gnu/installer/record.scm


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;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2018, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
;;; Copyright © 2020 Florian Pelz <pelzflorian@pelzflorian.de>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (gnu installer record)
  #:use-module (guix records)
  #:use-module (srfi srfi-1)
  #:export (<installer>
            installer
            make-installer
            installer?
            installer-name
            installer-init
            installer-exit
            installer-exit-error
            installer-final-page
            installer-keymap-page
            installer-locale-page
            installer-menu-page
            installer-network-page
            installer-substitutes-page
            installer-timezone-page
            installer-hostname-page
            installer-user-page
            installer-partition-page
            installer-services-page
            installer-welcome-page
            installer-parameters-menu
            installer-parameters-page))


;;;
;;; Installer record.
;;;

;; The <installer> record contains pages that will be run to prompt the user
;; for the system configuration. The goal of the installer is to produce a
;; complete <operating-system> record and install it.

(define-record-type* <installer>
  installer make-installer
  installer?
  ;; symbol
  (name installer-name)
  ;; procedure: void -> void
  (init installer-init)
  ;; procedure: void -> void
  (exit installer-exit)
  ;; procedure (key arguments) -> void
  (exit-error installer-exit-error)
  ;; procedure void -> void
  (final-page installer-final-page)
  ;; procedure (layouts context) -> (list layout variant options)
  (keymap-page installer-keymap-page)
  ;; procedure: (#:key supported-locales iso639-languages iso3166-territories)
  ;; -> glibc-locale
  (locale-page installer-locale-page)
  ;; procedure: (steps) -> step-id
  (menu-page installer-menu-page)
  ;; procedure void -> void
  (network-page installer-network-page)
  ;; procedure void -> void
  (substitutes-page installer-substitutes-page)
  ;; procedure (zonetab) -> posix-timezone
  (timezone-page installer-timezone-page)
  ;; procedure void -> void
  (hostname-page installer-hostname-page)
  ;; procedure void -> void
  (user-page installer-user-page)
  ;; procedure void -> void
  (partition-page installer-partition-page)
  ;; procedure void -> void
  (services-page installer-services-page)
  ;; procedure (logo) -> void
  (welcome-page installer-welcome-page)
  ;; procedure (menu-proc) -> void
  (parameters-menu installer-parameters-menu)
  ;; procedure (keyboard-layout-selection) -> void
  (parameters-page installer-parameters-page))
;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2016 Chris Marusich <cmmarusich@gmail.com>
;;; Copyright © 2017 Leo Famulari <leo@famulari.name>
;;; Copyright © 2017, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
;;; Copyright © 2019, 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
;;; Copyright © 2020 Stefan <stefan-guix@vodafonemail.de>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (gnu bootloader grub)
  #:use-module (guix build union)
  #:use-module (guix records)
  #:use-module (guix store)
  #:use-module (guix utils)
  #:use-module (guix gexp)
  #:use-module (gnu artwork)
  #:use-module (gnu bootloader)
  #:use-module (gnu system uuid)
  #:use-module (gnu system file-systems)
  #:use-module (gnu system keyboard)
  #:use-module (gnu packages bootloaders)
  #:autoload   (gnu packages gtk) (guile-cairo guile-rsvg)
  #:autoload   (gnu packages xorg) (xkeyboard-config)
  #:use-module (ice-9 match)
  #:use-module (ice-9 regex)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-2)
  #:export (grub-theme
            grub-theme?
            grub-theme-image
            grub-theme-resolution
            grub-theme-color-normal
            grub-theme-color-highlight
            grub-theme-gfxmode

            install-grub-efi-netboot

            grub-bootloader
            grub-efi-bootloader
            grub-efi-netboot-bootloader
            grub-mkrescue-bootloader
            grub-minimal-bootloader

            grub-configuration))

;;; Commentary:
;;;
;;; Configuration of GNU GRUB.
;;;
;;; Code:

(define* (normalize-file file mount-point store-directory-prefix)
  "Strip MOUNT-POINT and prepend STORE-DIRECTORY-PREFIX, if any, to FILE, a
G-expression or other lowerable object denoting a file name."

