bpo-39763: Add _bootsubprocess to build Python on AIX (GH-18872)

Add _bootsubprocess module to bootstrap Python: subprocess
implementation which only uses the os module.

On AIX, distutils.util uses _aix_support which calls
subprocess.check_output(), before the _posixsubprocess module is
built. Implement check_output() with os.system() in _bootsubprocess.

setup.py

@@ -16,53 +16,17 @@ try:
     del subprocess
     SUBPROCESS_BOOTSTRAP = False
 except ImportError:
-    SUBPROCESS_BOOTSTRAP = True
-
     # Bootstrap Python: distutils.spawn uses subprocess to build C extensions,
     # subprocess requires C extensions built by setup.py like _posixsubprocess.
     #
-    # Basic subprocess implementation for POSIX (setup.py is not used on
-    # Windows) which only uses os functions. Only implement features required
-    # by distutils.spawn.
+    # Use _bootsubprocess which only uses the os module.
     #
     # It is dropped from sys.modules as soon as all C extension modules
     # are built.
-    class Popen:
-        def __init__(self, cmd, env=None):
-            self._cmd = cmd
-            self._env = env
-            self.returncode = None
-
-        def wait(self):
-            pid = os.fork()
-            if pid == 0:
-                # Child process
-                try:
-                    if self._env is not None:
-                        os.execve(self._cmd[0], self._cmd, self._env)
-                    else:
-                        os.execv(self._cmd[0], self._cmd)
-                finally:
-                    os._exit(1)
-            else:
-                # Parent process
-                pid, status = os.waitpid(pid, 0)
-                if os.WIFSIGNALED(status):
-                    self.returncode = -os.WTERMSIG(status)
-                elif os.WIFEXITED(status):
-                    self.returncode = os.WEXITSTATUS(status)
-                elif os.WIFSTOPPED(status):
-                    self.returncode = -os.WSTOPSIG(status)
-                else:
-                    # Should never happen
-                    raise Exception("Unknown child exit status!")
-
-            return self.returncode
-
-    mod = type(sys)('subprocess')
-    mod.Popen = Popen
-    sys.modules['subprocess'] = mod
-    del mod
+    import _bootsubprocess
+    sys.modules['subprocess'] = _bootsubprocess
+    del _bootsubprocess
+    SUBPROCESS_BOOTSTRAP = True
 
 
 from distutils import log