Add nixos-generators devshell module

that injects our custom formats (at present, limited to `bootstrap-iso`)
into the nixos-generators format search path.

Additionally, introduce the `bootstrap-iso` command that wraps
`nixos-generate --format bootstrap-iso`.

fix: divnix#450

Author: tomeon

doc/start/iso.md

@@ -1,9 +1,12 @@
 # ISO
 
-Making and writing an installable iso for `hosts/bootstrap.nix` is as simple as:
+Making and writing an installable iso for a host is as simple as
+entering the devshell and running `boostrap-iso burn <host>`.  For instance, to
+burn an iso for the host defined in `hosts/bootstrap.nix`, run:
+
 ```sh
-bud build bootstrap bootstrapIso
-sudo -E $(which bud) burn
+nix develop
+bootstrap-iso burn bootstrap # note: uses sudo
 ```
 
 This works for any host.

examples/devos/flake.nix

@@ -11,7 +11,8 @@
       nixos.url = "github:nixos/nixpkgs/nixos-21.11";
       latest.url = "github:nixos/nixpkgs/nixos-unstable";
 
-      digga.url = "github:divnix/digga";
+      #digga.url = "github:divnix/digga";
+      digga.url = "path:/home/matt/git/packaging/nix/digga";
       digga.inputs.nixpkgs.follows = "nixos";
       digga.inputs.nixlib.follows = "nixos";
       digga.inputs.home-manager.follows = "home";

examples/devos/hosts/bootstrap.nix

@@ -1,6 +1,15 @@
 { profiles, ... }:
 {
-  # build with: `bud build bootstrap bootstrapIso`
+  # build (from within a devshell) with:
+  #
+  #   nixos-generate --format bootstrap-iso --flake '.#bootstrap'
+  #
+  # or:
+  #
+  #   bootstrap-iso build bootstrap
+  #
+  # which does the same thing.
+  #
   # reachable on the local link via ssh root@fe80::47%eno1
   # where 'eno1' is replaced by your own machine's network
   # interface that has the local link to the target machine
@@ -13,6 +22,7 @@
 
   boot.loader.systemd-boot.enable = true;
 
-  # will be overridden by the bootstrapIso instrumentation
+  # will be overridden by the bootstrap-iso `nixos-generate` format profile
+  # defined by the digga "nixos-generators" devshell module
   fileSystems."/" = { device = "/dev/disk/by-label/nixos"; };
 }

examples/devos/shell/default.nix

@@ -2,6 +2,7 @@
 {
   modules = with inputs; [
     bud.devshellModules.bud
+    digga.devshellModules.nixos-generators
   ];
   exportedModules = [
     ./devos.nix

examples/devos/shell/devos.nix

@@ -48,8 +48,14 @@ in
   ++ lib.optional
     (system != "i686-linux")
     (devos cachix)
-  ++ lib.optional
-    (system != "aarch64-darwin")
-    (devos inputs.nixos-generators.defaultPackage.${pkgs.system})
   ;
+
+  # digga-supplied extensions to nixos-generators.  Provides the
+  # "bootstrap-iso" nixos-generators format, plus the "bootstrap-iso" wrapper
+  # command.
+  digga.nixos-generators = {
+    enable = pkgs.system != "aarch64-darwin";
+    category = "devos";
+    package = inputs.nixos-generators.defaultPackage.${pkgs.system};
+  };
 }

flake.nix

@@ -116,7 +116,8 @@
 
       # a little extra service ...
       overlays = import ./overlays { inherit inputs; };
-      nixosModules = import ./modules;
+      nixosModules = import ./modules/nixos;
+      devshellModules = import ./modules/devShell;
 
       defaultTemplate = self.templates.devos;
       templates.devos.path = ./examples/devos;

modules/bootstrap-iso.nix

@@ -0,0 +1,3 @@
+{
+  nixos-generators = import ./nixos-generators;
+}

modules/default.nix

@@ -0,0 +1,64 @@
+#!/bin/sh
+
+set -eu
+
+if [ -z "${PRJ_ROOT:-}" ]; then
+  if ! PRJ_ROOT="$(git rev-parse --show-toplevel 2>/dev/null)"; then
+    # shellcheck disable=SC2016
+    printf 1>&2 -- '%s: %s. %s. %s.\n' \
+      "${0##*/}" 'unable to locate your project root ($PRJ_ROOT)' \
+      'bud relies on the project root for locating the bootstrap ISO' \
+      'Aborting'
+
+    exit 1
+  fi
+fi
+
+boostrapIsoRequireHost() {
+  if [ "$#" -lt 1 ]; then
+    printf 1>&2 -- 'Error: missing required <host> argument.\n' "${0##*/}"
+    bootstrapIsoHelp 1
+  fi
+}
+
+bootstrapIsoBuild() {
+  boostrapIsoRequireHost "$@" || return
+
+  local host="$1"
+  shift
+
+  # XXX the way nixos-generate handles flake specifications requires that we do
+  # not quote the host as we would with, e.g.:
+  #   nix build ".#nixosConfigurations.\"my.host.name\".config.system.build.toplevel"
+  nixos-generate "$@" --format bootstrap-iso --flake "${PRJ_ROOT}#${host}"
+}
+
+bootstrapIsoBurn() {
+  boostrapIsoRequireHost "$@" || return
+  bud="$(command -v bud)" || return
+  bootstrapIsoBuild "$@" --out-link "${PRJ_ROOT?}/result" || return
+  sudo -E "$bud" burn
+}
+
+bootstrapIsoHelp() {
+  printf 1>&2 -- 'Usage: %s {build,burn,help} <host> [<nixos-generate-arg> ...]\n' "${0##*/}"
+  return "${1:-0}"
+}
+
+cmd="${1:-}"
+shift 2>/dev/null || :
+
+case "$cmd" in
+  build)
+    bootstrapIsoBuild "$@"
+    ;;
+  burn)
+    bootstrapIsoBurn "$@"
+    ;;
+  help)
+    bootstrapIsoHelp
+    ;;
+  *)
+    bootstrapIsoHelp 1
+    ;;
+esac