gh-144503: Pass sys.argv to forkserver as real argv elements (GH-148194)

Avoid embedding the parent's sys.argv into the forkserver -c command
string via repr().  When sys.argv is large (e.g. thousands of file
paths from a pre-commit hook), the resulting single argument could
exceed the OS per-argument length limit (MAX_ARG_STRLEN on Linux,
typically 128 KiB), causing posix_spawn to fail and the parent to
observe a BrokenPipeError.

Instead, append the argv entries as separate command-line arguments
after -c; the forkserver child reads them back as sys.argv[1:].  This
cannot exceed any limit the parent itself did not already satisfy.

Regression introduced by gh-143706 / 298d544.

Author: gpshead

Lib/multiprocessing/forkserver.py

@@ -162,18 +162,25 @@ def ensure_running(self):
                 self._forkserver_alive_fd = None
                 self._forkserver_pid = None
 
-            cmd = ('from multiprocessing.forkserver import main; ' +
-                   'main(%d, %d, %r, **%r)')
+            # gh-144503: sys_argv is passed as real argv elements after the
+            # ``-c cmd`` rather than repr'd into main_kws so that a large
+            # parent sys.argv cannot push the single ``-c`` command string
+            # over the OS per-argument length limit (MAX_ARG_STRLEN on Linux).
+            # The child sees them as sys.argv[1:].
+            cmd = ('import sys; '
+                   'from multiprocessing.forkserver import main; '
+                   'main(%d, %d, %r, sys_argv=sys.argv[1:], **%r)')
 
             main_kws = {}
+            sys_argv = None
             if self._preload_modules:
                 data = spawn.get_preparation_data('ignore')
                 if 'sys_path' in data:
                     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']
+                    sys_argv = data['sys_argv']
                 if self._preload_on_error != 'ignore':
                     main_kws['on_error'] = self._preload_on_error
 
@@ -197,6 +204,8 @@ def ensure_running(self):
                     exe = spawn.get_executable()
                     args = [exe] + util._args_from_interpreter_flags()
                     args += ['-c', cmd]
+                    if sys_argv is not None:
+                        args += sys_argv
                     pid = util.spawnv_passfds(exe, args, fds_to_pass)
                 except:
                     os.close(alive_w)

Lib/test/_test_multiprocessing.py

@@ -207,10 +207,38 @@ def spawn_check_wrapper(*args, **kwargs):
     return decorator
 
 
+def only_run_in_forkserver_testsuite(reason):
+    """Returns a decorator: raises SkipTest unless fork is supported
+    and the current start method is forkserver.
+
+    Combines @support.requires_fork() with the single-run semantics of
+    only_run_in_spawn_testsuite(), but uses the forkserver testsuite as
+    the single-run target.  Appropriate for tests that exercise
+    os.fork() directly (raw fork or mp.set_start_method("fork") in a
+    subprocess) and don't vary by start method, since forkserver is
+    only available on platforms that support fork.
+    """
+
+    def decorator(test_item):
+
+        @functools.wraps(test_item)
+        def forkserver_check_wrapper(*args, **kwargs):
+            if not support.has_fork_support:
+                raise unittest.SkipTest("requires working os.fork()")
+            if (start_method := multiprocessing.get_start_method()) != "forkserver":
+                raise unittest.SkipTest(
+                    f"{start_method=}, not 'forkserver'; {reason}")
+            return test_item(*args, **kwargs)
+
+        return forkserver_check_wrapper
+
+    return decorator
+
+
 class TestInternalDecorators(unittest.TestCase):
     """Logic within a test suite that could errantly skip tests? Test it!"""
 