  (define (strip-mount-point mount-point file)
    (if mount-point
        (if (string=? mount-point "/")
            file
            #~(let ((file #$file))
                (if (string-prefix? #$mount-point file)
                    (substring #$file #$(string-length mount-point))
                    file)))
        file))

  (define (prepend-store-directory-prefix store-directory-prefix file)
    (if store-directory-prefix
        #~(string-append #$store-directory-prefix #$file)
        file))

  (prepend-store-directory-prefix store-directory-prefix
                                  (strip-mount-point mount-point file)))


(define-record-type* <grub-theme>
  ;; Default theme contributed by Felipe López.
  grub-theme make-grub-theme
  grub-theme?
  (image           grub-theme-image
                   (default (file-append %artwork-repository
                                         "/grub/GuixSD-fully-black-4-3.svg")))
  (resolution      grub-theme-resolution
                   (default '(1024 . 768)))
  (color-normal    grub-theme-color-normal
                   (default '((fg . light-gray) (bg . black))))
  (color-highlight grub-theme-color-highlight
                   (default '((fg . yellow) (bg . black))))
  (gfxmode         grub-theme-gfxmode
                   (default '("auto"))))          ;list of string


;;;
;;; Background image & themes.
;;;

(define (bootloader-theme config)
  "Return user defined theme in CONFIG if defined or a default theme
otherwise."
  (or (bootloader-configuration-theme config) (grub-theme)))

(define* (image->png image #:key width height)
  "Build a PNG of HEIGHT x WIDTH from IMAGE if its file suffix is \".svg\".
Otherwise the picture in IMAGE is just copied."
  (computed-file "grub-image.png"
                 (with-imported-modules '((gnu build svg))
                   (with-extensions (list guile-rsvg guile-cairo)
                     #~(if (string-suffix? ".svg" #+image)
                           (begin
                             (use-modules (gnu build svg))
                             (svg->png #+image #$output
                                       #:width #$width
                                       #:height #$height))
                           (copy-file #+image #$output))))))

(define* (grub-background-image config)
  "Return the GRUB background image defined in CONFIG or #f if none was found.
If the suffix of the image file is \".svg\", then it is converted into a PNG
file with the resolution provided in CONFIG."
  (let* ((theme (bootloader-theme config))
         (image (grub-theme-image theme)))
    (and image
         (match (grub-theme-resolution theme)
           (((? number? width) . (? number? height))
            (image->png image #:width width #:height height))
           (_ #f)))))

(define* (eye-candy config store-device store-mount-point
                    #:key store-directory-prefix port)
  "Return a gexp that writes to PORT (a port-valued gexp) the 'grub.cfg' part
concerned with graphics mode, background images, colors, and all that.
STORE-DEVICE designates the device holding the store, and STORE-MOUNT-POINT is
its mount point; these are used to determine where the background image and
fonts must be searched for.  STORE-DIRECTORY-PREFIX is a directory prefix to
prepend to any store file name."
  (define (setup-gfxterm config font-file)
    (if (memq 'gfxterm (bootloader-configuration-terminal-outputs config))
        #~(format #f "
if loadfont ~a; then
  set gfxmode=~a
  insmod all_video
  insmod gfxterm
fi~%"
                  #+font-file
                  #$(string-join
                     (grub-theme-gfxmode (bootloader-theme config))
                     ";"))
        ""))

  (define (theme-colors type)
    (let* ((theme  (bootloader-theme config))
           (colors (type theme)))
      (string-append (symbol->string (assoc-ref colors 'fg)) "/"
                     (symbol->string (assoc-ref colors 'bg)))))

  (define font-file
    (normalize-file (file-append grub "/share/grub/unicode.pf2")
                    store-mount-point
                    store-directory-prefix))

  (define image
    (normalize-file (grub-background-image config)
                    store-mount-point
                    store-directory-prefix))

  (and image
       #~(format #$port "
# Set 'root' to the partition that contains /gnu/store.
~a

