IO#close_write

static VALUE
rb_io_close_write(VALUE io)
{
    rb_io_t *fptr;
    VALUE write_io;

    write_io = GetWriteIO(io);
    fptr = rb_io_get_fptr(rb_io_taint_check(write_io));
    if (fptr->fd < 0) return Qnil;
    if (is_socket(fptr->fd, fptr->pathv)) {
#ifndef SHUT_WR
# define SHUT_WR 1
#endif
        /* Flush any buffered data before shutting down the write side.
         * Otherwise the buffered bytes are silently dropped here, and a
         * subsequent #close would try to flush them into the now
         * shutdown(SHUT_WR) socket and fail with EPIPE. This matches the
         * behaviour of the non-socket path below, which flushes via
         * rb_io_close(). */
        if (fptr->mode & FMODE_WRITABLE) {
            if (io_fflush(fptr) < 0)
                rb_sys_fail_on_write(fptr);
        }
        if (shutdown(fptr->fd, SHUT_WR) < 0)
            rb_sys_fail_path(fptr->pathv);
        fptr->mode &= ~FMODE_WRITABLE;
        if (!(fptr->mode & FMODE_READABLE))
            return rb_io_close(write_io);
        return Qnil;
    }

    if ((fptr->mode & (FMODE_DUPLEX|FMODE_READABLE)) == FMODE_READABLE) {
        rb_raise(rb_eIOError, "closing non-duplex IO for writing");
    }

    if (io != write_io) {
        fptr = rb_io_get_fptr(rb_io_taint_check(io));
        fptr->tied_io_for_writing = 0;
    }
    rb_io_close(write_io);
    return Qnil;
}