654204f455
read_buf_exact: on error, all read bytes are appended to the buffer Guarantee that when `read_buf_exact` returns, all bytes read will be appended to the buffer. Including the case when the operations fails. The motivating use case are operations on a non-blocking reader. When `read_buf_exact` fails with `ErrorKind::WouldBlock` error, the operation can be resumed at a later time.