os.forkpty() and pty.fork() return FD with O_CLOEXEC

Bug report

Bug description:

(pid, fd) = pty.fork() # os.forkpty()
print('os.get_inheritable(fd))

https://docs.python.org/3/library/os.html#os.forkpty
https://docs.python.org/3/library/pty.html#pty.fork

Say nothing about that.

Lib/pty.py:

def fork():
    """fork() -> (pid, master_fd)
    Fork and make the child a session leader with a controlling terminal."""

    try:
        pid, fd = os.forkpty()
    except (AttributeError, OSError):
        pass
    else:
        if pid == CHILD:
            try:
                os.setsid()
            except OSError:
                # os.forkpty() already set us session leader
                pass
        return pid, fd

    master_fd, slave_fd = openpty()
    pid = os.fork()
    if pid == CHILD:
        os.close(master_fd)
        os.login_tty(slave_fd)
    else:
        os.close(slave_fd)

    # Parent and child process.
    return pid, master_fd

Also uses openpty() which also does not set O_CLOEXEC

Meanwhile, there is a C function posix_openpt(int flags); which allows setting O_CLOEXEC.

Seems, fd-leak safe solution is to boilerplate all the actions by hand. i.e. same way as pty.fork() does, but also implement openpty() using posix_openpt() + ptsname() / ptsname_r().

Just setting O_CLOEXEC after openpty() or after calling C forkpty() is not enough because of race-condition and possible file descriptor leak.

So, CLOEXEC behavior should be documented and race-condition prevented

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-139408

Lib/pty.py と、文書化されているエントリポイント os.forkpty() および pty.fork() から開始し、続いてそれらの openpty() パスを、報告で言及されている posix_openpt(int flags) インターフェースと比較します。CLOEXEC の動作が文書化され、fork/openpty パスによって説明されている競合状態によるファイルディスクリプタのリークが防止されれば、作業は完了です。