nixos/activation/bootspec: init bootspec support (RFC-0125)

.github/CODEOWNERS

@@ -78,6 +78,7 @@
 /nixos/doc/manual/man-nixos-option.xml                @nbp
 /nixos/modules/installer/tools/nixos-option.sh        @nbp
 /nixos/modules/system                                 @dasJ
+/nixos/modules/system/activation/bootspec.nix         @grahamc @cole-h
 
 # NixOS integration test driver
 /nixos/lib/test-driver  @tfc

nixos/modules/system/activation/bootspec.nix

@@ -0,0 +1,50 @@
+# Note that these schemas are defined by RFC-0125.
+# This document is considered a stable API, and is depended upon by external tooling.
+# Changes to the structure of the document, or the semantics of the values should go through an RFC.
+#
+# See: https://github.com/NixOS/rfcs/pull/125
+{ config, pkgs, lib, children }:
+let
+  schemas = {
+    v1 = rec {
+      filename = "boot.v1.json";
+      json =
+        pkgs.writeText filename
+          (builtins.toJSON
+            {
+              schemaVersion = 1;
+
+              kernel = "${config.boot.kernelPackages.kernel}/${config.system.boot.loader.kernelFile}";
+              kernelParams = config.boot.kernelParams;
+              initrd = "${config.system.build.initialRamdisk}/${config.system.boot.loader.initrdFile}";
+              initrdSecrets = "${config.system.build.initialRamdiskSecretAppender}/bin/append-initrd-secrets";
+              label = "NixOS ${config.system.nixos.codeName} ${config.system.nixos.label} (Linux ${config.boot.kernelPackages.kernel.modDirVersion})";
+
+              specialisation = lib.mapAttrs
+                (childName: childToplevel: {
+                  bootspec = "${childToplevel}/${filename}";
+                })
+                children;
+            });
+
+      generator = ''
+        ${pkgs.jq}/bin/jq '
+          .toplevel = $toplevel |
+          .init = $init
+          ' \
+          --sort-keys \
+          --arg toplevel "$out" \
+          --arg init "$out/init" \
+          < ${json} \
+          > $out/${filename}
+      '';
+    };
+  };
+in
+{
+  # This will be run as a part of the `systemBuilder` in ./top-level.nix. This
+  # means `$out` points to the output of `config.system.build.toplevel` and can
+  # be used for a variety of things (though, for now, it's only used to report
+  # the path of the `toplevel` itself and the `init` executable).
+  writer = schemas.v1.generator;
+}

nixos/modules/system/activation/top-level.nix

@@ -9,6 +9,14 @@ let
         "${config.system.boot.loader.kernelFile}";
       initrdPath = "${config.system.build.initialRamdisk}/" +
         "${config.system.boot.loader.initrdFile}";
+
+      bootSpec = import ./bootspec.nix {
+        inherit
+          config
+          pkgs
+          lib
+          children;
+      };
     in ''
       mkdir $out
 
@@ -79,6 +87,8 @@ let
 
       echo -n "${toString config.system.extraDependencies}" > $out/extra-dependencies
 
+      ${bootSpec.writer}
+
       ${config.system.extraSystemBuilderCmds}
     '';