Explain more concretely the failure scenarios

[nh2]

From the README:

builtins.fetchurl or other network builtins are used (e.g. in nixpkgs-mozilla)

Can we have a concrete example in the README that explains what kind of change it wouldn't catch?

Thanks!

[xzfc]

In nix, builtins.fetchurl caches files for tarball-ttl seconds, default is 3600. C-n-s checks only local files, but do not check if tarball-ttl is expired.

This issue is only related to live URLs, e.g. nixpkgs-mozilla fetches channel-rust-nightly.toml which is updated daily.

Note that c-n-s stores a single cache entry per nix-shell invocation, and stale cache for cached-nix-shell ./foo.nix won't affect cache for cached-nix-shell ./bar.nix.

---

# 1.nix
let pkgs = import <nixpkgs> { };
in pkgs.mkShell {
  date = builtins.elemAt (builtins.split "\n" (builtins.readFile
    (builtins.fetchurl
      "https://static.rust-lang.org/dist/channel-rust-nightly.toml"))) 2;
}

nix-shell:

$ nix-shell ./1.nix --run 'echo $date'
date = "2020-12-25"

$ sleep 86400

$ nix-shell ./1.nix --run 'echo $date'
date = "2020-12-26"

cached-nix-shell:

$ cached-nix-shell ./1.nix --run 'echo $date'
date = "2020-12-25"

$ sleep 86400

$ cached-nix-shell ./1.nix --run 'echo $date' # will be cached forever, unless file contents changes
date = "2020-12-25"

$ echo "# something" >> ./1.nix  # file update invalidate the cache

$ cached-nix-shell ./1.nix --run 'echo $date'
date = "2020-12-26"

[nh2]

@xzfc That's great, thanks a lot. Could you add it to the README (or maybe link to a separate .md file if it's too long)?

This is much less constraining than I thought, so I think it'd be very good for everyone to know that.