gh-121381 Remove subprocess._USE_VFORK escape hatch (#121383)

This flag was added as an escape hatch in gh-91401 and backported to
Python 3.10. The flag broke at some point between its addition and now.
As there is currently no publicly known environments that require this,
remove it rather than work on fixing it.

This leaves the flag in the subprocess module to not break code which
may have used / checked the flag itself.

discussion: https://discuss.python.org/t/subprocess-use-vfork-escape-hatch-broken-fix-or-remove/56915/2

Author: cmaloney

Doc/library/subprocess.rst

@@ -1561,41 +1561,26 @@ runtime):
       Module which provides function to parse and escape command lines.
 
 
-.. _disable_vfork:
 .. _disable_posix_spawn:
 
-Disabling use of ``vfork()`` or ``posix_spawn()``
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Disable use of ``posix_spawn()``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 On Linux, :mod:`subprocess` defaults to using the ``vfork()`` system call
 internally when it is safe to do so rather than ``fork()``. This greatly
 improves performance.
 
-If you ever encounter a presumed highly unusual situation where you need to
-prevent ``vfork()`` from being used by Python, you can set the
-:const:`subprocess._USE_VFORK` attribute to a false value.
-
-::
-
-   subprocess._USE_VFORK = False  # See CPython issue gh-NNNNNN.
-
-Setting this has no impact on use of ``posix_spawn()`` which could use
-``vfork()`` internally within its libc implementation.  There is a similar
-:const:`subprocess._USE_POSIX_SPAWN` attribute if you need to prevent use of
-that.
-
 ::
 
    subprocess._USE_POSIX_SPAWN = False  # See CPython issue gh-NNNNNN.
 
-It is safe to set these to false on any Python version. They will have no
-effect on older versions when unsupported. Do not assume the attributes are
-available to read. Despite their names, a true value does not indicate that the
+It is safe to set this to false on any Python version. It will have no
+effect on older or newer versions where unsupported. Do not assume the attribute
+is available to read. Despite the name, a true value does not indicate the
 corresponding function will be used, only that it may be.
 
 Please file issues any time you have to use these private knobs with a way to
 reproduce the issue you were seeing. Link to that issue from a comment in your
 code.
 
 .. versionadded:: 3.8 ``_USE_POSIX_SPAWN``
-.. versionadded:: 3.11 ``_USE_VFORK``

Lib/multiprocessing/util.py

@@ -445,8 +445,7 @@ def spawnv_passfds(path, args, passfds):
         return _posixsubprocess.fork_exec(
             args, [path], True, passfds, None, None,
             -1, -1, -1, -1, -1, -1, errpipe_read, errpipe_write,
-            False, False, -1, None, None, None, -1, None,
-            subprocess._USE_VFORK)
+            False, False, -1, None, None, None, -1, None)
     finally:
         os.close(errpipe_read)
         os.close(errpipe_write)

Lib/subprocess.py

@@ -749,7 +749,6 @@ def _use_posix_spawn():
 # These are primarily fail-safe knobs for negatives. A True value does not
 # guarantee the given libc/syscall API will be used.
 _USE_POSIX_SPAWN = _use_posix_spawn()
-_USE_VFORK = True
 _HAVE_POSIX_SPAWN_CLOSEFROM = hasattr(os, 'POSIX_SPAWN_CLOSEFROM')
 
 
@@ -1902,7 +1901,7 @@ def _execute_child(self, args, executable, preexec_fn, close_fds,
                             errpipe_read, errpipe_write,
                             restore_signals, start_new_session,
                             process_group, gid, gids, uid, umask,
-                            preexec_fn, _USE_VFORK)
+                            preexec_fn)
                     self._child_created = True
                 finally:
                     # be sure the FD is closed no matter what

Lib/test/test_capi/test_misc.py

@@ -120,15 +120,15 @@ def __len__(self):
                 return 1
         with self.assertRaisesRegex(TypeError, 'indexing'):
             _posixsubprocess.fork_exec(
-                          1,Z(),True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22,False)
+                          1,Z(),True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22)
         # Issue #15736: overflow in _PySequence_BytesToCharpArray()
         class Z(object):
             def __len__(self):
                 return sys.maxsize
             def __getitem__(self, i):
                 return b'x'
         self.assertRaises(MemoryError, _posixsubprocess.fork_exec,
-                          1,Z(),True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22,False)
+                          1,Z(),True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22)
 
     @unittest.skipUnless(_posixsubprocess, '_posixsubprocess required for this test.')
     def test_subprocess_fork_exec(self):
@@ -138,7 +138,7 @@ def __len__(self):
 
