Start the given command in a child process in a pseudo terminal.

        This does all the fork/exec type of stuff for a pty, and returns an
        instance of PtyProcess.

        If preexec_fn is supplied, it will be called with no arguments in the
        child process before exec-ing the specified command.
        It may, for instance, set signal handlers to SIG_DFL or SIG_IGN.

        Dimensions of the psuedoterminal used for the subprocess can be
        specified as a tuple (rows, cols), or the default (24, 80) will be used.

def spawn(
            cls, argv, cwd=None, env=None, echo=True, preexec_fn=None,
            dimensions=(24, 80)):
        '''Start the given command in a child process in a pseudo terminal.

        This does all the fork/exec type of stuff for a pty, and returns an
        instance of PtyProcess.

        If preexec_fn is supplied, it will be called with no arguments in the
        child process before exec-ing the specified command.
        It may, for instance, set signal handlers to SIG_DFL or SIG_IGN.

        Dimensions of the psuedoterminal used for the subprocess can be
        specified as a tuple (rows, cols), or the default (24, 80) will be used.
        '''
        # Note that it is difficult for this method to fail.
        # You cannot detect if the child process cannot start.
        # So the only way you can tell if the child process started
        # or not is to try to read from the file descriptor. If you get
        # EOF immediately then it means that the child is already dead.
        # That may not necessarily be bad because you may have spawned a child
        # that performs some task; creates no stdout output; and then dies.

        if not isinstance(argv, (list, tuple)):
            raise TypeError("Expected a list or tuple for argv, got %r" % argv)

        # Shallow copy of argv so we can modify it
        argv = argv[:]
        command = argv[0]

        command_with_path = which(command)
        if command_with_path is None:
            raise FileNotFoundError('The command was not found or was not ' +
                                    'executable: %s.' % command)
        command = command_with_path
        argv[0] = command

        # [issue #119] To prevent the case where exec fails and the user is
        # stuck interacting with a python child process instead of whatever
        # was expected, we implement the solution from
        # http://stackoverflow.com/a/3703179 to pass the exception to the
        # parent process

        # [issue #119] 1. Before forking, open a pipe in the parent process.
        exec_err_pipe_read, exec_err_pipe_write = os.pipe()

        if use_native_pty_fork:
            pid, fd = pty.fork()
        else:
            # Use internal fork_pty, for Solaris
            pid, fd = _fork_pty.fork_pty()

        # Some platforms must call setwinsize() and setecho() from the
        # child process, and others from the master process. We do both,
        # allowing IOError for either.

        if pid == CHILD:
            # set window size
            try:
                _setwinsize(STDIN_FILENO, *dimensions)
            except IOError as err:
                if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
                    raise

            # disable echo if spawn argument echo was unset
            if not echo:
                try:
                    _setecho(STDIN_FILENO, False)
                except (IOError, termios.error) as err:
                    if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
                        raise

            # [issue #119] 3. The child closes the reading end and sets the
            # close-on-exec flag for the writing end.
            os.close(exec_err_pipe_read)
            fcntl.fcntl(exec_err_pipe_write, fcntl.F_SETFD, fcntl.FD_CLOEXEC)

            # Do not allow child to inherit open file descriptors from parent,
            # with the exception of the exec_err_pipe_write of the pipe
            # Impose ceiling on max_fd: AIX bugfix for users with unlimited
            # nofiles where resource.RLIMIT_NOFILE is 2^63-1 and os.closerange()
            # occasionally raises out of range error
            max_fd = min(1048576, resource.getrlimit(resource.RLIMIT_NOFILE)[0])
            os.closerange(3, exec_err_pipe_write)
            os.closerange(exec_err_pipe_write+1, max_fd)

            if cwd is not None:
                os.chdir(cwd)

            if preexec_fn is not None:
                try:
                    preexec_fn()
                except Exception as e:
                    ename = type(e).__name__
                    tosend = '{}:0:{}'.format(ename, str(e))
                    if PY3:
                        tosend = tosend.encode('utf-8')

                    os.write(exec_err_pipe_write, tosend)
                    os.close(exec_err_pipe_write)
                    os._exit(1)

            try:
                if env is None:
                    os.execv(command, argv)
                else:
                    os.execvpe(command, argv, env)
            except OSError as err:
                # [issue #119] 5. If exec fails, the child writes the error
                # code back to the parent using the pipe, then exits.
                tosend = 'OSError:{}:{}'.format(err.errno, str(err))
                if PY3:
                    tosend = tosend.encode('utf-8')
                os.write(exec_err_pipe_write, tosend)
                os.close(exec_err_pipe_write)
                os._exit(os.EX_OSERR)

        # Parent
        inst = cls(pid, fd)
        
        # Set some informational attributes
        inst.argv = argv
        if env is not None:
            inst.env = env
        if cwd is not None:
            inst.launch_dir = cwd

        # [issue #119] 2. After forking, the parent closes the writing end
        # of the pipe and reads from the reading end.
        os.close(exec_err_pipe_write)
        exec_err_data = os.read(exec_err_pipe_read, 4096)
        os.close(exec_err_pipe_read)

        # [issue #119] 6. The parent reads eof (a zero-length read) if the
        # child successfully performed exec, since close-on-exec made
        # successful exec close the writing end of the pipe. Or, if exec
        # failed, the parent reads the error code and can proceed
        # accordingly. Either way, the parent blocks until the child calls
        # exec.
        if len(exec_err_data) != 0:
            try:
                errclass, errno_s, errmsg = exec_err_data.split(b':', 2)
                exctype = getattr(builtins, errclass.decode('ascii'), Exception)

                exception = exctype(errmsg.decode('utf-8', 'replace'))
                if exctype is OSError:
                    exception.errno = int(errno_s)
            except:
                raise Exception('Subprocess failed, got bad error data: %r'
                                    % exec_err_data)
            else:
                raise exception

        try:
            inst.setwinsize(*dimensions)
        except IOError as err:
            if err.args[0] not in (errno.EINVAL, errno.ENOTTY, errno.ENXIO):
                raise

        return inst