Exceptions from Future do not contain original call site

[sethladd]

Consider this code:

import 'dart:async';

Future doItLater() {
  //return new Future.immediate('dart');
  return new Future.immediateError(new StateError('doh!'));
}

main() {
  doItLater().then(print);
}

When you run it, you get:

sethladd:~/dart/simpletests/bin$ dart asyncexception.dart
Uncaught Error: Bad state: doh!
Unhandled exception:
Bad state: doh!

­0 _FutureImpl._scheduleUnhandledError.<anonymous closure> (dart:async:334:9)

­1 Timer.Timer.<anonymous closure> (dart:async-patch:11:15)

­2 _Timer._createTimerHandler._handleTimeout (dart:io:7240:28)

­3 _Timer._createTimerHandler._handleTimeout (dart:io:7248:7)

­4 _Timer._createTimerHandler.<anonymous closure> (dart:io:7256:23)

­5 _ReceivePortImpl._handleMessage (dart:isolate-patch:40:92)

Problem:

The stack trace does not include any reference to the line of code that actually generated the error.

This makes debugging Dart really difficult.

(submitted on behalf of a partner who is building a Dart app for his job)

[sethladd]

(not sure how to triage, starting with VM as first guess)

[gbracha]

Added Duplicate label.
Marked as being merged into #4061.

[munificent]

Marked as being merged into #7040.

[a-siva]

Something like this could be done to get more context about the original call site:

import 'dart:async';

String getCurrentStackTrace() {
  try {
    throw "";
  } catch(e, s) {
    return "$s";
  }
}

Future doItLater() {
  var s = getCurrentStackTrace();
  return new Future.immediateError(new StateError('doh! @­ $s'));
}

main() {
  doItLater().then(print);
}

Executing this I get:
Uncaught Error: Bad state: doh! @­ #­0 getCurrentStackTrace (file:///tmp/junk2.dart:5:5)
#­0 doItLater (file:///tmp/junk2.dart:12:31)
#­1 main (file:///tmp/junk2.dart:17:12)

Unhandled exception:
Bad state: doh! @­ #­0 getCurrentStackTrace (file:///tmp/junk2.dart:5:5)
#­0 doItLater (file:///tmp/junk2.dart:12:31)
#­1 main (file:///tmp/junk2.dart:17:12)

#­0 _FutureImpl._scheduleUnhandledError.<anonymous closure> (dart:async:348:9)
#­1 Timer.run.<anonymous closure> (dart:async:2172:21)
#­2 Timer.run.<anonymous closure> (dart:async:2179:13)
#­3 Timer.Timer.<anonymous closure> (dart:async-patch:11:15)
#­4 _Timer._createTimerHandler._handleTimeout (dart:io:5930:28)
#­5 _Timer._createTimerHandler._handleTimeout (dart:io:5938:7)
#­6 _Timer._createTimerHandler.<anonymous closure> (dart:io:5946:23)
#­7 _ReceivePortImpl._handleMessage (dart:isolate-patch:40:92)

[sethladd]

Hi Siva,

How does this work when you can't edit doItLater? That is, you don't own it and it's a 3rd party library. (I understand that all Dart apps are just text files, but it's not good practice to start editing dependencies.)

Also, doesn't this assume I know where the exception is happening in the first place? If I knew that, I wouldn't have to add the special getCurrentStackTrace() function. :)

Thoughts?

Thanks!

[a-siva]

For the more generic case that you describe one would have to implement a subclass of Future which would have the additional functionality of storing the stack trace for later use. I think a mechanism like that is outside the scope of the Dart VM.

[sethladd]

Thanks Siva, I'll ask the lib team about this.