* Called repeatedly until 0 is returned. Must assign the
* head of the next block of data to &*dst and return the length
* of the block or 0 if at the end. consumed == 0 indicates
* Called repeatedly until 0 is returned. Must assign the
* head of the next block of data to &*dst and return the length
* of the block or 0 if at the end. consumed == 0 indicates