* returned when returning at least one byte of data to the
* caller, i.e. this must be returned when there's
* absolutely nothing left; should the request size be
* larger than what's left in the file, this function must
* return what's left, setting \p buffer_sz to the number of
* remaining bytes, and return
* returned when returning at least one byte of data to the
* caller, i.e. this must be returned when there's
* absolutely nothing left; should the request size be
* larger than what's left in the file, this function must
* return what's left, setting \p buffer_sz to the number of
* remaining bytes, and return