refactor(server): Move package source management logic to server

Introduces three new types representing each of the possible package
sources and moves the logic for specifying the package source to the
server.

Concrete changes:

* Determining whether a specified git reference is a commit vs. a
  branch/tag is now done in the server, and is done more precisely by
  using a regular expression.

* Package sources now have a new `CacheKey` function which can be used
  to retrieve a key under which a build manifest can be cached *if*
  the package source is not a moving target (i.e. a full git commit
  hash of either nixpkgs or a private repository).

  This function is not yet used.

* Users *must* now specify a package source, Nixery no longer defaults
  to anything and will fail to launch if no source is configured.

2 files changed, 16 insertions, 43 deletions

diff --git a/tools/nixery/build-image/default.nix b/tools/nixery/build-image/default.nix
index 0d3002cb404e..6b1cea6f0ca2 100644
--- a/tools/nixery/build-image/default.nix
+++ b/tools/nixery/build-image/default.nix
@@ -20,11 +20,12 @@
 
   # Because of the insanity occuring below, this function must mirror
   # all arguments of build-image.nix.
-, pkgSource ? "nixpkgs!nixos-19.03"
+, srcType ? "nixpkgs"
+, srcArgs ? "nixos-19.03"
 , tag ? null, name ? null, packages ? null, maxLayers ? null
 }@args:
 
-let pkgs = import ./load-pkgs.nix { inherit pkgSource; };
+let pkgs = import ./load-pkgs.nix { inherit srcType srcArgs; };
 in with pkgs; rec {
 
   groupLayers = buildGoPackage {
diff --git a/tools/nixery/build-image/load-pkgs.nix b/tools/nixery/build-image/load-pkgs.nix
index 3e8b450c45d2..cceebfc14dae 100644
--- a/tools/nixery/build-image/load-pkgs.nix
+++ b/tools/nixery/build-image/load-pkgs.nix
@@ -12,17 +12,9 @@
 # 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 ? { } }:
+# Load a Nix package set from one of the supported source types
+# (nixpkgs, git, path).
+{ srcType, srcArgs, importArgs ? { } }:
 
 with builtins;
 let
@@ -32,42 +24,22 @@ let
     let
       url =
         "https://github.com/NixOS/nixpkgs-channels/archive/${channel}.tar.gz";
-    in import (fetchTarball url) args;
+    in import (fetchTarball url) importArgs;
 
   # 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;
+  fetchImportGit = spec: import (fetchGit spec) importArgs;
 
   # 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)
+  importPath = path: import (toPath path) importArgs;
+in if srcType == "nixpkgs" then
+  fetchImportChannel srcArgs
+else if srcType == "git" then
+  fetchImportGit (fromJSON srcArgs)
+else if srcType == "path" then
+  importPath srcArgs
 else
-  throw ("Invalid package set source specification: ${pkgSource}")
+  throw ("Invalid package set source specification: ${srcType} (${srcArgs})")