gh-82616: Add process_group support to subprocess.Popen (#23930)

One more thing that can help prevent people from using `preexec_fn`.

Also adds conditional skips to two tests exposing ASAN flakiness on the Ubuntu 20.04 Address Sanitizer Github CI system. When that build is run on more modern systems the "problem" does not show up. It seems ASAN implementation related.

Co-authored-by: Zackery Spytz <zspytz@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Doc/library/subprocess.rst

@@ -344,7 +344,8 @@ functions.
                  startupinfo=None, creationflags=0, restore_signals=True, \
                  start_new_session=False, pass_fds=(), *, group=None, \
                  extra_groups=None, user=None, umask=-1, \
-                 encoding=None, errors=None, text=None, pipesize=-1)
+                 encoding=None, errors=None, text=None, pipesize=-1, \
+                 process_group=None)
 
    Execute a child program in a new process.  On POSIX, the class uses
    :meth:`os.execvpe`-like behavior to execute the child program.  On Windows,
@@ -500,18 +501,16 @@ functions.
 
    .. warning::
 
-      The *preexec_fn* parameter is not safe to use in the presence of threads
+      The *preexec_fn* parameter is NOT SAFE to use in the presence of threads
       in your application.  The child process could deadlock before exec is
       called.
-      If you must use it, keep it trivial!  Minimize the number of libraries
-      you call into.
 
    .. note::
 
       If you need to modify the environment for the child use the *env*
       parameter rather than doing it in a *preexec_fn*.
-      The *start_new_session* parameter can take the place of a previously
+      The *start_new_session* and *process_group* parameters should take the place of
-      common use of *preexec_fn* to call os.setsid() in the child.
+      code using *preexec_fn* to call :func:`os.setsid` or :func:`os.setpgid` in the child.
 
    .. versionchanged:: 3.8
 
@@ -568,12 +567,20 @@ functions.
    .. versionchanged:: 3.2
       *restore_signals* was added.
 
-   If *start_new_session* is true the setsid() system call will be made in the
+   If *start_new_session* is true the ``setsid()`` system call will be made in the
-   child process prior to the execution of the subprocess.  (POSIX only)
+   child process prior to the execution of the subprocess.
 
+   .. availability:: POSIX
    .. versionchanged:: 3.2
       *start_new_session* was added.
 
+   If *process_group* is a non-negative integer, the ``setpgid(0, value)`` system call will
+   be made in the child process prior to the execution of the subprocess.
+
+   .. availability:: POSIX
+   .. versionchanged:: 3.11
+      *process_group* was added.
+
    If *group* is not ``None``, the setregid() system call will be made in the
    child process prior to the execution of the subprocess. If the provided
    value is a string, it will be looked up via :func:`grp.getgrnam()` and

Lib/multiprocessing/util.py

@@ -453,7 +453,7 @@ 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, None, None, None, -1, None,
+            False, False, -1, None, None, None, -1, None,
             subprocess._USE_VFORK)
     finally:
         os.close(errpipe_read)

Lib/subprocess.py

@@ -769,6 +769,8 @@ class Popen:
 
       start_new_session (POSIX only)
 
+      process_group (POSIX only)
+
       group (POSIX only)
 
       extra_groups (POSIX only)
@@ -794,7 +796,8 @@ class Popen:
                  startupinfo=None, creationflags=0,
                  restore_signals=True, start_new_session=False,
                  pass_fds=(), *, user=None, group=None, extra_groups=None,
-                 encoding=None, errors=None, text=None, umask=-1, pipesize=-1):
+                 encoding=None, errors=None, text=None, umask=-1, pipesize=-1,
+                 process_group=None):
         """Create new Popen instance."""
         _cleanup()
         # Held while anything is calling waitpid before returncode has been
@@ -900,6 +903,9 @@ class Popen:
             else:
                 line_buffering = False
 
+        if process_group is None:
+            process_group = -1  # The internal APIs are int-only
+
         gid = None
         if group is not None:
             if not hasattr(os, 'setregid'):
@@ -1003,7 +1009,7 @@ class Popen:
                                 errread, errwrite,
                                 restore_signals,
                                 gid, gids, uid, umask,
-                                start_new_session)
+                                start_new_session, process_group)
         except:
             # Cleanup if the child failed starting.
             for f in filter(None, (self.stdin, self.stdout, self.stderr)):
@@ -1387,7 +1393,7 @@ class Popen:
                            unused_restore_signals,
                            unused_gid, unused_gids, unused_uid,
                            unused_umask,
-                           unused_start_new_session):
+                           unused_start_new_session, unused_process_group):
             """Execute program (MS Windows version)"""
 
             assert not pass_fds, "pass_fds not supported on Windows."
@@ -1719,7 +1725,7 @@ class Popen:
                            errread, errwrite,
                            restore_signals,
                            gid, gids, uid, umask,
-                           start_new_session):
+                           start_new_session, process_group):
             """Execute program (POSIX version)"""
 
