Check if the file read/write in the dependency cache can cause issues in the solver

[mpizenberg]

The solver read and writes files from a cache in the elm home. We need to check if many parallel calls cannot create problems. And in such case, check if using atomic read/writes can solve this problem, or if something else is needed.

[lydell]

I ran a few parallel invocations, with no elm home and no elm-stuff and couldn’t cause it, which is good at least.

If concurrent writes is a problem, we should end up with a jumbled JSON file, right? I simulated that by manually editing the JSON file to be invalid. elm-test then immediately exits (but only if it actually needs to read the cache, depending on your elm-stuff):

$ elm-test

Failed to parse the cache file /Users/lydell/.elm/pubgrub/versions_cache.json.
Unexpected token : in JSON at position 23

So if this happens, there’s a chance users will report it. It’s good that it’s not a silent error we would never have noticed!

Maybe fs.writeFileSync as we have now is good enough until we hear otherwise, and we should be good to go to release?

[mpizenberg]

Thanks a lot for checking this @lydell .
It should be good to go then IMO. And that also means @jfmengels can take advantage of reusing the same cache if he ever wants to also use that dependency solver in elm-review.