         # Issue #15738: crash in subprocess_fork_exec()
         self.assertRaises(TypeError, _posixsubprocess.fork_exec,
-                          Z(),[b'1'],True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22,False)
+                          Z(),[b'1'],True,(1, 2),5,6,7,8,9,10,11,12,13,14,True,True,17,False,19,20,21,22)
 
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")

Lib/test/test_subprocess.py

@@ -3278,7 +3278,7 @@ def __int__(self):
                         1, 2, 3, 4,
                         True, True, 0,
                         None, None, None, -1,
-                        None, True)
+                        None)
                 self.assertIn('fds_to_keep', str(c.exception))
         finally:
             if not gc_enabled:
@@ -3413,25 +3413,6 @@ def __del__(self):
         self.assertEqual(out.strip(), b"OK")
         self.assertIn(b"preexec_fn not supported at interpreter shutdown", err)
 
-    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
-                     "vfork() not enabled by configure.")
-    @mock.patch("subprocess._fork_exec")
-    @mock.patch("subprocess._USE_POSIX_SPAWN", new=False)
-    def test__use_vfork(self, mock_fork_exec):
-        self.assertTrue(subprocess._USE_VFORK)  # The default value regardless.
-        mock_fork_exec.side_effect = RuntimeError("just testing args")
-        with self.assertRaises(RuntimeError):
-            subprocess.run([sys.executable, "-c", "pass"])
-        mock_fork_exec.assert_called_once()
-        # NOTE: These assertions are *ugly* as they require the last arg
-        # to remain the have_vfork boolean. We really need to refactor away
-        # from the giant "wall of args" internal C extension API.
-        self.assertTrue(mock_fork_exec.call_args.args[-1])
-        with mock.patch.object(subprocess, '_USE_VFORK', False):
-            with self.assertRaises(RuntimeError):
-                subprocess.run([sys.executable, "-c", "pass"])
-            self.assertFalse(mock_fork_exec.call_args_list[-1].args[-1])
-
     @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
                      "vfork() not enabled by configure.")
     @unittest.skipIf(sys.platform != "linux", "Linux only, requires strace.")
@@ -3478,7 +3459,6 @@ def test_vfork_used_when_expected(self):
         # Test that each individual thing that would disable the use of vfork
         # actually disables it.
         for sub_name, preamble, sp_kwarg, expect_permission_error in (
-                ("!use_vfork", "subprocess._USE_VFORK = False", "", False),
                 ("preexec", "", "preexec_fn=lambda: None", False),
                 ("setgid", "", f"group={os.getgid()}", True),
                 ("setuid", "", f"user={os.getuid()}", True),

Misc/NEWS.d/next/Core and Builtins/2024-06-28-23-17-22.gh-issue-121381.i2xL7P.rst

@@ -0,0 +1,2 @@
+Remove ``subprocess._USE_VFORK`` escape hatch code and documentation.
+It was added just in case, and doesn't have any known cases that require it.

Modules/_posixsubprocess.c

@@ -977,7 +977,6 @@ _posixsubprocess.fork_exec as subprocess_fork_exec
     uid as uid_object: object
     child_umask: int
     preexec_fn: object
-    allow_vfork: bool
     /
 
 Spawn a fresh new child process.
@@ -1014,8 +1013,8 @@ subprocess_fork_exec_impl(PyObject *module, PyObject *process_args,
                           pid_t pgid_to_set, PyObject *gid_object,
                           PyObject *extra_groups_packed,
                           PyObject *uid_object, int child_umask,
-                          PyObject *preexec_fn, int allow_vfork)
-/*[clinic end generated code: output=7ee4f6ee5cf22b5b input=51757287ef266ffa]*/
+                          PyObject *preexec_fn)
+/*[clinic end generated code: output=288464dc56e373c7 input=f311c3bcb5dd55c8]*/
 {
     PyObject *converted_args = NULL, *fast_args = NULL;
     PyObject *preexec_fn_args_tuple = NULL;
@@ -1218,7 +1217,7 @@ subprocess_fork_exec_impl(PyObject *module, PyObject *process_args,
 #ifdef VFORK_USABLE
     /* Use vfork() only if it's safe. See the comment above child_exec(). */
     sigset_t old_sigs;
-    if (preexec_fn == Py_None && allow_vfork &&
+    if (preexec_fn == Py_None &&
         uid == (uid_t)-1 && gid == (gid_t)-1 && extra_group_size < 0) {
         /* Block all signals to ensure that no signal handlers are run in the
          * child process while it shares memory with us. Note that signals