system-version: add system.stateVersion option

default.nix

@@ -31,6 +31,7 @@ let
         ./modules/system/applications.nix
         ./modules/system/etc.nix
         ./modules/system/launchd.nix
+        ./modules/system/version.nix
         ./modules/time
         ./modules/networking
         ./modules/nix

modules/system/default.nix

@@ -42,11 +42,6 @@ in
       '';
     };
 
-    system.darwinLabel = mkOption {
-      type = types.str;
-      default = pkgs.lib.nixpkgsVersion;
-    };
-
     assertions = mkOption {
       type = types.listOf types.unspecified;
       internal = true;

modules/system/version.nix

@@ -0,0 +1,76 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.system;
+
+  gitCommitId = lib.substring 0 7 (commitIdFromGitRepo gitRepo);
+  gitRepo = "${toString pkgs.path}/.git";
+  releaseFile = "${toString pkgs.path}/.version";
+  revisionFile = "${toString pkgs.path}/.git-revision";
+  suffixFile = "${toString pkgs.path}/.version-suffix";
+in
+
+{
+  options = {
+    system.stateVersion = mkOption {
+      type = types.int;
+      default = 2;
+      description = ''
+        Every once in a while, a new NixOS release may change
+        configuration defaults in a way incompatible with stateful
+        data. For instance, if the default version of PostgreSQL
+        changes, the new version will probably be unable to read your
+        existing databases. To prevent such breakage, you can set the
+        value of this option to the NixOS release with which you want
+        to be compatible. The effect is that NixOS will option
+        defaults corresponding to the specified release (such as using
+        an older version of PostgreSQL).
+      '';
+    };
+
+    system.darwinLabel = mkOption {
+      type = types.str;
+      default = cfg.nixpkgsVersion;
+      description = "Label to be used in the names of generated outputs.";
+    };
+
+    system.nixpkgsRelease = mkOption {
+      readOnly = true;
+      type = types.str;
+      default = fileContents releaseFile;
+      description = "The nixpkgs release (e.g. <literal>16.03</literal>).";
+    };
+
+    system.nixpkgsVersion = mkOption {
+      internal = true;
+      type = types.str;
+      description = "The full nixpkgs version (e.g. <literal>16.03.1160.f2d4ee1</literal>).";
+    };
+
+    system.nixpkgsVersionSuffix = mkOption {
+      internal = true;
+      type = types.str;
+      default = "pre-git";
+      description = "The nixpkgs version suffix (e.g. <literal>1160.f2d4ee1</literal>).";
+    };
+
+    system.nixpkgsRevision = mkOption {
+      internal = true;
+      type = types.str;
+      default = "master";
+      description = "The nixpkgs git revision from which this configuration was built.";
+    };
+  };
+
+  config = {
+
+    # These defaults are set here rather than up there so that
+    # changing them would not rebuild the manual
+    system.nixpkgsVersion = mkDefault (cfg.nixpkgsRelease + cfg.nixpkgsVersionSuffix);
+    system.nixpkgsRevision = mkIf (pathIsDirectory gitRepo) (mkDefault gitCommitId);
+    system.nixpkgsVersionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
+
+  };
+}