bootloader: grub: Add removable grub-efi bootloader option.

For single-disk installs (external USBs) and for amnesiac UEFIs, Guix is not
found using its default Grub EFI bootloader location of
/boot/efi/EFI/Guix/grubx64.efi means the Guix install will not be found.

To handle this, we can place the bootloader file in the UEFI specification
location, overwriting any other OS that may have placed a file there. This
location is namely /boot/efi/EFI/BOOT/BOOTX64.efi. Grub has explicit support
for this location/situation with the `--removable` flag.

* gnu/bootloader/grub.scm (install-grub-efi-removable)
(grub-efi-removable-bootloader): New variables.
* doc/guix.texi (Bootloader Configuration): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 34 insertions, 0 deletions

diff --git a/gnu/bootloader/grub.scm b/gnu/bootloader/grub.scm
index 120cd55012b..65d7171432c 100644
--- a/gnu/bootloader/grub.scm
+++ b/gnu/bootloader/grub.scm
@@ -7,6 +7,7 @@
 ;;; Copyright © 2019, 2020 Miguel Ángel Arruga Vivas <rosen644835@gmail.com>
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2020 Stefan <stefan-guix@vodafonemail.de>
+;;; Copyright © 2022 Karl Hallsby <karl@hallsby.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -50,10 +51,12 @@
             grub-theme-color-highlight
             grub-theme-gfxmode
 
+            install-grub-efi-removable
             install-grub-efi-netboot
 
             grub-bootloader
             grub-efi-bootloader
+            grub-efi-removable-bootloader
             grub-efi-netboot-bootloader
             grub-mkrescue-bootloader
             grub-minimal-bootloader
@@ -608,6 +611,31 @@ fi~%"))))
                         "--bootloader-id=Guix"
                         "--efi-directory" target-esp)))))
 
+(define install-grub-efi-removable
+  #~(lambda (bootloader efi-dir mount-point)
+      ;; NOTE: mount-point is /mnt in guix system init /etc/config.scm /mnt/point
+      ;; NOTE: efi-dir comes from target list of booloader configuration
+      ;; There is nothing useful to do when called in the context of a disk
+      ;; image generation.
+      (when efi-dir
+        ;; 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
+                        "--removable"
+                        ;; "--no-nvram"
+                        "--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."
@@ -734,6 +762,12 @@ considered for security aspects."
    (name 'grub-efi)
    (package grub-efi)))
 
+(define grub-efi-removable-bootloader
+  (bootloader
+   (inherit grub-efi-bootloader)
+   (name 'grub-efi-removable-bootloader)
+   (installer install-grub-efi-removable)))
+
 (define grub-efi-netboot-bootloader
   (bootloader
    (inherit grub-efi-bootloader)