bpo-40094: Add os.waitstatus_to_exitcode() (GH-19201)

Add os.waitstatus_to_exitcode() function to convert a wait status to an
exitcode.

Suggest waitstatus_to_exitcode() usage in the documentation when
appropriate.

Use waitstatus_to_exitcode() in:

* multiprocessing, os, subprocess and _bootsubprocess modules;
* test.support.wait_process();
* setup.py: run_command();
* and many tests.

Doc/library/os.rst

@@ -3665,6 +3665,11 @@ written in Python, such as a mail server's external command delivery program.
    subprocess was killed.)  On Windows systems, the return value
    contains the signed integer return code from the child process.
 
+   On Unix, :func:`waitstatus_to_exitcode` can be used to convert the ``close``
+   method result (exit status) into an exit code if it is not ``None``. On
+   Windows, the ``close`` method result is directly the exit code
+   (or ``None``).
+
    This is implemented using :class:`subprocess.Popen`; see that class's
    documentation for more powerful ways to manage and communicate with
    subprocesses.
@@ -3968,6 +3973,10 @@ written in Python, such as a mail server's external command delivery program.
    to using this function.  See the :ref:`subprocess-replacements` section in
    the :mod:`subprocess` documentation for some helpful recipes.
 
+   On Unix, :func:`waitstatus_to_exitcode` can be used to convert the result
+   (exit status) into an exit code. On Windows, the result is directly the exit
+   code.
+
    .. audit-event:: os.system command os.system
 
    .. availability:: Unix, Windows.
@@ -4008,8 +4017,16 @@ written in Python, such as a mail server's external command delivery program.
    number is zero); the high bit of the low byte is set if a core file was
    produced.
 
+   :func:`waitstatus_to_exitcode` can be used to convert the exit status into an
+   exit code.
+
    .. availability:: Unix.
 
+   .. seealso::
+
+      :func:`waitpid` can be used to wait for the completion of a specific
+      child process and has more options.
+
 .. function:: waitid(idtype, id, options)
 
    Wait for the completion of one or more child processes.
@@ -4105,6 +4122,9 @@ written in Python, such as a mail server's external command delivery program.
    id is known, not necessarily a child process. The :func:`spawn\* <spawnl>`
    functions called with :const:`P_NOWAIT` return suitable process handles.
 
+   :func:`waitstatus_to_exitcode` can be used to convert the exit status into an
+   exit code.
+
    .. versionchanged:: 3.5
       If the system call is interrupted and the signal handler does not raise an
       exception, the function now retries the system call instead of raising an
@@ -4120,6 +4140,9 @@ written in Python, such as a mail server's external command delivery program.
    information.  The option argument is the same as that provided to
    :func:`waitpid` and :func:`wait4`.
 
+   :func:`waitstatus_to_exitcode` can be used to convert the exit status into an
+   exitcode.
+
    .. availability:: Unix.
 
 
@@ -4131,9 +4154,42 @@ written in Python, such as a mail server's external command delivery program.
    resource usage information.  The arguments to :func:`wait4` are the same
    as those provided to :func:`waitpid`.
 
+   :func:`waitstatus_to_exitcode` can be used to convert the exit status into an
+   exitcode.
+
    .. availability:: Unix.
 
 
+.. function:: waitstatus_to_exitcode(status)
+
+   Convert a wait status to an exit code.
+
+   On Unix:
+
+   * If the process exited normally (if ``WIFEXITED(status)`` is true),
+     return the process exit status (return ``WEXITSTATUS(status)``):
+     result greater than or equal to 0.
+   * If the process was terminated by a signal (if ``WIFSIGNALED(status)`` is
+     true), return ``-signum`` where *signum* is the number of the signal that
+     caused the process to terminate (return ``-WTERMSIG(status)``):
+     result less than 0.
+   * Otherwise, raise a :exc:`ValueError`.
+
+   On Windows, return *status* shifted right by 8 bits.
+
+   On Unix, if the process is being traced or if :func:`waitpid` was called
+   with :data:`WUNTRACED` option, the caller must first check if
+   ``WIFSTOPPED(status)`` is true. This function must not be called if
+   ``WIFSTOPPED(status)`` is true.
+
+   .. seealso::
+
+      :func:`WIFEXITED`, :func:`WEXITSTATUS`, :func:`WIFSIGNALED`,
+      :func:`WTERMSIG`, :func:`WIFSTOPPED`, :func:`WSTOPSIG` functions.
+
+   .. versionadded:: 3.9
+
+
 .. data:: WNOHANG
 
    The option for :func:`waitpid` to return immediately if no child process status

Doc/library/pty.rst

@@ -69,6 +69,11 @@ The :mod:`pty` module defines the following functions:
    *select* throws an error on your platform when passed three empty lists. This
    is a bug, documented in `issue 26228 <https://bugs.python.org/issue26228>`_.
 
+   Return the exit status value from :func:`os.waitpid` on the child process.
+
+   :func:`waitstatus_to_exitcode` can be used to convert the exit status into
+   an exit code.
+
    .. audit-event:: pty.spawn argv pty.spawn
 
    .. versionchanged:: 3.4

Doc/whatsnew/3.9.rst

@@ -322,6 +322,10 @@ The :func:`os.putenv` and :func:`os.unsetenv` functions are now always
 available.
 (Contributed by Victor Stinner in :issue:`39395`.)
 
