RFC 0027: Calling a product worker from its app

[BigTava]

Summary

• Proposes one Worker method, call_worker, letting a product's rendered executable invoke a named export of that same product's worker.

[johnthecat]

I think we should generalize into some generic mechanism for cross-executables communications.
API that looks suitable for it:

1. product public log. product can subscribe to own log, product A can subscribe to products B log, under explicit permission
2. event broadcasting inside product. other executables of this product can catch this message and broadcast response.