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Add pure evaluation mode

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In this mode, the following restrictions apply:

* The builtins currentTime, currentSystem and storePath throw an
  error.

* $NIX_PATH and -I are ignored.

* fetchGit and fetchMercurial require a revision hash.

* fetchurl and fetchTarball require a sha256 attribute.

* No file system access is allowed outside of the paths returned by
  fetch{Git,Mercurial,url,Tarball}. Thus 'nix build -f ./foo.nix' is
  not allowed.

Thus, the evaluation result is completely reproducible from the
command line arguments. E.g.

  nix build --pure-eval '(
    let
      nix = fetchGit { url = https://github.com/NixOS/nixpkgs.git; rev = "9c927de4b179a6dd210dd88d34bda8af4b575680"; };
      nixpkgs = fetchGit { url = https://github.com/NixOS/nixpkgs.git; ref = "release-17.09"; rev = "66b4de79e3841530e6d9c6baf98702aa1f7124e4"; };
    in (import (nix + "/release.nix") { inherit nix nixpkgs; }).build.x86_64-linux
  )'

The goal is to enable completely reproducible and traceable
evaluation. For example, a NixOS configuration could be fully
described by a single Git commit hash. 'nixos-rebuild' would do
something like

  nix build --pure-eval '(
    (import (fetchGit { url = file:///my-nixos-config; rev = "..."; })).system
  ')

where the Git repository /my-nixos-config would use further fetchGit
calls or Git externals to fetch Nixpkgs and whatever other
dependencies it has. Either way, the commit hash would uniquely
identify the NixOS configuration and allow it to reproduced.
This commit is contained in:
Eelco Dolstra 2018-01-16 18:50:38 +01:00
parent 23fa7e3606
commit d4dcffd643
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GPG key ID: 8170B4726D7198DE
19 changed files with 159 additions and 53 deletions
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7
tests/fetchGit.sh
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@ -29,10 +29,17 @@ rev2=$(git -C $repo rev-parse HEAD)
path=$(nix eval --raw "(builtins.fetchGit file://$repo).outPath")
[[ $(cat $path/hello) = world ]]
# In pure eval mode, fetchGit without a revision should fail.
[[ $(nix eval --raw "(builtins.readFile (fetchGit file://$repo + \"/hello\"))") = world ]]
(! nix eval --pure-eval --raw "(builtins.readFile (fetchGit file://$repo + \"/hello\"))")
# Fetch using an explicit revision hash.
path2=$(nix eval --raw "(builtins.fetchGit { url = file://$repo; rev = \"$rev2\"; }).outPath")
[[ $path = $path2 ]]
# In pure eval mode, fetchGit with a revision should succeed.
[[ $(nix eval --pure-eval --raw "(builtins.readFile (fetchGit { url = file://$repo; rev = \"$rev2\"; } + \"/hello\"))") = world ]]
# Fetch again. This should be cached.
mv $repo ${repo}-tmp
path2=$(nix eval --raw "(builtins.fetchGit file://$repo).outPath")