~a
~a

insmod png
if background_image ~a; then
  set color_normal=~a
  set color_highlight=~a
else
  set menu_color_normal=cyan/blue
  set menu_color_highlight=white/blue
fi~%"
                 #$(grub-root-search store-device font-file)
                 #$(setup-gfxterm config font-file)
                 #$(grub-setup-io config)

                 #$image
                 #$(theme-colors grub-theme-color-normal)
                 #$(theme-colors grub-theme-color-highlight))))


;;;
;;; Configuration file.
;;;

(define* (keyboard-layout-file layout
                               #:key
                               (grub grub))
  "Process the X keyboard layout description LAYOUT, a <keyboard-layout> record,
and return a file in the format for GRUB keymaps.  LAYOUT must be present in
the 'share/X11/xkb/symbols/' directory of 'xkeyboard-config'."
  (define builder
    (with-imported-modules '((guix build utils))
      #~(begin
          (use-modules (guix build utils))

          ;; 'grub-kbdcomp' passes all its arguments but '-o' to 'ckbcomp'
          ;; (from the 'console-setup' package).
          (invoke #+(file-append grub "/bin/grub-mklayout")
                  "-i" #+(keyboard-layout->console-keymap layout)
                  "-o" #$output))))

  (computed-file (string-append "grub-keymap."
                                (string-map (match-lambda
                                              (#\, #\-)
                                              (chr chr))
                                            (keyboard-layout-name layout)))
                 builder))

(define (grub-setup-io config)
  "Return GRUB commands to configure the input / output interfaces.  The result
is a string that can be inserted in grub.cfg."
  (let* ((symbols->string (lambda (list)
                           (string-join (map symbol->string list) " ")))
         (outputs (bootloader-configuration-terminal-outputs config))
         (inputs (bootloader-configuration-terminal-inputs config))
         (unit (bootloader-configuration-serial-unit config))
         (speed (bootloader-configuration-serial-speed config))

         ;; Respectively, GRUB_TERMINAL_OUTPUT and GRUB_TERMINAL_INPUT,
         ;; as documented in GRUB manual section "Simple Configuration
         ;; Handling".
         (valid-outputs '(console serial serial_0 serial_1 serial_2 serial_3
                          gfxterm vga_text mda_text morse spkmodem))
         (valid-inputs '(console serial serial_0 serial_1 serial_2 serial_3
                         at_keyboard usb_keyboard))

         (io (string-append
               "terminal_output "
               (symbols->string
                 (map
                   (lambda (output)
                     (if (memq output valid-outputs) output #f)) outputs)) "\n"
               (if (null? inputs)
                 ""
                 (string-append
                   "terminal_input "
                   (symbols->string
                     (map
                       (lambda (input)
                         (if (memq input valid-inputs) input #f)) inputs)) "\n"))
               ;; UNIT and SPEED are arguments to the same GRUB command
               ;; ("serial"), so we process them together.
               (if (or unit speed)
                 (string-append
                   "serial"
                   (if unit
                     ;; COM ports 1 through 4
                     (if (and (exact-integer? unit) (<= unit 3) (>= unit 0))
                       (string-append " --unit=" (number->string unit))
                       #f)
                     "")
                   (if speed
                     (if (exact-integer? speed)
                       (string-append " --speed=" (number->string speed))
                       #f)
                     ""))
                 ""))))
    (format #f "~a" io)))

