No complaint while obtaining a service from multiple services involves ambiguity

[kitchoi]

    contributed_service_offers = List(contributes_to=envisage_ids.SERVICE_OFFERS)

    def _contributed_service_offers_default(self):
        service_offer1 = ServiceOffer(
            protocol=SomeClass, factory=factory1
        )
        service_offer2 = ServiceOffer(
            protocol=SomeClass, factory=factory2
        )
        return [service_offer1, service_offer2]

If a plugin, or two plugins in conjunction, contributes two service offers that have the same protocol but different factory, then I'd expect this to be at least a warning, or possibly an error, to the maintainer of a plugin manager. This is because it is ambiguous which of factory1 or factory2 would have been used when one obtains a service via application.get_service(SomeClass), so this likely indicates programming errors or oversights.

Currently no such warnings or errors would occur.

(Sorry for a somewhat curt description as I did not want this observation to go unreported.)

[kitchoi]

@mdickinson For awareness, this issue is tangentially related to the awkwardness with having to subclass (say the SomeClass here) just to offer two services of the same type but used for different purposes.

[kitchoi]

At the very least, a warning in here would be good:

envisage/envisage/service_registry.py

Lines 88 to 89 in d62af08

	if len(services) > 0:
	service = services[0]