nix-daemon.nix: Add option nix.registry

This allows you to specify the system-wide flake registry. One use is
to pin 'nixpkgs' to the Nixpkgs version used to build the system:

  nix.registry.nixpkgs.flake = nixpkgs;

where 'nixpkgs' is a flake input. This ensures that commands like

  $ nix run nixpkgs#hello

pull in a minimum of additional store paths.

You can also use this to redirect flakes, e.g.

  nix.registry.nixpkgs.to = {
    type = "github";
    owner = "my-org";
    repo = "my-nixpkgs";
  };

1 files changed, 58 insertions, 0 deletions

diff --git a/nixos/modules/services/misc/nix-daemon.nix b/nixos/modules/services/misc/nix-daemon.nix
index 17c3582db0f..8203cb13e50 100644
--- a/nixos/modules/services/misc/nix-daemon.nix
+++ b/nixos/modules/services/misc/nix-daemon.nix
@@ -376,6 +376,59 @@ in
           If enabled (the default), checks that Nix can parse the generated nix.conf.
         '';
       };
+
+      registry = mkOption {
+        type = types.attrsOf (types.submodule (
+          let
+            inputAttrs = types.attrsOf (types.oneOf [types.str types.int types.bool types.package]);
+          in
+          { config, name, ... }:
+          { options = {
+              from = mkOption {
+                type = inputAttrs;
+                example = { type = "indirect"; id = "nixpkgs"; };
+                description = "The flake reference to be rewritten.";
+              };
+              to = mkOption {
+                type = inputAttrs;
+                example = { type = "github"; owner = "my-org"; repo = "my-nixpkgs"; };
+                description = "The flake reference to which <option>from></option> is to be rewritten.";
+              };
+              flake = mkOption {
+                type = types.unspecified;
+                default = null;
+                example = literalExample "nixpkgs";
+                description = ''
+                  The flake input to which <option>from></option> is to be rewritten.
+                '';
+              };
+              exact = mkOption {
+                type = types.bool;
+                default = true;
+                description = ''
+                  Whether the <option>from</option> reference needs to match exactly. If set,
+                  a <option>from</option> reference like <literal>nixpkgs</literal> does not
+                  match with a reference like <literal>nixpkgs/nixos-20.03</literal>.
+                '';
+              };
+            };
+            config = {
+              from = mkDefault { type = "indirect"; id = name; };
+              to = mkIf (config.flake != null)
+                ({ type = "path";
+                   path = config.flake.outPath;
+                 } // lib.filterAttrs
+                   (n: v: n == "lastModified" || n == "rev" || n == "revCount" || n == "narHash")
+                   config.flake);
+            };
+          }
+        ));
+        default = {};
+        description = ''
+          A system-wide flake registry.
+        '';
+      };
+
     };
 
   };
@@ -390,6 +443,11 @@ in
 
     environment.etc."nix/nix.conf".source = nixConf;
 
+    environment.etc."nix/registry.json".text = builtins.toJSON {
+      version = 2;
+      flakes = mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+    };
+
     # List of machines for distributed Nix builds in the format
     # expected by build-remote.pl.
     environment.etc."nix/machines" =