Add LoadBalancer.errors stream

[lexaknyazev]

A simple implementation of #23 (basically a port of IsolateRunner.errors).

LoadBalancer class lacks unit tests, so it might be reasonable to add some before merging.

[lrhn]

I'd prefer to not have special-casing for isolate runners inside the LoadBalancer class.

I'd rather make an error handling wrapper for isolate runners in general, then you can capture the errors of all the isolate runners before passing them to the LoadBalancer constructor.

[lexaknyazev]

OK, I'll update the PR this week.

[lexaknyazev]

an error handling wrapper for isolate runners in general

After looking at LoadBalancer and IsolateRunner public interfaces, I'm not sure how this wrapper should look like.

In a case when a LoadBalancer is created with a set of pre-existing IsolateRunners, there's no need for an additional wrapper because each IsolateRunner exposes its own .errors stream.

final isolateRunners =
    await Future.wait(Iterable.generate(16, (_) => IsolateRunner.spawn()));

for (final isolateRunner in isolateRunners) {
  // subscribe to error streams  
}

final balancer = LoadBalancer(isolateRunners);

On the other hand, when a LoadBalancer is created with a static create method, individual runners cannot be accessed from the outside, so there's no way to subscribe to their error streams.

So I can think of a few options on how to go forward from here.

• Don't change the API, add some documentation that suggests creating isolate runners manually for enhanced functionality; or

• add a new static method similar to LoadBalancer.create that creates only isolate runners and

  • calls an optional user-provided callback to pass each runners's errors stream; or
  • combines all runners' errors streams into one stream and passes it when all runners are ready.

[lrhn]

What I would do is to make a create function wrapper so that you can intercept the created runners. For example:

Future<Runner> createIsolateRunner(void whenCreated(IsolateRunner runner)) async {
  var runner = await IsolateRunner.spawn();
  whenCreated(runner);
  return runner;
}

Then you can collect errors by doing:

var poolUncaughtErrors = StreamController();
var pool = await LoadBalancer.create(createIsolateRunner((runner) {
   runner.isolate.errors.forEach(errors.add);
});

[lrhn]

Nor planning on using this approach.