ssh client is setting O_NONBLOCK on a pipe shared with other processes

[Doug Graham]

> Dumb question: shouldn't whatever is calling fork() (here 'make' I believe)
> be dup()'ing the FDs just in case the called program does something odd
> with them like set O_NONBLOCK?

Doesn't work.  That's where I think POSIX is downright weird.  Ssh
actually does dup
descriptors 0, 1, and 2, and then only sets O_NONBLOCK on the new descriptors:

        if (stdin_null_flag) {
                in = open(_PATH_DEVNULL, O_RDONLY);
        } else {
                in = dup(STDIN_FILENO);
        }
        out = dup(STDOUT_FILENO);
        err = dup(STDERR_FILENO);

        /* enable nonblocking unless tty */
        if (!isatty(in))
                set_nonblock(in);
        if (!isatty(out))
                set_nonblock(out);
        if (!isatty(err))
                set_nonblock(err);

The problem, I've learned, it that the original descriptor and the new
copy of it refer to the same
POSIX "file description", so setting O_NONBLOCK on one affects the
other as well.  I find this
quite counter-intuitive and it's why I'm starting to believe that the
bug is in POSIX.  I also
find it odd a child process can affect this flag in the parent, but
that again is because both the
parent and child's descriptors refer to the same POSIX "file description".