Control over "isRealData" during Data-MC Mixing
When mixing Data events into MC, or vice-versa, the “isRealData” flag assumes the value of the primary stream. Various uses of isRealData leave the current behaviour not necessarily the desired behaviour.
Sometimes isRealData=true decides to not look for the MC event products which would be mixed in. Other times, isRealData might decide how to retrieve spill info, etc...
Given that any experiment's code was not developed to be mindful of Data-MC mixing in the use of isRealData, it would be nice to introduce a handle on setting the isRealData flag during mixing only. This way an experiment can choose the lesser evil when mitigating uses of isRealData.
I imagine maintaining default behaviour, but just adding a fcl option to assume the secondary stream event's value rather than the primary. But I also imagine you may have better ideas.