(define (grub-root-search device file)
  "Return the GRUB 'search' command to look for DEVICE, which contains FILE,
a gexp.  The result is a gexp that can be inserted in the grub.cfg-generation
code."
  ;; Usually FILE is a file name gexp like "/gnu/store/…-linux/vmlinuz", but
  ;; it can also be something like "(hd0,msdos1)/vmlinuz" in the case of
  ;; custom menu entries.  In the latter case, don't emit a 'search' command.
  (if (and (string? file) (not (string-prefix? "/" file)))
      ""
      (match device
        ;; Preferably refer to DEVICE by its UUID or label.  This is more
        ;; efficient and less ambiguous, see <http://bugs.gnu.org/22281>.
        ((? uuid? uuid)
         (format #f "search --fs-uuid --set ~a"
                 (uuid->string device)))
        ((? file-system-label? label)
         (format #f "search --label --set ~a"
                 (file-system-label->string label)))
        ((? (lambda (device)
              (and (string? device) (string-contains device ":/"))) nfs-uri)
         ;; If the device is an NFS share, then we assume that the expected
         ;; file on that device (e.g. the GRUB background image or the kernel)
         ;; has to be loaded over the network.  Otherwise we would need an
         ;; additional device information for some local disk to look for that
         ;; file, which we do not have.
         ;;
         ;; We explicitly set "root=(tftp)" here even though if grub.cfg
         ;; had been loaded via TFTP, Grub would have set "root=(tftp)"
         ;; automatically anyway.  The reason is if you have a system that
         ;; used to be on NFS but now is local, root would be set to local
         ;; disk.  If you then selected an older system generation that is
         ;; supposed to boot from network in the Grub boot menu, Grub still
         ;; wouldn't load those files from network otherwise.
         ;;
         ;; TFTP is preferred to HTTP because it is used more widely and
         ;; specified in standards more widely--especially BOOTP/DHCPv4
         ;; defines a TFTP server for DHCP option 66, but not HTTP.
         ;;
         ;; Note: DHCPv6 specifies option 59 to contain a boot-file-url,
         ;; which can contain a HTTP or TFTP URL.
         ;;
         ;; Note: It is assumed that the file paths are of a similar
         ;; setup on both the TFTP server and the NFS server (it is
         ;; not possible to search for files on TFTP).
         ;;
         ;; TODO: Allow HTTP.
         "set root=(tftp)")
        ((or #f (? string?))
         #~(format #f "search --file --set ~a" #$file)))))

(define* (grub-configuration-file config entries
                                  #:key
                                  (system (%current-system))
                                  (old-entries '())
                                  store-directory-prefix)
  "Return the GRUB configuration file corresponding to CONFIG, a
<bootloader-configuration> object, and where the store is available at
STORE-FS, a <file-system> object.  OLD-ENTRIES is taken to be a list of menu
entries corresponding to old generations of the system.
STORE-DIRECTORY-PREFIX may be used to specify a store prefix, as is required
when booting a root file system on a Btrfs subvolume."
  (define all-entries
    (append entries (bootloader-configuration-menu-entries config)))
  (define (menu-entry->gexp entry)
    (let ((label (menu-entry-label entry))
          (linux (menu-entry-linux entry))
          (device (menu-entry-device entry))
          (device-mount-point (menu-entry-device-mount-point entry)))
      (if linux
          (let ((arguments (menu-entry-linux-arguments entry))
                (linux (normalize-file linux
                                       device-mount-point
                                       store-directory-prefix))
                (initrd (normalize-file (menu-entry-initrd entry)
                                        device-mount-point
                                        store-directory-prefix)))
         ;; Here DEVICE is the store and DEVICE-MOUNT-POINT is its mount point.
         ;; Use the right file names for LINUX and INITRD in case
         ;; DEVICE-MOUNT-POINT is not "/", meaning that the store is on a
         ;; separate partition.

         ;; When BTRFS-SUBVOLUME-FILE-NAME is defined, prepend it the linux and
         ;; initrd paths, to allow booting from a Btrfs subvolume.
         #~(format port "menuentry ~s {
  ~a
  linux ~a ~a
  initrd ~a
}~%"
                   #$label
                   #$(grub-root-search device linux)
                   #$linux (string-join (list #$@arguments))
                   #$initrd))
          (let ((kernel (menu-entry-multiboot-kernel entry))
                (arguments (menu-entry-multiboot-arguments entry))
                (modules (menu-entry-multiboot-modules entry))
                (root-index 1))            ; XXX EFI will need root-index 2
        #~(format port "
menuentry ~s {
  multiboot ~a root=device:hd0s~a~a~a
}~%"
                  #$label
                  #$kernel
                  #$root-index (string-join (list #$@arguments) " " 'prefix)
                  (string-join (map string-join '#$modules)
                               "\n  module " 'prefix))))))

