Evaluation dependence on external tools: docs, purity, fallbacks

[dtzWill]

Various functionality provided by Nix tools and builtins relies on the presence and "correct" functionality of external tools.

This seems... unfortunate 😇 .

The biggest issue is one of simple usability: evaluation failures due to missing tools are a bad experience and it's not always obvious, trivial, or convenient to manipulate the environment of the Nix-based process you're interacting with to properly resolve them.

As an example, nix-based services (such as the auto-upgrader) will fail to evaluate expressions using builtins.fetchGit and fixing it requires modifying the service configuration to add required tools to the service's PATH.

Additional related issues:

• The full list of tools required to make use of builtin Nix functionality is not available AFAICT (although this is easily fixed)
• Reliance on these tools makes Nix evaluation "brittle" in a very (IMHO) non-Nix way--consider attempting to use Nix itself in a sandbox.
• Reliability and purity: tools found may have different behavior than those expected and tested against (newer, older, bugged, malicious replacements).

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I'm not sure of the best solution here, especially considering concerns about closure size and ease of deployment (being self-contained is rather useful here).

It's not perfect but best proposal I have is to replace all external programs with use of (mandatory?) libraries.

Thoughts? :)

[lheckemann]

As a sort of compromise, maybe we could produce wrappers for nix that set add the appropriate tools to PATH, and make the default nix in nixpkgs such a wrapped one? This would allow a reduced closure size while enabling all the functionality to work by default.

[copumpkin]

I submitted this a while back, which could be relevant: #1345

[copumpkin]

One thing that might be nice is a first-class building block for optional external dependencies in Nix code, that would let us enumerate them and detect early if they're available or not, to emit warnings/errors.

[stale]

I marked this as stale due to inactivity. → More info