system: Add store-directory-prefix to boot-parameters.

Fixes <http://issues.guix.gnu.org/44196>

* gnu/machine/ssh.scm (roll-back-managed-host): Use
boot-parameters-store-directory-prefix.
* gnu/system.scm (define-module): Export
boot-parameters-store-directory-prefix.
(<boot-parameters>)[store-directory-prefix]: New field. It is used to
generate the correct paths when /gnu/store is installed on a btrfs
subvolume whose name doesn't match the final runtime path, as the
bootloader doesn't have knowledge about the final mounting points.
[boot-parameters-store-directory-prefix]: New accessor.
(read-boot-parameters): Read directory-prefix from store field.
(operating-system-boot-parameters-file): Add directory-prefix to
store field.
* guix/scripts/system.scm (reinstall-bootloader): Use
boot-parameters-store-directory-prefix.
* test/boot-parameters.scm (%default-btrfs-subvolume,
%default-store-directory-prefix): New variables.
(%grub-boot-parameters): Use %default-store-directory-prefix.
(%default-operating-system): Use %default-btrfs-subvolume.
(test-boot-parameters): Add directory-prefix.
(test optional fields): Add test for directory-prefix.
(test os store-directory-prefix): New test.

2 files changed, 25 insertions, 1 deletions

diff --git a/gnu/machine/ssh.scm b/gnu/machine/ssh.scm
index 5020bd362f..a3a12fb54b 100644
--- a/gnu/machine/ssh.scm
+++ b/gnu/machine/ssh.scm
@@ -482,6 +482,8 @@ an environment type of 'managed-host."
                                         (list (second boot-parameters))))
                        (locale -> (boot-parameters-locale
                                    (second boot-parameters)))
+                       (store-dir -> (boot-parameters-store-directory-prefix
+                                      (second boot-parameters)))
                        (old-entries -> (map boot-parameters->menu-entry
                                             (drop boot-parameters 2)))
                        (bootloader -> (operating-system-bootloader
@@ -492,6 +494,7 @@ an environment type of 'managed-host."
                                     bootloader))
                                   bootloader entries
                                   #:locale locale
+                                  #:store-directory-prefix store-dir
                                   #:old-entries old-entries)))
                        (remote-result (machine-remote-eval machine remote-exp)))
     (when (eqv? 'error remote-result)
diff --git a/gnu/system.scm b/gnu/system.scm
index a3122eaa65..ff9ab18f22 100644
--- a/gnu/system.scm
+++ b/gnu/system.scm
@@ -148,6 +148,7 @@
             boot-parameters-bootloader-name
             boot-parameters-bootloader-menu-entries
             boot-parameters-store-device
+            boot-parameters-store-directory-prefix
             boot-parameters-store-mount-point
             boot-parameters-locale
             boot-parameters-kernel
@@ -293,12 +294,17 @@ directly by the user."
   ;; understand that.  The 'root-device', on the other hand, corresponds
   ;; exactly to the device field of the <file-system> object representing the
   ;; OS's root file system, so it might be a device path like "/dev/sda3".
+  ;; The 'store-directory-prefix' field contains #f or the store path inside
+  ;; the 'store-device' as it is seen by GRUB, e.g. it would contain
+  ;; "/storefs" if the store is located in that subvolume of a btrfs
+  ;; partition.
   (root-device      boot-parameters-root-device)
   (bootloader-name  boot-parameters-bootloader-name)
   (bootloader-menu-entries                        ;list of <menu-entry>
    boot-parameters-bootloader-menu-entries)
   (store-device     boot-parameters-store-device)
   (store-mount-point boot-parameters-store-mount-point)
+  (store-directory-prefix boot-parameters-store-directory-prefix)
   (locale           boot-parameters-locale)
   (kernel           boot-parameters-kernel)
   (kernel-arguments boot-parameters-kernel-arguments)
@@ -394,6 +400,17 @@ file system labels."
           (_                                      ;the old format
            root-device))))
 
+      (store-directory-prefix
+       (match (assq 'store rest)
+         (('store . store-data)
+          (match (assq 'directory-prefix store-data)
+            (('directory-prefix prefix) prefix)
+            ;; No directory-prefix found.
+            (_ #f)))
+         (_
+          ;; No store found, old format.
+          #f)))
+
       (store-mount-point
        (match (assq 'store rest)
          (('store ('device _) ('mount-point mount-point) _ ...)
@@ -1294,6 +1311,7 @@ such as '--root' and '--load' to <boot-parameters>."
   (let* ((initrd          (and (not (operating-system-hurd os))
                                (operating-system-initrd-file os)))
          (store           (operating-system-store-file-system os))
+         (file-systems    (operating-system-file-systems os))
          (locale          (operating-system-locale os))
          (bootloader      (bootloader-configuration-bootloader
                            (operating-system-bootloader os)))
@@ -1315,6 +1333,7 @@ such as '--root' and '--load' to <boot-parameters>."
       (bootloader-configuration-menu-entries (operating-system-bootloader os)))
      (locale locale)
      (store-device (ensure-not-/dev (file-system-device store)))
+     (store-directory-prefix (btrfs-store-subvolume-file-name file-systems))
      (store-mount-point (file-system-mount-point store)))))
 
 (define (device->sexp device)
@@ -1371,7 +1390,9 @@ being stored into the \"parameters\" file)."
                       (device
                        #$(device->sexp (boot-parameters-store-device params)))
                       (mount-point #$(boot-parameters-store-mount-point
-                                      params))))
+                                      params))
+                      (directory-prefix
+                       #$(boot-parameters-store-directory-prefix params))))
                   #:set-load-path? #f)))
 
 (define-gexp-compiler (operating-system-compiler (os <operating-system>)