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.

tests/fetchGit.sh

@@ -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")

tests/fetchMercurial.sh

@@ -29,10 +29,17 @@ rev2=$(hg log --cwd $repo -r tip --template '{node}')
 path=$(nix eval --raw "(builtins.fetchMercurial file://$repo).outPath")
 [[ $(cat $path/hello) = world ]]
 
+# In pure eval mode, fetchGit without a revision should fail.
+[[ $(nix eval --raw "(builtins.readFile (fetchMercurial file://$repo + \"/hello\"))") = world ]]
+(! nix eval --pure-eval --raw "(builtins.readFile (fetchMercurial file://$repo + \"/hello\"))")
+
 # Fetch using an explicit revision hash.
 path2=$(nix eval --raw "(builtins.fetchMercurial { url = file://$repo; rev = \"$rev2\"; }).outPath")
 [[ $path = $path2 ]]
 
+# In pure eval mode, fetchGit with a revision should succeed.
+[[ $(nix eval --pure-eval --raw "(builtins.readFile (fetchMercurial { url = file://$repo; rev = \"$rev2\"; } + \"/hello\"))") = world ]]
+
 # Fetch again. This should be cached.
 mv $repo ${repo}-tmp
 path2=$(nix eval --raw "(builtins.fetchMercurial file://$repo).outPath")

tests/local.mk

@@ -20,7 +20,8 @@ nix_tests = \
   fetchMercurial.sh \
   signing.sh \
   run.sh \
-  brotli.sh
+  brotli.sh \
+  pure-eval.sh
   # parallel.sh
 
 install-tests += $(foreach x, $(nix_tests), tests/$(x))

tests/pure-eval.nix

@@ -0,0 +1,3 @@
+{
+  x = 123;
+}

tests/pure-eval.sh

@@ -0,0 +1,18 @@
+source common.sh
+
+clearStore
+
+nix eval --pure-eval '(assert 1 + 2 == 3; true)'
+
+[[ $(nix eval '(builtins.readFile ./pure-eval.sh)') =~ clearStore ]]
+
+(! nix eval --pure-eval '(builtins.readFile ./pure-eval.sh)')
+
+(! nix eval --pure-eval '(builtins.currentTime)')
+(! nix eval --pure-eval '(builtins.currentSystem)')
+
+(! nix-instantiate --pure-eval ./simple.nix)
+
+[[ $(nix eval "((import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; })).x)") == 123 ]]
+(! nix eval --pure-eval "((import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; })).x)")
+nix eval --pure-eval "((import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; sha256 = \"$(nix hash-file pure-eval.nix --type sha256)\"; })).x)"

tests/restricted.nix

@@ -0,0 +1 @@
+1 + 2

tests/restricted.sh

@@ -3,7 +3,8 @@ source common.sh
 clearStore
 
 nix-instantiate --restrict-eval --eval -E '1 + 2'
-(! nix-instantiate --restrict-eval ./simple.nix)
+(! nix-instantiate --restrict-eval ./restricted.nix)
+(! nix-instantiate --eval --restrict-eval <(echo '1 + 2'))
 nix-instantiate --restrict-eval ./simple.nix -I src=.
 nix-instantiate --restrict-eval ./simple.nix -I src1=simple.nix -I src2=config.nix -I src3=./simple.builder.sh
 
@@ -28,3 +29,10 @@ nix eval --raw "(builtins.fetchurl file://$(pwd)/restricted.sh)" --restrict-eval
 (! nix eval --raw "(builtins.fetchurl https://github.com/NixOS/patchelf/archive/master.tar.gz)" --restrict-eval)
 (! nix eval --raw "(builtins.fetchTarball https://github.com/NixOS/patchelf/archive/master.tar.gz)" --restrict-eval)
 (! nix eval --raw "(fetchGit git://github.com/NixOS/patchelf.git)" --restrict-eval)
+
+ln -sfn $(pwd)/restricted.nix $TEST_ROOT/restricted.nix
+[[ $(nix-instantiate --eval $TEST_ROOT/restricted.nix) == 3 ]]
+(! nix-instantiate --eval --restrict-eval $TEST_ROOT/restricted.nix)
+(! nix-instantiate --eval --restrict-eval $TEST_ROOT/restricted.nix -I $TEST_ROOT)
+(! nix-instantiate --eval --restrict-eval $TEST_ROOT/restricted.nix -I .)
+nix-instantiate --eval --restrict-eval $TEST_ROOT/restricted.nix -I $TEST_ROOT -I .