fix(core): errored initial snapshot throws sync w/o observers

[boneskull]

This modifies the error-handling logic for when an error is thrown during the creation of an Actor's initial snapshot (during start()). If the actor has no observers, the error will now be thrown synchronously out of start() instead of to the global error handler.

Example use case:

const machine = createMachine({
  context: () => {
    throw new Error('egad!');
  }
});

const actor = createActor(machine);

try {
  await toPromise(actor.start());
} catch (err) {
  err.message === 'egad!' // true
}

Note that this does not impact child actors, but it might want to!

Fixes: #4928

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[boneskull]

For reference, the current workaround is this:

const machine = createMachine({
  context: () => {
    throw new Error('egad!');
  }
});
const actor = createActor(machine);

const p = toPromise(actor);
actor.start();
try {
  await p;
} catch (err) {
  err.message === 'egad!' // true
}

[boneskull]

I don't know if the new behavior is desirable to the maintainers, but I find it more ergonomic. Thought I would restart that conversation here!

[boneskull]

My main concern is that it is somewhat inconsistent, since if the example machine was spawned as a child actor, it would reach the global error handler.

...that being said, if using spawnChild(), there's really nowhere else for the error to go.

[boneskull]

This looks like a breaking change to me unless you feel this is actually a "bug fix" and not just a "revision of intent" 😜

[Andarist]

...that being said, if using spawnChild(), there's really nowhere else for the error to go.

Could you show the exact situation you are talking about here?

[boneskull]

...that being said, if using spawnChild(), there's really nowhere else for the error to go.

Could you show the exact situation you are talking about here?

@Andarist

something like this:

it('handled sync errors thrown when starting a child actor should be reported globally when spawned via spawnChild()', (done) => {
  const badMachine = createMachine({
    context: () => {
      throw new Error('handled_sync_error_in_actor_start');
    }
  });
  
  const machine = createMachine({
    entry: [
      spawnChild(badMachine)
    ]
  });

  const actorRef = createActor(machine);
  actorRef.start();
  installGlobalOnErrorHandler((ev) => {
    ev.preventDefault();
    expect(ev.error.message).toEqual('handled_sync_error_in_actor_start');
    done();
  });
});

If a xstate.error.actor.badMachine (or w/e) event handler in machine is the canonical way to trap this error, then I don't think changes to the above behavior are warranted (i.e. changing it so the error is thrown out of actor.start(), which I think is only possible because it's thrown synchronously?). It's just that there's no other reasonable way to catch it out of the root actor (e.g. if badMachine was the root actor), which is what this PR solves.