[BUGFIX release] Avoid instantiation errors when app/router.js injects the router service.

[Ravenstine]

Related to my previous PR: #19385

And issue: #17791

@rwjblue Since we talked about this with @snewcomer recently, maybe you can weigh in on this approach?

The intention is to prevent an infinite recursion when service:router is invoked before router:main has a chance to be initialized and cached. This PR does it by making service:router more "passive" so that the responsibility of forwarding routeWillChange and routeDidChange events is handled by router:main, as opposed to looking up(and possibly instantiating) router:main and listening to events on it from service:router. The active behavior of service:router instantiating router:main seems unexpected, and doesn't appear to be required in any tests.

Specifically, the service:router has been changed to only use router:main if it's already available and router:main now forwards those two route events to service:router if that service has been instantiated. That way there's no possibility of getting caught in a loop if service:router happens to be used during initialization of router:main, which can happen unexpectedly when using a service from an add-on that references service:router.

Definitely open to any kind of feedback.

[rwjblue]

Since we only need to forward the events when someone calls routerService.on('routeWillChange'), why don't we implement on and setup the wiring there?

[rwjblue]

Let's go the route of #19454, and look up the router service during init. That way we can avoid relying on { instantiate: false }. It does add some small amount of overhead (automatic creation of service:router), but in practice I don't think that will matter much and the resulting simplicity of the implementation will be easier to maintain.

What do you think @Ravenstine?

[Ravenstine]

@rwjblue That makes sense. Doing { instantiate: false } doesn't seem to really save us much. @snewcomer and I will work on this tomorrow.

[rwjblue]

kk, awesome thanks for taking a look @Ravenstine!

[sly7-7]

Thanks a lot to both of you @snewcomer @Ravenstine. This looks good to me. I just don't know what should be the choice for the setupRouter() call. I have my opinion, but I'm so new to the router area that I probably miss something.

[rwjblue]

Awesome, this is great work. Thank you @Ravenstine @snewcomer @sly7-7!