refactor(build-image): Extract package set loading into helper

Some upcoming changes might require the Nix build to be split into
multiple separate nix-build invocations of different expressions, thus
splitting this out is useful.

It also fixes an issue where `build-image/default.nix` might be called
in an environment where no Nix channels are configured.

1 files changed, 73 insertions, 0 deletions

diff --git a/tools/nixery/build-image/load-pkgs.nix b/tools/nixery/build-image/load-pkgs.nix
new file mode 100644
index 0000000000..3e8b450c45
--- /dev/null
+++ b/tools/nixery/build-image/load-pkgs.nix
@@ -0,0 +1,73 @@
+# Copyright 2019 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Load a Nix package set from a source specified in one of the following
+# formats:
+#
+# 1. nixpkgs!$channel (e.g. nixpkgs!nixos-19.03)
+# 2. git!$repo!$rev (e.g. git!git@github.com:NixOS/nixpkgs.git!master)
+# 3. path!$path (e.g. path!/var/local/nixpkgs)
+#
+# '!' was chosen as the separator because `builtins.split` does not
+# support regex escapes and there are few other candidates. It
+# doesn't matter much because this is invoked by the server.
+{ pkgSource, args ? { } }:
+
+with builtins;
+let
+  # If a nixpkgs channel is requested, it is retrieved from Github (as
+  # a tarball) and imported.
+  fetchImportChannel = channel:
+    let
+      url =
+        "https://github.com/NixOS/nixpkgs-channels/archive/${channel}.tar.gz";
+    in import (fetchTarball url) args;
+
+  # If a git repository is requested, it is retrieved via
+  # builtins.fetchGit which defaults to the git configuration of the
+  # outside environment. This means that user-configured SSH
+  # credentials etc. are going to work as expected.
+  fetchImportGit = url: rev:
+    let
+      # builtins.fetchGit needs to know whether 'rev' is a reference
+      # (e.g. a branch/tag) or a revision (i.e. a commit hash)
+      #
+      # Since this data is being extrapolated from the supplied image
+      # tag, we have to guess if we want to avoid specifying a format.
+      #
+      # There are some additional caveats around whether the default
+      # branch contains the specified revision, which need to be
+      # explained to users.
+      spec = if (stringLength rev) == 40 then {
+        inherit url rev;
+      } else {
+        inherit url;
+        ref = rev;
+      };
+    in import (fetchGit spec) args;
+
+  # No special handling is used for paths, so users are expected to pass one
+  # that will work natively with Nix.
+  importPath = path: import (toPath path) args;
+
+  source = split "!" pkgSource;
+  sourceType = elemAt source 0;
+in if sourceType == "nixpkgs" then
+  fetchImportChannel (elemAt source 2)
+else if sourceType == "git" then
+  fetchImportGit (elemAt source 2) (elemAt source 4)
+else if sourceType == "path" then
+  importPath (elemAt source 2)
+else
+  throw ("Invalid package set source specification: ${pkgSource}")