IO#wait

static VALUE
io_wait(int argc, VALUE *argv, VALUE io)
{
    VALUE timeout = Qundef;
    enum rb_io_event events = 0;
    int return_io = 0;

    if (argc != 2 || (RB_SYMBOL_P(argv[0]) || RB_SYMBOL_P(argv[1]))) {
        // We'd prefer to return the actual mask, but this form would return the io itself:
        return_io = 1;

        // Slow/messy path:
        for (int i = 0; i < argc; i += 1) {
            if (RB_SYMBOL_P(argv[i])) {
                events |= wait_mode_sym(argv[i]);
            }
            else if (UNDEF_P(timeout)) {
                rb_time_interval(timeout = argv[i]);
            }
            else {
                rb_raise(rb_eArgError, "timeout given more than once");
            }
        }

        if (UNDEF_P(timeout)) timeout = Qnil;

        if (events == 0) {
            events = RUBY_IO_READABLE;
        }
    }
    else /* argc == 2 and neither are symbols */ {
        // This is the fast path:
        events = io_event_from_value(argv[0]);
        timeout = argv[1];
    }

    if (events & RUBY_IO_READABLE) {
        rb_io_t *fptr = NULL;
        RB_IO_POINTER(io, fptr);

        if (rb_io_read_pending(fptr)) {
            // This was the original behaviour:
            if (return_io) return Qtrue;
            // New behaviour always returns an event mask:
            else return RB_INT2NUM(RUBY_IO_READABLE);
        }
    }

    return io_wait_event(io, events, timeout, return_io);
}