             if isinstance(args, (str, bytes)):
@@ -1755,6 +1761,7 @@ class Popen:
                     and (c2pwrite == -1 or c2pwrite > 2)
                     and (errwrite == -1 or errwrite > 2)
                     and not start_new_session
+                    and process_group == -1
                     and gid is None
                     and gids is None
                     and uid is None
@@ -1812,7 +1819,7 @@ class Popen:
                             errread, errwrite,
                             errpipe_read, errpipe_write,
                             restore_signals, start_new_session,
-                            gid, gids, uid, umask,
+                            process_group, gid, gids, uid, umask,
                             preexec_fn, _USE_VFORK)
                     self._child_created = True
                 finally:

Lib/test/test_asyncio/test_subprocess.py

@@ -15,6 +15,9 @@ from test.support import os_helper
 if sys.platform != 'win32':
     from asyncio import unix_events
 
+if support.check_sanitizer(address=True):
+    raise unittest.SkipTest("Exposes ASAN flakiness in GitHub CI")
+
 # Program blocking
 PROGRAM_BLOCKED = [sys.executable, '-c', 'import time; time.sleep(3600)']
 

Lib/test/test_capi.py

@@ -140,7 +140,7 @@ class CAPITest(unittest.TestCase):
             def __len__(self):
                 return 1
         self.assertRaises(TypeError, _posixsubprocess.fork_exec,
-                          1,Z(),3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22)
+                          1,Z(),3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)
         # Issue #15736: overflow in _PySequence_BytesToCharpArray()
         class Z(object):
             def __len__(self):
@@ -148,7 +148,7 @@ class CAPITest(unittest.TestCase):
             def __getitem__(self, i):
                 return b'x'
         self.assertRaises(MemoryError, _posixsubprocess.fork_exec,
-                          1,Z(),3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22)
+                          1,Z(),3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)
 
     @unittest.skipUnless(_posixsubprocess, '_posixsubprocess required for this test.')
     def test_subprocess_fork_exec(self):
@@ -158,7 +158,7 @@ class CAPITest(unittest.TestCase):
 
         # Issue #15738: crash in subprocess_fork_exec()
         self.assertRaises(TypeError, _posixsubprocess.fork_exec,
-                          Z(),[b'1'],3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22)
+                          Z(),[b'1'],3,(1, 2),5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)
 
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")

Lib/test/test_distutils.py

@@ -23,6 +23,8 @@ def load_tests(*_):
 def tearDownModule():
     support.reap_children()
 
+if support.check_sanitizer(address=True):
+    raise unittest.SkipTest("Exposes ASAN flakiness in GitHub CI")
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_subprocess.py

@@ -1905,14 +1905,32 @@ class POSIXProcessTestCase(BaseTestCase):
             output = subprocess.check_output(
                     [sys.executable, "-c", "import os; print(os.getsid(0))"],
                     start_new_session=True)
-        except OSError as e:
+        except PermissionError as e:
             if e.errno != errno.EPERM:
-                raise
+                raise  # EACCES?
         else:
             parent_sid = os.getsid(0)
             child_sid = int(output)
             self.assertNotEqual(parent_sid, child_sid)
 
+    @unittest.skipUnless(hasattr(os, 'setpgid') and hasattr(os, 'getpgid'),
+                         'no setpgid or getpgid on platform')
+    def test_process_group_0(self):
+        # For code coverage of calling setpgid().  We don't care if we get an
+        # EPERM error from it depending on the test execution environment, that
+        # still indicates that it was called.
+        try:
+            output = subprocess.check_output(
+                    [sys.executable, "-c", "import os; print(os.getpgid(0))"],
+                    process_group=0)
+        except PermissionError as e:
+            if e.errno != errno.EPERM:
+                raise  # EACCES?
+        else:
+            parent_pgid = os.getpgid(0)
+            child_pgid = int(output)
+            self.assertNotEqual(parent_pgid, child_pgid)
+
     @unittest.skipUnless(hasattr(os, 'setreuid'), 'no setreuid on platform')
     def test_user(self):
         # For code coverage of the user parameter.  We don't care if we get an
@@ -3134,7 +3152,7 @@ class POSIXProcessTestCase(BaseTestCase):
                         True, (), cwd, env_list,
                         -1, -1, -1, -1,
                         1, 2, 3, 4,
-                        True, True,
+                        True, True, 0,
                         False, [], 0, -1,
                         func, False)
                 # Attempt to prevent
@@ -3183,7 +3201,7 @@ class POSIXProcessTestCase(BaseTestCase):
                         True, fds_to_keep, None, [b"env"],
                         -1, -1, -1, -1,
                         1, 2, 3, 4,
-                        True, True,
+                        True, True, 0,
                         None, None, None, -1,
                         None, "no vfork")
                 self.assertIn('fds_to_keep', str(c.exception))

Misc/NEWS.d/next/Library/2020-12-24-19-11-53.bpo-38435.rEHTAR.rst

@@ -0,0 +1,2 @@
+Add a ``process_group`` parameter to :class:`subprocess.Popen` to help move
+more things off of the unsafe ``preexec_fn`` parameter.

