Enum Source 

enum Source {
    Read(Result<Option<usize>, TransportError>),
    ResetUnobservable,
}

What one poll of the source stream saw.

The two pipe loops that queue what they read — pipe_control_mutating and pipe_data_framed — poll their source through observe_source rather than calling recv.read directly, and this is what it hands back.

Variants

Read(Result<Option<usize>, TransportError>)

recv.read’s own result, verbatim: Ok(Some(n)) bytes into the caller’s buffer, Ok(None) a clean FIN, Err a failure.

A reset seen by the reset-only observer arrives here too, as Err(TransportError::StreamReset(code)) — byte-identical to what recv.read would have produced — so propagate_reset mirrors the same code down the same path and neither pipe loop has to know which observer was live.

ResetUnobservable

The source can no longer be reset, and the reset-only observer must not be polled again: it resolves immediately every time (see RecvStream::received_reset), so re-polling it spins. The caller latches it off and the branch parks for the rest of the stream, which is exactly the disabled read branch this replaced.