Impurity: Hash of relative paths depends on parent directory name

[rickynils]

Consider the following Nix expression, stored in default.nix:

with (import <nixpkgs> {});

writeText "out" ''
  ${./.}
''

Then look at the following console session:

/tmp/nixtest $ nix-build --no-out-link
these derivations will be built:
  /nix/store/wvarg2d7f0dhfgz0wvwhm8ibzkb1pfrc-out.drv
building path(s) `/nix/store/1664687s61dyki8b1pqw8fa1k2314pwz-out'
�building /nix/store/1664687s61dyki8b1pqw8fa1k2314pwz-out
�/nix/store/1664687s61dyki8b1pqw8fa1k2314pwz-out
/tmp/nixtest $ cd ..
/tmp $ mv nixtest/ nixtest2
/tmp $ cd nixtest2/
/tmp/nixtest2 $ nix-build --no-out-link
these derivations will be built:
  /nix/store/piwxsvbn6myy4xajazg0pwlh7k46sfzq-out.drv
building path(s) `/nix/store/4z5vzja4gk0mifrn4dxd2d17fwfsy86w-out'
�building /nix/store/4z5vzja4gk0mifrn4dxd2d17fwfsy86w-out
�/nix/store/4z5vzja4gk0mifrn4dxd2d17fwfsy86w-out
/tmp/nixtest2 $ cd ..
/tmp $ mv nixtest2/ nixtest
/tmp $ cd nixtest/
/tmp/nixtest $ nix-build --no-out-link
/nix/store/1664687s61dyki8b1pqw8fa1k2314pwz-out

As you can see, the hash of the build depends on the name of directory that contains default.nix. I had expected that only the files and directories contained inside this directory would matter. This is an impurity, I would say, since you can't recreate a build if you don't know the name of the directory that the build was originally performed in.

[edolstra]

That's not really an impurity. As long as the name and contents of the file are the same, you should get the same hash. That the name might change is simply a reason not to use ./. in this case.

[rickynils]

But the parent directory is not really part of the content, I think. If I clone Nix sources from github, should it really impact my build what I named the target directory?

My example may seem contrived, but I discovered this when building NixOS. I keep different NixOS configurations in the following directory layout:

/.../machine1/default.nix
/.../machine1/configuration.nix
/.../machine1/nixos
/.../machine1/nixpkgs
/.../machine2/default.nix
/.../machine2/configuration.nix
/.../machine2/nixos
/.../machine2/nixpkgs

My default.nix files contains the following:

let
  nixos = import ./nixos {
    nixpkgs = ./nixpkgs;
    configuration = ./configuration.nix;
  };
in {
  inherit (nixos) system;
}

Then I can build a system in the following way:

cd machine1
nix-build -A system

But if I change the names of the machine1 or machine2 directories, I get a rebuild even though nothing that should have an impact on the build has changed.

[edolstra]

Closing since this is intended (if not always desirable) behaviour.