+Add :func:`os.waitstatus_to_exitcode` function:
+convert a wait status to an exit code.
+(Contributed by Victor Stinner in :issue:`40094`.)
+
 pathlib
 -------
 

Lib/_bootsubprocess.py

@@ -6,15 +6,6 @@
 import os
 
 
-def _waitstatus_to_exitcode(status):
-    if os.WIFEXITED(status):
-        return os.WEXITSTATUS(status)
-    elif os.WIFSIGNALED(status):
-        return -os.WTERMSIG(status)
-    else:
-        raise ValueError(f"invalid wait status: {status!r}")
-
-
 # distutils.spawn used by distutils.command.build_ext
 # calls subprocess.Popen().wait()
 class Popen:
@@ -37,7 +28,7 @@ def wait(self):
         else:
             # Parent process
             _, status = os.waitpid(pid, 0)
-            self.returncode = _waitstatus_to_exitcode(status)
+            self.returncode = os.waitstatus_to_exitcode(status)
 
         return self.returncode
 
@@ -87,7 +78,7 @@ def check_output(cmd, **kwargs):
     try:
         # system() spawns a shell
         status = os.system(cmd)
-        exitcode = _waitstatus_to_exitcode(status)
+        exitcode = os.waitstatus_to_exitcode(status)
         if exitcode:
             raise ValueError(f"Command {cmd!r} returned non-zero "
                              f"exit status {exitcode!r}")

Lib/multiprocessing/forkserver.py

@@ -237,14 +237,8 @@ def sigchld_handler(*_unused):
                             break
                         child_w = pid_to_fd.pop(pid, None)
                         if child_w is not None:
-                            if os.WIFSIGNALED(sts):
-                                returncode = -os.WTERMSIG(sts)
-                            else:
-                                if not os.WIFEXITED(sts):
-                                    raise AssertionError(
-                                        "Child {0:n} status is {1:n}".format(
-                                            pid,sts))
-                                returncode = os.WEXITSTATUS(sts)
+                            returncode = os.waitstatus_to_exitcode(sts)
+
                             # Send exit code to client process
                             try:
                                 write_signed(child_w, returncode)

Lib/multiprocessing/popen_fork.py

@@ -30,11 +30,7 @@ def poll(self, flag=os.WNOHANG):
                 # e.errno == errno.ECHILD == 10
                 return None
             if pid == self.pid:
-                if os.WIFSIGNALED(sts):
-                    self.returncode = -os.WTERMSIG(sts)
-                else:
-                    assert os.WIFEXITED(sts), "Status is {:n}".format(sts)
-                    self.returncode = os.WEXITSTATUS(sts)
+                self.returncode = os.waitstatus_to_exitcode(sts)
         return self.returncode
 
     def wait(self, timeout=None):

Lib/os.py

@@ -864,12 +864,8 @@ def _spawnvef(mode, file, args, env, func):
                 wpid, sts = waitpid(pid, 0)
                 if WIFSTOPPED(sts):
                     continue
-                elif WIFSIGNALED(sts):
-                    return -WTERMSIG(sts)
-                elif WIFEXITED(sts):
-                    return WEXITSTATUS(sts)
-                else:
-                    raise OSError("Not stopped, signaled or exited???")
+
+                return waitstatus_to_exitcode(sts)
 
     def spawnv(mode, file, args):
         """spawnv(mode, file, args) -> integer

Lib/subprocess.py

@@ -1838,23 +1838,17 @@ def _execute_child(self, args, executable, preexec_fn, close_fds,
                 raise child_exception_type(err_msg)
 
 
-        def _handle_exitstatus(self, sts, _WIFSIGNALED=os.WIFSIGNALED,
-                _WTERMSIG=os.WTERMSIG, _WIFEXITED=os.WIFEXITED,
-                _WEXITSTATUS=os.WEXITSTATUS, _WIFSTOPPED=os.WIFSTOPPED,
-                _WSTOPSIG=os.WSTOPSIG):
+        def _handle_exitstatus(self, sts,
+                               waitstatus_to_exitcode=os.waitstatus_to_exitcode,
+                               _WIFSTOPPED=os.WIFSTOPPED,
+                               _WSTOPSIG=os.WSTOPSIG):
             """All callers to this function MUST hold self._waitpid_lock."""
             # This method is called (indirectly) by __del__, so it cannot
             # refer to anything outside of its local scope.
-            if _WIFSIGNALED(sts):
-                self.returncode = -_WTERMSIG(sts)
-            elif _WIFEXITED(sts):
-                self.returncode = _WEXITSTATUS(sts)
-            elif _WIFSTOPPED(sts):
+            if _WIFSTOPPED(sts):
                 self.returncode = -_WSTOPSIG(sts)
             else:
-                # Should never happen
-                raise SubprocessError("Unknown child exit status!")
-
+                self.returncode = waitstatus_to_exitcode(sts)
 
         def _internal_poll(self, _deadstate=None, _waitpid=os.waitpid,
                 _WNOHANG=os.WNOHANG, _ECHILD=errno.ECHILD):

