Adding john; I think he and I had a discussion about this?

I had thought about doing this, but it would require teaching the recipe engine to look at rolls and understand that changes to the recipes.cfg file. That seemed like a fair amount of extra knowledge that the engine would have to understand.

I do like the idea of a presubmit builder blocking the roll though. That's how rolls into chromium work IIRC. webrtc, v8, etc... all roll into chromium, and their autorollers use the CQ to test the changes. If any of the changes break, the roll fails.

My biggest worry about this is that the recipe roller doesn't really have a good way to deal with rolls breaking right now, I think. Usually it just kinda sits there and stays broken for several days. 

I think I would rather spend time improving the roller monitoring/reporting (which we need ANYWAY), if the main concern is having a stagnant roller.

I would just take any change to recipes.cfg as a cue to run the led/chromium builder. I don't think I'd spend any time trying to be smarter than that.