  (define (sugar)
    (let* ((entry (first all-entries))
           (device (menu-entry-device entry))
           (mount-point (menu-entry-device-mount-point entry)))
      (eye-candy config
                 device
                 mount-point
                 #:store-directory-prefix store-directory-prefix
                 #:port #~port)))

  (define keyboard-layout-config
    (let* ((layout (bootloader-configuration-keyboard-layout config))
           (grub   (bootloader-package
                    (bootloader-configuration-bootloader config)))
           (keymap* (and layout
                         (keyboard-layout-file layout #:grub grub)))
           (keymap (and keymap*
                        (if store-directory-prefix
                            #~(string-append #$store-directory-prefix
                                             #$keymap*)
                            keymap*))))
      #~(when #$keymap
          (format port "\
insmod keylayouts
keymap ~a~%" #$keymap))))

  (define builder
    #~(call-with-output-file #$output
        (lambda (port)
          (format port
                  "# This file was generated from your Guix configuration.  Any changes
# will be lost upon reconfiguration.
")
          #$(sugar)
          #$keyboard-layout-config
          (format port "
set default=~a
set timeout=~a~%"
                  #$(bootloader-configuration-default-entry config)
                  #$(bootloader-configuration-timeout config))
          #$@(map menu-entry->gexp all-entries)

          #$@(if (pair? old-entries)
                 #~((format port "
submenu \"GNU system, old configurations...\" {~%")
                    #$@(map menu-entry->gexp old-entries)
                    (format port "}~%"))
                 #~())
          (format port "
if [ \"${grub_platform}\" == efi ]; then
  menuentry \"Firmware setup\" {
    fwsetup
  }
fi~%"))))

  ;; Since this file is rather unique, there's no point in trying to
  ;; substitute it.
  (computed-file "grub.cfg" builder
                 #:options '(#:local-build? #t
                             #:substitutable? #f)))


;;;
;;; Install procedures.
;;;

(define install-grub
  #~(lambda (bootloader device mount-point)
      (let ((grub (string-append bootloader "/sbin/grub-install"))
            (install-dir (string-append mount-point "/boot")))
        ;; Install GRUB on DEVICE which is mounted at MOUNT-POINT. If DEVICE
        ;; is #f, then we populate the disk-image rooted at MOUNT-POINT.
        (if device
            (begin
              ;; Tell 'grub-install' that there might be a LUKS-encrypted
              ;; /boot or root partition.
              (setenv "GRUB_ENABLE_CRYPTODISK" "y")

              ;; Hide potentially confusing messages from the user, such as
              ;; "Installing for i386-pc platform."
              (invoke/quiet grub "--no-floppy" "--target=i386-pc"
                            "--boot-directory" install-dir
                            device))
            ;; When creating a disk-image, only install GRUB modules.
            (copy-recursively (string-append bootloader "/lib/")
                              install-dir)))))

(define install-grub-disk-image
  #~(lambda (bootloader root-index image)
      ;; Install GRUB on the given IMAGE. The root partition index is
      ;; ROOT-INDEX.
      (let ((grub-mkimage
             (string-append bootloader "/bin/grub-mkimage"))
            (modules '("biosdisk" "part_msdos" "fat" "ext2"))
            (grub-bios-setup
             (string-append bootloader "/sbin/grub-bios-setup"))
            (root-device (format #f "hd0,msdos~a" root-index))
            (boot-img (string-append bootloader "/lib/grub/i386-pc/boot.img"))
            (device-map "device.map"))

        ;; Create a minimal, standalone GRUB image that will be written
        ;; directly in the MBR-GAP (space between the end of the MBR and the
        ;; first partition).
        (apply invoke grub-mkimage
               "-O" "i386-pc"
               "-o" "core.img"
               "-p" (format #f "(~a)/boot/grub" root-device)
               modules)