Modules/_posixsubprocess.c

@@ -517,7 +517,7 @@ child_exec(char *const exec_array[],
            int errread, int errwrite,
            int errpipe_read, int errpipe_write,
            int close_fds, int restore_signals,
-           int call_setsid,
+           int call_setsid, pid_t pgid_to_set,
            int call_setgid, gid_t gid,
            int call_setgroups, size_t groups_size, const gid_t *groups,
            int call_setuid, uid_t uid, int child_umask,
@@ -611,6 +611,11 @@ child_exec(char *const exec_array[],
         POSIX_CALL(setsid());
 #endif
 
+#ifdef HAVE_SETPGID
+    if (pgid_to_set >= 0)
+        POSIX_CALL(setpgid(0, pgid_to_set));
+#endif
+
 #ifdef HAVE_SETGROUPS
     if (call_setgroups)
         POSIX_CALL(setgroups(groups_size, groups));
@@ -716,7 +721,7 @@ do_fork_exec(char *const exec_array[],
              int errread, int errwrite,
              int errpipe_read, int errpipe_write,
              int close_fds, int restore_signals,
-             int call_setsid,
+             int call_setsid, pid_t pgid_to_set,
              int call_setgid, gid_t gid,
              int call_setgroups, size_t groups_size, const gid_t *groups,
              int call_setuid, uid_t uid, int child_umask,
@@ -769,7 +774,7 @@ do_fork_exec(char *const exec_array[],
     child_exec(exec_array, argv, envp, cwd,
                p2cread, p2cwrite, c2pread, c2pwrite,
                errread, errwrite, errpipe_read, errpipe_write,
-               close_fds, restore_signals, call_setsid,
+               close_fds, restore_signals, call_setsid, pgid_to_set,
                call_setgid, gid, call_setgroups, groups_size, groups,
                call_setuid, uid, child_umask, child_sigmask,
                py_fds_to_keep, preexec_fn, preexec_fn_args_tuple);
@@ -791,6 +796,7 @@ subprocess_fork_exec(PyObject *module, PyObject *args)
     int p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite;
     int errpipe_read, errpipe_write, close_fds, restore_signals;
     int call_setsid;
+    pid_t pgid_to_set = -1;
     int call_setgid = 0, call_setgroups = 0, call_setuid = 0;
     uid_t uid;
     gid_t gid, *groups = NULL;
@@ -806,13 +812,13 @@ subprocess_fork_exec(PyObject *module, PyObject *args)
     int allow_vfork;
 
     if (!PyArg_ParseTuple(
-            args, "OOpO!OOiiiiiiiiiiOOOiOp:fork_exec",
+            args, "OOpO!OOiiiiiiiiii" _Py_PARSE_PID "OOOiOp:fork_exec",
             &process_args, &executable_list,
             &close_fds, &PyTuple_Type, &py_fds_to_keep,
             &cwd_obj, &env_list,
             &p2cread, &p2cwrite, &c2pread, &c2pwrite,
             &errread, &errwrite, &errpipe_read, &errpipe_write,
-            &restore_signals, &call_setsid,
+            &restore_signals, &call_setsid, &pgid_to_set,
             &gid_object, &groups_list, &uid_object, &child_umask,
             &preexec_fn, &allow_vfork))
         return NULL;
@@ -1016,7 +1022,7 @@ subprocess_fork_exec(PyObject *module, PyObject *args)
     pid = do_fork_exec(exec_array, argv, envp, cwd,
                        p2cread, p2cwrite, c2pread, c2pwrite,
                        errread, errwrite, errpipe_read, errpipe_write,
-                       close_fds, restore_signals, call_setsid,
+                       close_fds, restore_signals, call_setsid, pgid_to_set,
                        call_setgid, gid, call_setgroups, num_groups, groups,
                        call_setuid, uid, child_umask, old_sigmask,
                        py_fds_to_keep, preexec_fn, preexec_fn_args_tuple);
@@ -1081,7 +1087,7 @@ PyDoc_STRVAR(subprocess_fork_exec_doc,
 "fork_exec(args, executable_list, close_fds, pass_fds, cwd, env,\n\
           p2cread, p2cwrite, c2pread, c2pwrite,\n\
           errread, errwrite, errpipe_read, errpipe_write,\n\
-          restore_signals, call_setsid,\n\
+          restore_signals, call_setsid, pgid_to_set,\n\
           gid, groups_list, uid,\n\
           preexec_fn)\n\
 \n\