fix: prevent one-sided auth when stream teardown races connection drop

[gusinacio]

end_of_auth awaited send.stopped() and propagated any error up the call stack. In practice this raced with the peer dropping its Connection as soon as it finished auth, so the slower side frequently observed connection lost from stopped().await and returned an error before calling add_authenticated. The faster side ended up authenticated, the slower side did not, and the real application connection timed out in after_handshake waiting for the watcher to tick.

Two changes fix this:

1. end_of_auth becomes best-effort. All SPAKE2 tokens and HKDF-derived keys are already written and ACKed at the QUIC layer by the time we reach this function; the stream FIN is cosmetic and should not be able to fail auth.

2. add_authenticated is moved before end_of_auth in both auth_accept and auth_open. Even if teardown is ever made non-best-effort again, the authenticated set is committed first.

The error branches in both paths also call end_of_auth best-effort so the stream is still finalized on mismatch without masking the real ...FailedAndBlock error.

All existing tests continue to pass.

[rustonbsd]

Hi @gusinacio, thank you for the PR!

The issue in end_of_auth is definitely a bug. I have already implemented a fix along with a lot of stability improvements based on my network degradation testing in #5.

I suggest that after I push #5 to main we rebase this branch and keep your ordering changes and if required adopt the the infallible end_of_auth approach. I agree that add_authenticated should happen before we call end_of_auth and if after the changes from #5 we still fail the end_of_auth function asymmetrically, then we use your approach if still required.

My solution was to use recv.read_to_end(usize::MAX).await instead of stopp. This should not fail even if the other stream is already closed, but if the read_to_end or the finish call still fail, we use your version.

iroh-auth/src/lib.rs

Lines 317 to 331 in 08e8930

	if let Err(err) = recv.read_to_end(usize::MAX).await.map_err(|err| {
	if open {
	AuthenticatorError::OpenFailed(format!(
	"Failed to wait for stream stopped: {}",
	err
	))
	} else {
	AuthenticatorError::AcceptFailed(format!(
	"Failed to wait for stream stopped: {}",
	err
	))
	}
	}) {
	warn!("[end_of_auth] {}", err);
	}

I will write you here once I have updated main. Do you want to rebase it after or should I? (don't want to mess around with your fork if you don't agree ^^). Let me know what you think!

[gusinacio]

Feel free to rebase it.