Function sys_fill_exact 

pub(crate) fn sys_fill_exact(
    buf: &mut [MaybeUninit<u8>],
    sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> ssize_t,
) -> Result<(), Error>

Fill a buffer by repeatedly invoking sys_fill.

The sys_fill function:

• should return -1 and set errno on failure
• should return the number of bytes written on success