Armeria: Refuse to use a decorating `Client` and `Service` if `as()` or `serviceAdded()` is not overriden.

[trustin]

A common mistake a user makes when writing a decorator is:

ServerBuilder sb = new ServerBuilder();
sb.service("/svc", myService.decorate(delegate -> (ctx, req) -> {
    ...
    return delegate.serve(ctx, req);
}));

The above decorator is not correct because the returned service does not override the default as() and serviceAdded() implementation. A correct usage is:

sb.service("/svc", myService.decorate((delegate, ctx, req) -> {
    return delegate.serve(ctx, req);
});
// or
sb.service("/svc", myService.decorate(delegate -> new SimpleDecoratingService(delegate) {
    ...
    return delegate().serve(ctx, req);
    ...
}));

We can detect such a mistake by checking if the decorator overrides the default as() and serviceAdded() implementation using Java reflection API and raise an exception.

[thinkgruen]

I would like to also tackle this one :)

[trustin]

Thank you! :bowing_man:

[thinkgruen]

Sorry guys, a month has already passed and I yet couldn't get to it. I will try to work on it over the course of the next week. 😞

[trustin]

No worries. I really appreciate that you're trying to help us! :bowing_man:

[jacobis]

@trustin Could I try this issue?

[ikhoon]

I'm not trustin, but I could tell "Yes, please!" 😄

[trustin]

:rofl: Yes, please!

[trustin]

I've just realized this stops a user from using Function.identity() or other creative use of functions, like returning a completely different Client or Service which is not a decorator.

That means, I think we can't validate anything this way. :sob: