14.1. Root caused by Aaron Wieczorek,
+test provided by Thomas Watson, thanks!
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