Don't download/fetch if the output folder is already up to date

[bweggersen]

Currently, backfill will create a hash and then try to download the content belonging to that hash. In many cases, this is unnecessary as the local output folder might already be up to date. Is there a way to figure out whether the output folder is up to date and then skip fetching?

We could create a local mapping between the hash of the output-folder and the hash of the package. Then, on a later run, after we have created a hash of the package, we could create a hash of the output folder and then do a lookup to see if the output folder hash points to the package hash. In which case we don't have to even do a fetch.

This problem might be related to the clearOutputFolder config option. This value is false by default, which means the output folder doesn't get cleared between runs and between scripts that might be outputting to the same output folder in the same package (i.e. prepare and build). It is possible that hashing the output folder might not work well, as the output folder might change too often.