-    @unittest.skipIf(sys.platform == "win32", "test requires that fork exists.")
+    @support.requires_fork()
     def test_only_run_in_spawn_testsuite(self):
         if multiprocessing.get_start_method() != "spawn":
             raise unittest.SkipTest("only run in test_multiprocessing_spawn.")
@@ -234,6 +262,30 @@ def return_four_if_spawn():
         finally:
             multiprocessing.set_start_method(orig_start_method, force=True)
 
+    @support.requires_fork()
+    def test_only_run_in_forkserver_testsuite(self):
+        if multiprocessing.get_start_method() != "forkserver":
+            raise unittest.SkipTest("only run in test_multiprocessing_forkserver.")
+
+        try:
+            @only_run_in_forkserver_testsuite("testing this decorator")
+            def return_four_if_forkserver():
+                return 4
+        except Exception as err:
+            self.fail(f"expected decorated `def` not to raise; caught {err}")
+
+        orig_start_method = multiprocessing.get_start_method(allow_none=True)
+        try:
+            multiprocessing.set_start_method("forkserver", force=True)
+            self.assertEqual(return_four_if_forkserver(), 4)
+            multiprocessing.set_start_method("spawn", force=True)
+            with self.assertRaises(unittest.SkipTest) as ctx:
+                return_four_if_forkserver()
+            self.assertIn("testing this decorator", str(ctx.exception))
+            self.assertIn("start_method=", str(ctx.exception))
+        finally:
+            multiprocessing.set_start_method(orig_start_method, force=True)
+
 
 #
 # Creates a wrapper for a function which records the time it takes to finish
@@ -7133,6 +7185,23 @@ def test_preload_main_sys_argv(self):
             '',
         ])
 
+    @only_run_in_forkserver_testsuite("forkserver specific test.")
+    def test_preload_main_large_sys_argv(self):
+        # gh-144503: a very large parent sys.argv must not prevent the
+        # forkserver from starting (it previously overflowed the OS
+        # per-argument length limit when repr'd into the -c command string).
+        name = os.path.join(os.path.dirname(__file__),
+                            'mp_preload_large_sysargv.py')
+        _, out, err = test.support.script_helper.assert_python_ok(name)
+        self.assertEqual(err, b'')
+
+        out = out.decode().split("\n")
+        self.assertEqual(out, [
+            'preload:5002:sentinel',
+            'worker:5002:sentinel',
+            '',
+        ])
+
 #
 # Mixins
 #

Lib/test/mp_preload_large_sysargv.py

@@ -0,0 +1,30 @@
+# gh-144503: Test that the forkserver can start when the parent process has
+# a very large sys.argv.  Prior to the fix, sys.argv was repr'd into the
+# forkserver ``-c`` command string which could exceed the OS limit on the
+# length of a single argv element (MAX_ARG_STRLEN on Linux, ~128 KiB),
+# causing posix_spawn to fail and the parent to see a BrokenPipeError.
+
+import multiprocessing
+import sys
+
+EXPECTED_LEN = 5002  # argv[0] + 5000 padding entries + sentinel
+
+
+def fun():
+    print(f"worker:{len(sys.argv)}:{sys.argv[-1]}")
+
+
+if __name__ == "__main__":
+    # Inflate sys.argv well past 128 KiB before the forkserver is started.
+    sys.argv[1:] = ["x" * 50] * 5000 + ["sentinel"]
+    assert len(sys.argv) == EXPECTED_LEN
+
+    ctx = multiprocessing.get_context("forkserver")
+    p = ctx.Process(target=fun)
+    p.start()
+    p.join()
+    sys.exit(p.exitcode)
+else:
+    # This branch runs when the forkserver preloads this module as
+    # __mp_main__; confirm the large argv was propagated intact.
+    print(f"preload:{len(sys.argv)}:{sys.argv[-1]}")

Misc/NEWS.d/next/Library/2026-04-07-01-04-00.gh-issue-144503.argvfs.rst

@@ -0,0 +1,6 @@
+Fix a regression introduced in 3.14.3 and 3.13.12 where the
+:mod:`multiprocessing` ``forkserver`` start method would fail with
+:exc:`BrokenPipeError` when the parent process had a very large
+:data:`sys.argv`.  The argv is now passed to the forkserver as separate
+command-line arguments rather than being embedded in the ``-c`` command
+string, avoiding the operating system's per-argument length limit.