nixos/…/kexec-boot.nix: move into netboot.nix, rename to kexecTree

`nixos/modules/installer/kexec/kexec-boot.nix` doesn't contain any
custom NixOS config, other than importing `netboot-minimal.nix` (which
imports `netboot-base.nix`, which imports `netboot.nix`.

`netboot.nix` really is just describing a self-contained system config,
running entirely off kernel and initrd, so we might as well move the
kexec script generation there as well.

`netboot.nix` already contains some `system.build` attributes.
Provide a `system.build.kexecTree` attribute (and `kexecScript` for
composability).

nixos/modules/installer/kexec/kexec-boot.nix

@@ -1,51 +0,0 @@
-# This module exposes a config.system.build.kexecBoot attribute,
-# which returns a directory with kernel, initrd and a shell script
-# running the necessary kexec commands.
-
-# It's meant to be scp'ed to a machine with working ssh and kexec binary
-# installed.
-
-# This is useful for (cloud) providers where you can't boot a custom image, but
-# get some Debian or Ubuntu installation.
-
-{ pkgs
-, modulesPath
-, config
-, ...
-}:
-{
-  imports = [
-    (modulesPath + "/installer/netboot/netboot-minimal.nix")
-  ];
-
-  config = {
-    system.build.kexecBoot =
-      let
-        kexecScript = pkgs.writeScript "kexec-boot" ''
-          #!/usr/bin/env bash
-          if ! kexec -v >/dev/null 2>&1; then
-            echo "kexec not found: please install kexec-tools" 2>&1
-            exit 1
-          fi
-          SCRIPT_DIR=$( cd -- "$( dirname -- "''${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
-          kexec --load ''${SCRIPT_DIR}/bzImage \
-            --initrd=''${SCRIPT_DIR}/initrd.gz \
-            --command-line "init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}"
-          kexec -e
-        ''; in
-      pkgs.linkFarm "kexec-tree" [
-        {
-          name = "initrd.gz";
-          path = "${config.system.build.netbootRamdisk}/initrd";
-        }
-        {
-          name = "bzImage";
-          path = "${config.system.build.kernel}/${config.system.boot.loader.kernelFile}";
-        }
-        {
-          name = "kexec-boot";
-          path = kexecScript;
-        }
-      ];
-  };
-}

nixos/modules/installer/netboot/netboot.nix

@@ -101,6 +101,37 @@ with lib;
       boot
     '';
 
+    # A script invoking kexec on ./bzImage and ./initrd.gz.
+    # Usually used through system.build.kexecTree, but exposed here for composability.
+    system.build.kexecScript = pkgs.writeScript "kexec-boot" ''
+      #!/usr/bin/env bash
+      if ! kexec -v >/dev/null 2>&1; then
+        echo "kexec not found: please install kexec-tools" 2>&1
+        exit 1
+      fi
+      SCRIPT_DIR=$( cd -- "$( dirname -- "''${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+      kexec --load ''${SCRIPT_DIR}/bzImage \
+        --initrd=''${SCRIPT_DIR}/initrd.gz \
+        --command-line "init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}"
+      kexec -e
+    '';
+
+    # A tree containing initrd.gz, bzImage and a kexec-boot script.
+    system.build.kexecTree = pkgs.linkFarm "kexec-tree" [
+      {
+        name = "initrd.gz";
+        path = "${config.system.build.netbootRamdisk}/initrd";
+      }
+      {
+        name = "bzImage";
+        path = "${config.system.build.kernel}/${config.system.boot.loader.kernelFile}";
+      }
+      {
+        name = "kexec-boot";
+        path = config.system.build.kexecScript;
+      }
+    ];
+
     boot.loader.timeout = 10;
 
     boot.postBootCommands =

nixos/tests/kexec.nix

@@ -18,8 +18,7 @@ import ./make-test-python.nix ({ pkgs, lib, ... }: {
       virtualisation.vlans = [ ];
       environment.systemPackages = [ pkgs.hello ];
       imports = [
-        "${modulesPath}/installer/kexec/kexec-boot.nix"
-        "${modulesPath}/profiles/minimal.nix"
+        "${modulesPath}/installer/netboot/netboot-minimal.nix"
       ];
     };
   };
@@ -33,14 +32,14 @@ import ./make-test-python.nix ({ pkgs, lib, ... }: {
     node1.connect()
     node1.wait_for_unit("multi-user.target")
 
-    # Check if the machine with kexec-boot.nix profile boots up
+    # Check if the machine with netboot-minimal.nix profile boots up
     node2.wait_for_unit("multi-user.target")
     node2.shutdown()
 
     # Kexec node1 to the toplevel of node2 via the kexec-boot script
     node1.succeed('touch /run/foo')
     node1.fail('hello')
-    node1.execute('${nodes.node2.config.system.build.kexecBoot}/kexec-boot', check_return=False)
+    node1.execute('${nodes.node2.config.system.build.kexecTree}/kexec-boot', check_return=False)
     node1.succeed('! test -e /run/foo')
     node1.succeed('hello')
     node1.succeed('[ "$(hostname)" = "node2" ]')