Function copy_until 

pub fn copy_until(
    reader: &mut dyn Read,
    sink: &mut dyn Write,
    cancel: &dyn Fn() -> bool,
    cap_remaining: u64,
    buf: &mut [u8],
) -> (CopyEnd, u64)

Copy from reader to sink until EOF, cancellation, the byte cap, or an error. cancel is checked between reads — a read already in flight finishes first, so this is cooperative (paired with a short read timeout on the real stream). Always returns the bytes written (even on error), so the caller’s cap and totals stay correct across reconnects. Never writes past cap_remaining.