drain

        fn drain(io_w: *Io.Writer, data: []const []const u8, splat: usize) Io.Writer.Error!usize {
            const w: *Writer = @alignCast(@fieldParentPtr("interface", io_w));
            const io = w.io;
            const buffered = io_w.buffered();
            const handle = w.stream.socket.handle;
            const result = io.operate(.{ .net_write = .{
                .socket_handle = handle,
                .header = buffered,
                .data = data,
                .splat = splat,
            } }) catch |err| {
                w.err = err;
                return error.WriteFailed;
            };
            const n = result.net_write catch |err| {
                w.err = err;
                return error.WriteFailed;
            };
            return io_w.consume(n);
        }