ComputedFn (and createTransformer) can leak memory in certain scenarios

[realyze]

computedFn uses _isComputingDerivation() to determine whether it should cache or not. However, when getting value of a computed from within an action, _isComputingDerivation() will return true and computedFn will cache the arguments it's called with. But since we're in an action, onBecomeUnobserved will never get called to evict the arguments from the cache.

Here's the smallest reproducible scenario I can come up with: https://codesandbox.io/s/computedfn-memory-leak-5xvsg6

I think it's similar to #116 (which has been mostly—but apparently not completely—fixed by #228).

Should there be a more reliable way than _isComputingDerivation() to determine whether we're running inside an action?

[urugator]

But since we're in an action, onBecomeUnobserved will never get called to evict the arguments from the cache.

Could also relate to this mobxjs/mobx#2309 (comment)

determine whether we're running inside an action?

I think you can (and probably should) do it the other way around - by checking you don't run in Reaction, in which case globalState.trackingContext === null IIRC.

Or you may consider caching the value for the duration of the batch, like computed does, but I don't know how you would go about that.