Function write_or_flush 

pub(super) async fn write_or_flush<W: AsyncWrite + Unpin>(
    write: &mut W,
    pending: &[u8],
    needs_flush: bool,
) -> Result<Wrote>

Writes one chunk of pending to write, or flushes when pending is empty.

Cancel safety

This is cancellation-safe. It performs a single poll_write (or a single poll_flush) and returns; it never tracks progress across awaits, so dropping the future before it resolves writes nothing the caller is not told about. The caller advances its own cursor by calling consume(n) only on a returned Wrote::Bytes. This is why the loop uses write rather than write_all: write_all keeps the progress cursor inside the future, which a cancellation would lose.