Make Evaluator Closeable

[alexarchambault]

Cleaning-up the worker cache upon close

[alexarchambault]

I decided to open a PR for that for #3579 (#3579 (comment) in particular), but I'm now realizing this might not be needed for it after all. Don't know what you think of that, I think it's a good idea to clean up things when the Evaluator isn't needed anymore.

[alexarchambault]

(This makes Evaluator extends AutoCloseable, so this breaks bin compat…)

[lihaoyi]

I'm not sure this is the right thing to do. Right now in production, the workerCache is this long lived thing that is shared across main0 invocations, and I think can out-live the individual evaluator that created it to be used in the next Mill command's evaluator. If that's the case, then having the workerCache be closed together with the evaluator at end of life would be wrong.

That code around sharing stuff across main0 commands is pretty gnarly, but that's what I last remember it being like

[alexarchambault]

It seems this works at the end (CI is green), as in the watch loop, the previous RunnerState isn't closed, so that underlying Evaluators aren't either, and workerCaches are kept intact and can keep being used. But I don't think I did that on purpose, that's more an oversight 😅

[lihaoyi]

Yeah that's why it's green, and then the next question is more what should the logic be here? I don't actually know, but although the current logic of injecting the workerCache into the Evaluator is pretty weird and roundabout, I'm inclined to leave it for now. I suspect that refactoring things enough to do things "properly" will require breaking bincompat, so we should wait until after 0.12.0 has landed and we're working towards 0.13.x and have the flexibility to make more sweeping changes without being restricted by bincompat requirements