File manager

"""
Basic subprocess implementation for POSIX which only uses os functions. Only
implement features required by setup.py to build C extension modules when
subprocess is unavailable. setup.py is not used on Windows.
"""
import os


# distutils.spawn used by distutils.command.build_ext
# calls subprocess.Popen().wait()
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
            _, status = os.waitpid(pid, 0)
            self.returncode = os.waitstatus_to_exitcode(status)

        return self.returncode


def _check_cmd(cmd):
    # Use regex [a-zA-Z0-9./-]+: reject empty string, space, etc.
    safe_chars = []
    for first, last in (("a", "z"), ("A", "Z"), ("0", "9")):
        for ch in range(ord(first), ord(last) + 1):
            safe_chars.append(chr(ch))
    safe_chars.append("./-")
    safe_chars = ''.join(safe_chars)

    if isinstance(cmd, (tuple, list)):
        check_strs = cmd
    elif isinstance(cmd, str):
        check_strs = [cmd]
    else:
        return False

    for arg in check_strs:
        if not isinstance(arg, str):
            return False
        if not arg:
            # reject empty string
            return False
        for ch in arg:
            if ch not in safe_chars:
                return False

    return True


# _aix_support used by distutil.util calls subprocess.check_output()
def check_output(cmd, **kwargs):
    if kwargs:
        raise NotImplementedError(repr(kwargs))

    if not _check_cmd(cmd):
        raise ValueError(f"unsupported command: {cmd!r}")

    tmp_filename = "check_output.tmp"
    if not isinstance(cmd, str):
        cmd = " ".join(cmd)
    cmd = f"{cmd} >{tmp_filename}"

    try:
        # system() spawns a shell
        status = os.system(cmd)
        exitcode = os.waitstatus_to_exitcode(status)
        if exitcode:
            raise ValueError(f"Command {cmd!r} returned non-zero "
                             f"exit status {exitcode!r}")

        try:
            with open(tmp_filename, "rb") as fp:
                stdout = fp.read()
        except FileNotFoundError:
            stdout = b''
    finally:
        try:
            os.unlink(tmp_filename)
        except OSError:
            pass

    return stdout