Lib/test/support/__init__.py

@@ -3442,18 +3442,11 @@ def wait_process(pid, *, exitcode, timeout=None):
 
             sleep = min(sleep * 2, max_sleep)
             time.sleep(sleep)
-
-        if os.WIFEXITED(status):
-            exitcode2 = os.WEXITSTATUS(status)
-        elif os.WIFSIGNALED(status):
-            exitcode2 = -os.WTERMSIG(status)
-        else:
-            raise ValueError(f"invalid wait status: {status!r}")
     else:
         # Windows implementation
         pid2, status = os.waitpid(pid, 0)
-        exitcode2 = (status >> 8)
 
+    exitcode2 = os.waitstatus_to_exitcode(status)
     if exitcode2 != exitcode:
         raise AssertionError(f"process {pid} exited with code {exitcode2}, "
                              f"but exit code {exitcode} is expected")

Lib/test/test_os.py

@@ -2794,6 +2794,35 @@ def test_waitpid(self):
         pid = os.spawnv(os.P_NOWAIT, FakePath(args[0]), args)
         support.wait_process(pid, exitcode=0)
 
+    def test_waitstatus_to_exitcode(self):
+        exitcode = 23
+        filename = support.TESTFN
+        self.addCleanup(support.unlink, filename)
+
+        with open(filename, "w") as fp:
+            print(f'import sys; sys.exit({exitcode})', file=fp)
+            fp.flush()
+        args = [sys.executable, filename]
+        pid = os.spawnv(os.P_NOWAIT, args[0], args)
+
+        pid2, status = os.waitpid(pid, 0)
+        self.assertEqual(os.waitstatus_to_exitcode(status), exitcode)
+        self.assertEqual(pid2, pid)
+
+    # Skip the test on Windows
+    @unittest.skipUnless(hasattr(signal, 'SIGKILL'), 'need signal.SIGKILL')
+    def test_waitstatus_to_exitcode_kill(self):
+        signum = signal.SIGKILL
+        args = [sys.executable, '-c',
+                f'import time; time.sleep({support.LONG_TIMEOUT})']
+        pid = os.spawnv(os.P_NOWAIT, args[0], args)
+
+        os.kill(pid, signum)
+
+        pid2, status = os.waitpid(pid, 0)
+        self.assertEqual(os.waitstatus_to_exitcode(status), -signum)
+        self.assertEqual(pid2, pid)
+
 
 class SpawnTests(unittest.TestCase):
     def create_args(self, *, with_env=False, use_bytes=False):

Lib/test/test_popen.py

@@ -44,10 +44,11 @@ def test_popen(self):
 
     def test_return_code(self):
         self.assertEqual(os.popen("exit 0").close(), None)
+        status = os.popen("exit 42").close()
         if os.name == 'nt':
-            self.assertEqual(os.popen("exit 42").close(), 42)
+            self.assertEqual(status, 42)
         else:
-            self.assertEqual(os.popen("exit 42").close(), 42 << 8)
+            self.assertEqual(os.waitstatus_to_exitcode(status), 42)
 
     def test_contextmanager(self):
         with os.popen("echo hello") as f:

Lib/test/test_pty.py

@@ -200,8 +200,8 @@ def test_fork(self):
             ##    raise TestFailed("Unexpected output from child: %r" % line)
 
             (pid, status) = os.waitpid(pid, 0)
-            res = status >> 8
-            debug("Child (%d) exited with status %d (%d)." % (pid, res, status))
+            res = os.waitstatus_to_exitcode(status)
+            debug("Child (%d) exited with code %d (status %d)." % (pid, res, status))
             if res == 1:
                 self.fail("Child raised an unexpected exception in os.setsid()")
             elif res == 2:

Lib/test/test_wait3.py

@@ -30,8 +30,7 @@ def wait_impl(self, cpid, *, exitcode):
             time.sleep(0.1)
 
         self.assertEqual(spid, cpid)
-        self.assertEqual(status, exitcode << 8,
-                         "cause = %d, exit = %d" % (status&0xff, status>>8))
+        self.assertEqual(os.waitstatus_to_exitcode(status), exitcode)
         self.assertTrue(rusage)
 
     def test_wait3_rusage_initialized(self):

Lib/test/test_wait4.py

@@ -29,8 +29,7 @@ def wait_impl(self, cpid, *, exitcode):
                 break
             time.sleep(0.1)
         self.assertEqual(spid, cpid)
-        self.assertEqual(status, exitcode << 8,
-                         "cause = %d, exit = %d" % (status&0xff, status>>8))
+        self.assertEqual(os.waitstatus_to_exitcode(status), exitcode)
         self.assertTrue(rusage)
 
 def tearDownModule():

Misc/NEWS.d/next/Library/2020-03-28-18-25-49.bpo-40094.v-wQIU.rst

@@ -0,0 +1,2 @@
+Add :func:`os.waitstatus_to_exitcode` function:
+convert a wait status to an exit code.