Filecache is not safe for concurrent use and can lead to broken files

[hlubek]

We sometimes experience this error when using Devbox:

Error: invalid character '"' after top-level value

The system is not usable after this. We debugged the issue and found broken JSON files in the filecache to be the culprit.

It seems the filecache is prone to break with concurrent writes. It should use proper locking to:

• prevent reading incomplete cache entries
• exclusively writing cache entries so files are not interleaved / wrongly truncated