flake: disable eval cache on local repos #12102
Conversation
Eval cache is useless when the cache key is the content of the repo, and the repo is being actively worked on. This is a first change towards not copying the local repo to the /nix/store on every eval.
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What happens in case directory names become part of the evaluation and make evaluation between two different machines not reproducible? |
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This can be solved by extending the string context to add non-store paths. Then have the derivation complain/abort if any of those are leaking into the build. |
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Disabling the eval cache does nothing, since (without lazy trees) the tree is always copied to the store, regardless of eval caching. Note that since #11992, calculating a fingerprint for dirty Git trees is pretty cheap and doesn't read the entire repo. For non-dirty Git trees, it was already cheap.
How do you propose to do that without lazy trees while still maintaining hermetic evaluation and not changing any evaluation results? |
Motivation
The most common complaint about flakes is that it copies the repo to the
/nix/store on each eval. This is especially annoying on large monorepos where
copying multiple GiB to the /nix/store outweighs whatever gains the eval cache
could bring.
Context
To solve this, turn off eval caching on local repositories.
I am aware of the lazy tree PR. This is a much simpler solution.
The current PR only implements 1/2 of the solution and turns off the eval
cache. The second change will be to stop copying the paths to the store.
Making this change will also automatically solve:
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