        ;; Create a device mapping file.
        (call-with-output-file device-map
          (lambda (port)
            (format port "(hd0) ~a~%" image)))

        ;; Copy the default boot.img, that will be written on the MBR sector
        ;; by GRUB-BIOS-SETUP.
        (copy-file boot-img "boot.img")

        ;; Install both the "boot.img" and the "core.img" files on the given
        ;; IMAGE. On boot, the MBR sector will execute the minimal GRUB
        ;; written in the MBR-GAP. GRUB configuration and missing modules will
        ;; be read from ROOT-DEVICE.
        (invoke grub-bios-setup
                "-m" device-map
                "-r" root-device
                "-d" "."
                image))))

(define install-grub-efi
  #~(lambda (bootloader efi-dir mount-point)
      ;; Install GRUB onto the EFI partition mounted at EFI-DIR, for the
      ;; system whose root is mounted at MOUNT-POINT.
      (let ((grub-install (string-append bootloader "/sbin/grub-install"))
            (install-dir (string-append mount-point "/boot"))
            ;; When installing Guix, it's common to mount EFI-DIR below
            ;; MOUNT-POINT rather than /boot/efi on the live image.
            (target-esp (if (file-exists? (string-append mount-point efi-dir))
                            (string-append mount-point efi-dir)
                            efi-dir)))
        ;; Tell 'grub-install' that there might be a LUKS-encrypted /boot or
        ;; root partition.
        (setenv "GRUB_ENABLE_CRYPTODISK" "y")
        (invoke/quiet grub-install "--boot-directory" install-dir
                      "--bootloader-id=Guix"
                      "--efi-directory" target-esp))))

(define (install-grub-efi-netboot subdir)
  "Define a grub-efi-netboot bootloader installer for installation in SUBDIR,
which is usually efi/Guix or efi/boot."
  (let* ((system (string-split (nix-system->gnu-triplet
                                (or (%current-target-system)
                                    (%current-system)))
                               #\-))
         (arch (first system))
         (boot-efi-link (match system
                          ;; These are the supportend systems and the names
                          ;; defined by the UEFI standard for removable media.
                          (("i686" _ ...)        "/bootia32.efi")
                          (("x86_64" _ ...)      "/bootx64.efi")
                          (("arm" _ ...)         "/bootarm.efi")
                          (("aarch64" _ ...)     "/bootaa64.efi")
                          (("riscv" _ ...)       "/bootriscv32.efi")
                          (("riscv64" _ ...)     "/bootriscv64.efi")
                          ;; Other systems are not supported, although defined.
                          ;; (("riscv128" _ ...) "/bootriscv128.efi")
                          ;; (("ia64" _ ...)     "/bootia64.efi")
                          ((_ ...)               #f)))
         (core-efi (string-append
                    ;; This is the arch dependent file name of GRUB, e.g.
                    ;; i368-efi/core.efi or arm64-efi/core.efi.
                    (match arch
                      ("i686"    "i386")
                      ("aarch64" "arm64")
                      ("riscv"   "riscv32")
                      (_         arch))
                    "-efi/core.efi")))
    (with-imported-modules
     '((guix build union))
     #~(lambda (bootloader target mount-point)
         "Install the BOOTLOADER, which must be the package grub, as e.g.
bootx64.efi or bootaa64.efi into SUBDIR, which is usually efi/Guix or efi/boot,
below the directory TARGET for the system whose root is mounted at MOUNT-POINT.

MOUNT-POINT is the last argument in 'guix system init /etc/config.scm mnt/point'
or '/' for other 'guix system' commands.

TARGET is the target argument given to the bootloader-configuration in

(operating-system
 (bootloader (bootloader-configuration
              (target \"/boot\")
              …))
 …)

TARGET is required to be an absolute directory name, usually mounted via NFS,
and finally needs to be provided by a TFTP server as the TFTP root directory.

GRUB will load tftp://server/SUBDIR/grub.cfg and this file will instruct it to
load more files from the store like tftp://server/gnu/store/…-linux…/Image.

