Add pure evaluation mode

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.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index af72f7b1e..81bb24a4e 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -232,6 +232,9 @@ public:
         "Whether to restrict file system access to paths in $NIX_PATH, "
         "and network access to the URI prefixes listed in 'allowed-uris'."};
 
+    Setting<bool> pureEval{this, false, "pure-eval",
+        "Whether to restrict file system and network access to files specified by cryptographic hash."};
+
     Setting<size_t> buildRepeat{this, 0, "repeat",
         "The number of times to repeat a build in order to verify determinism.",
         {"build-repeat"}};