Implementation of BEP2 Proposal 2

[tsmanner]

If all Products produced by a Step have already been set, this will not actually call the target function. In the case of a Step that produces no Products, the Step always calls it's target.

[tsmanner]

Should this be the normal functionality? I'm honestly torn about whether this belongs in Step directly or if there's some work we should do to enable/disable this behavior when a Step is constructed. I think, if it is not in the base node, it should be another bolt-on bit of functionality, similar to the serializers.

Maybe we implement it as decorators that can be enabled/disabled by a library config API.

Another idea is to implement several run functions and use the same pattern as the serialization framework to allow users to select the run function that they want. i.e. by default steps just run the target once their consumes are set, but user code could call something like bazaarci.set_run_strategy(Step, Step.run_with_short_circuit).

[tsmanner]

@dansvo @manistal I'm thinking about writing this up as another BEP, since it seems to me like another potentially disruptive framework discussion.

[tsmanner]

I wrote up my idea here, mostly just as an exercise to think through the problem. I'm not sure it's necessary still, I think we need to come up with more potential use cases and more ways to solve it.

[tsmanner]

@dansvo @manistal We may have already talked about it, but it popped into my head just now. Another use case for the ability to inject code before/after the run step, regardless of type is that it could be used to add interactions with a cache like Artifactory.