UTF8 decoding error when decoding disconnection reason

[matehat]

I see the following error report from iOS, very occasionally, correlated with other connection issues.

FormatException: FormatException: Missing extension byte (at offset 5)
#0      _Utf8Decoder.convertSingle (dart:convert-patch/convert_patch.dart:1855)
#1      Utf8Decoder.convert (dart:convert/utf.dart:351)
#2      Utf8Codec.decode (dart:convert/utf.dart:64)
#3      CupertinoWebSocket._connectionClosed (package:cupertino_http/src/cupertino_web_socket.dart:189)
#4      CupertinoWebSocket._closeConnectionWithError (package:cupertino_http/src/cupertino_web_socket.dart:181)
#5      _FutureListener.handleError (dart:async/future_impl.dart:245)
#6      Future._propagateToListeners.handleError (dart:async/future_impl.dart:963)
#7      Future._propagateToListeners (dart:async/future_impl.dart:984)
#8      Future._completeErrorObject (dart:async/future_impl.dart:741)
#9      Future._asyncCompleteErrorObject.<fn> (dart:async/future_impl.dart:835)
#10      _microtaskLoop (dart:async/schedule_microtask.dart:40)
#11      _startMicrotaskLoop (dart:async/schedule_microtask.dart:49)

Looking at the code, decoding the reason is a little risky:

final closeReason = reason == null ? '' : utf8.decode(reason.toList());

Would it be acceptable for it to fallback to '' if the reason is not a decodable utf8 string?

[brianquinlan]

Are you seeing this when communicating with a real server? RFC-6455 says that the reason must be UTF-8 so my first though is that it should be an exception if it is not.

[matehat]

Yes, a live Phoenix server (that uses Cowboy under the hood), same we use in production where we don't have issues 99.99% of the time.

Just occasionally, when there are connection drops due to internet connectivity issues, that error pops up.

[brianquinlan]

Before we land your PR, could we spend a bit more time investigating. Do you know what the actual bytes being sent by Phoenix are?