* @io: function to call when fd becomes read/writable, returns 0 to be
* called again, 1 if it's finished, and -1 on error (fd will be closed)
* @next: the next function which is called if io returns 1.
* @io: function to call when fd becomes read/writable, returns 0 to be
* called again, 1 if it's finished, and -1 on error (fd will be closed)
* @next: the next function which is called if io returns 1.