Isolate errors are not reported, if there is no error listener

[alsemenov]

It is essential for any Dart environment to report all uncaught errors.
However, errors that occur inside running Isolate are not reported, the isolate is just suspended without any message to console. This is not programmer-friendly and complicates writing simple programs.
Consider the following example:

import "dart:isolate";
import "dart:async";

entryPoint(message) {
  print(await new Future.value(message));
}

main() async {
  await Isolate.spawn(entryPoint, "hello");
  await new Future.delayed(new Duration(seconds:1));
}

The code contains a mistake, await is used in a function entryPoint that is not marked as async.
The programmer would expect to get a message about the mistake. Instead he/she gets "nothing happens" situation and he/she has no clue about what is wrong. To investigate the problem it is necessary to add error listener to the isolate, like this:

import "dart:isolate";
import "dart:async";

entryPoint(message) {
  print(await new Future.value(message));
}

main() async {
  ReceivePort errorPort = new ReceivePort();
  errorPort.listen((e) => print(e));
  await Isolate.spawn(entryPoint, "hello", onError:errorPort.sendPort);
  await new Future.delayed(new Duration(seconds:1));
  errorPort.close();
}

Output:

['file:///D:/dev/dart/test4.dart': error: line 5 pos 15: ')' expected
print(await new Future.value(message));
^
, null]

The function main here has 2.5 times more lines of code.

Dart VM version: 1.16.1 (Tue May 24 12:27:14 2016) on "windows_x64"

[lrhn]

That is surprising.
I think VM isolates used to print uncaught errors before terminating the isolate, even for spawned isolates. It should only omit printing the error if the isolate has an error listener.

The problem here might be that syntax errors are not normal errors, and they immediately terminate the isolate without going through the normal steps (#23684). Still, the error is reported if you do have an error listener, so it should be printed if you don't.

[a-siva]

The embedder (command line dart) gets back control only from the main isolate when uncaught errors happen. So when the main isolate has such uncaught errors they are printed by the embedder.

In this example the error is happening in the spawned isolate and the embedder does not get any control back after the error happens and hence nothing is being reported. I think the error maybe written to the console if --trace-isolates flag is used.

[lrhn]

Ok, so this is expected behavior?
I think all isolates should report uncaught errors on the console if they don't have an error listener added. If there is one or more error listeners, then the error should be considered handled that way and there is no need to print it.
(If the error listener has died or closed, that's a programming error, not something we can fix, although maybe the observatory knows enough to report when an isolate has attached error/exit listeners that aren't listening any more.)

[floitschG]

I can't find any --trace-isolates.

We had another user running into this. When there are no error-listeners we should just report the error on the console.

[aam]

--trace-isolates is only available in debug build of dart vm: https://github.com/dart-lang/sdk/blob/master/runtime/vm/flag_list.h#L169

[aam]

cc @rmacnak-google