feat: add env_logger

[olebedev]

Hi there 👋 First I want to say thank you for such a good project, this is a big deal when we talk about extensive use of nix shells.

Currently, the executable prints something like:

cached-nix-shell: updating cache
cached-nix-shell: done in 8.912511625s

And an interactive shell session being restored in macOS, which the system-wide bash will let users know. It looks like:

Restore session: ...

Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.

Which defers from the original behavior.

Motivation

To make the cached-nix-shell a full drop-in-replacement it requires that all the standard errors, outputs and inputs are equivalent to the original program.

In this PR

I intended to put the cached-nix-shell specific logging into stderr, as well as any other logging, behind an environment variable so by default it doesn't add anything extra. But it's still possible to print that information when needed by exporting CNS_LOG_LEVEL=trace, for example. In this PR I introduce a standardized way to for logging.

---

To make this a true drop-in-replacement more work is needed in make the cached-nix-shell bound to dependencies coming from /nix/store instead of exploring users' environment, like is done here.

Happy to chat about implementation details, about the approach in general and update my changes if we decided to, please let me know what do you think!

Kind regards, Oleg

cc @fwouts, @uri-canva

[xzfc]

To make this a true drop-in-replacement more work is needed in make the cached-nix-shell bound to dependencies coming from /nix/store instead of exploring users' environment, like is done here.

Yes, pinning $PATH to /nix/store paths would be helpful to reduce cache misses.

However, I'm not sure how it's related to "true drop-in-replacement" since the nix-shell itself uses these dependencies from the environment, and what CNS does is remove everything from the $PATH except things that nix-shell uses. E.g., if we remove git from the $PATH, then the upstream nix-shell would fail.

$ rm -rf ~/.cache/nix
$ PATH= $(which nix-shell) -p 'import (fetchGit https://github.com/xzfc/cached-nix-shell) {}'
error: executing 'git': No such file or directory
warning: could not read HEAD ref from repo at 'https://github.com/xzfc/cached-nix-shell', using 'master'
error: executing 'git': No such file or directory
error:
       … while calling the 'derivationStrict' builtin

         at /builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'shell'
         whose name attribute is located at /nix/store/n3hlkkgv04bfikw5kxc1694spmg1rjih-nixos/nixos/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'buildInputs' of derivation 'shell'

         at /nix/store/n3hlkkgv04bfikw5kxc1694spmg1rjih-nixos/nixos/pkgs/stdenv/generic/make-derivation.nix:395:7:

          394|       depsHostHost                = elemAt (elemAt dependencies 1) 0;
          395|       buildInputs                 = elemAt (elemAt dependencies 1) 1;
             |       ^
          396|       depsTargetTarget            = elemAt (elemAt dependencies 2) 0;

       error: program 'git' failed with exit code 1

[olebedev]

However, I'm not sure how it's related to "true drop-in-replacement" since the nix-shell itself uses these dependencies from the environment, and what CNS does is remove everything from the $PATH except things that nix-shell uses. E.g., if we remove git from the $PATH, then the upstream nix-shell would fail.

Ah, yeah, you're right. Nix depends on system-wide git. By "true drop-in-replacement" I mean make the tool behave and print outputs similar to what would vanilla nix-shell do. But it's obviously not totally possible and not even needed, for example in case of some unpinned dependencies. Also, if we don't evaluate expressions all the time we also missing a lot of similarity with the vanilla nix-shell and this is totally fine!

I don't want to replicate everything but rather do my best to make the cached-nix-shell be a a tool that I can seamlessly switch to and see that most (all 🙏) of our workflows at @canva won't fail and benefit from caching. We have a quite huge use of nix shells so it's important to have a tool that does the caching magic as this one along with mimic to the vanilla behavior up to certain extent but not totally :)

[olebedev]

Would supporting --quiet / --verbose options in CNS (with the same semantics as in the nix-shell) solve your case? I.e., keep the "updating cache" message by default, but suppress them if the --quiet option is given.

An explicit CLI argument wouldn't help much since it requires changes to entire codebase to respect that so the codebase become incompatible with the vanilla nix-shell which I don't won't to do. Also, we have cases where we use nested nix shells. In this case we would want to propagate the option all the way down to the depth, which is not quite possible without code change at the only top level.

I would rather go with an env var to reach similar behaviour as you propose. That sounds good to me as a starting point, so I made the change that doesn't suppress the messages and they ar eprinted as they were before the change, but now I can disable them by setting CNS_LOG_LEVEL=, which is not ideal but pretty much acceptable for my use case.

[olebedev]

The nix-shell itself is not silent by default e.g., it prints to stderr during building or fetching paths from the internet.

100%, as long as I capture stderr and stdout it's crucial for me to keep the output similar to what the nix-shell itself produces so the tooling around it can survive substitution of nix-shell with cached-nix-shell.

[olebedev]

Hey @xzfc, any chance you can have another look at this work?