To make this possible two symlinks will be created. The first symlink points
relatively form MOUNT-POINT/TARGET/SUBDIR/grub.cfg to
MOUNT-POINT/boot/grub/grub.cfg, and the second symlink points relatively from
MOUNT-POINT/TARGET/%store-prefix to MOUNT-POINT/%store-prefix.

It is important to note that these symlinks need to be relativ, as the absolute
paths on the TFTP server side are unknown.

It is also important to note that both symlinks will point outside the TFTP root
directory and that the TARGET/%store-prefix symlink makes the whole store
accessible via TFTP. Possibly the TFTP server must be configured
to allow accesses outside its TFTP root directory. This may need to be
considered for security aspects."
         (use-modules ((guix build union) #:select (symlink-relative)))
         (let* ((net-dir (string-append mount-point target "/"))
                (sub-dir (string-append net-dir #$subdir "/"))
                (store (string-append mount-point (%store-prefix)))
                (store-link (string-append net-dir (%store-prefix)))
                (grub-cfg (string-append mount-point "/boot/grub/grub.cfg"))
                (grub-cfg-link (string-append sub-dir (basename grub-cfg)))
                (boot-efi-link (string-append sub-dir #$boot-efi-link)))
           ;; Prepare the symlink to the store.
           (mkdir-p (dirname store-link))
           (false-if-exception (delete-file store-link))
           (symlink-relative store store-link)
           ;; Prepare the symlink to the grub.cfg, which points into the store.
           (mkdir-p (dirname grub-cfg-link))
           (false-if-exception (delete-file grub-cfg-link))
           (symlink-relative grub-cfg grub-cfg-link)
           ;; Install GRUB, which refers to the grub.cfg, with support for
           ;; encrypted partitions,
           (setenv "GRUB_ENABLE_CRYPTODISK" "y")
           (invoke/quiet (string-append bootloader "/bin/grub-mknetdir")
                         (string-append "--net-directory=" net-dir)
                         (string-append "--subdir=" #$subdir))
           ;; Prepare the bootloader symlink, which points to core.efi of GRUB.
           (false-if-exception (delete-file boot-efi-link))
           (symlink #$core-efi boot-efi-link))))))


;;;
;;; Bootloader definitions.
;;;
;;; For all these grub-bootloader variables the path to /boot/grub/grub.cfg
;;; is fixed.  Inheriting and overwriting the field 'configuration-file' will
;;; break 'guix system delete-generations', 'guix system switch-generation',
;;; and 'guix system roll-back'.

(define grub-bootloader
  (bootloader
   (name 'grub)
   (package grub)
   (installer install-grub)
   (disk-image-installer install-grub-disk-image)
   (configuration-file "/boot/grub/grub.cfg")
   (configuration-file-generator grub-configuration-file)))

(define grub-minimal-bootloader
  (bootloader
   (inherit grub-bootloader)
   (package grub-minimal)))

(define grub-efi-bootloader
  (bootloader
   (inherit grub-bootloader)
   (installer install-grub-efi)
   (disk-image-installer #f)
   (name 'grub-efi)
   (package grub-efi)))

(define grub-efi-netboot-bootloader
  (bootloader
   (inherit grub-efi-bootloader)
   (name 'grub-efi-netboot-bootloader)
   (installer (install-grub-efi-netboot "efi/Guix"))))

(define grub-mkrescue-bootloader
  (bootloader
   (inherit grub-efi-bootloader)
   (package grub-hybrid)))


;;;
;;; Compatibility macros.
;;;

(define-syntax grub-configuration
  (syntax-rules (grub)
                ((_ (grub package) fields ...)
                 (if (eq? package grub)
                     (bootloader-configuration
                      (bootloader grub-bootloader)
                      fields ...)
                   (bootloader-configuration
                    (bootloader grub-efi-bootloader)
                    fields ...)))
                ((_ fields ...)
                 (bootloader-configuration
                  (bootloader grub-bootloader)
                  fields ...))))

;;; grub.scm ends here