linux-boot: Refactor boot-system.

The --root option can now be omitted, and inferred from the root file system
declaration instead.

* gnu/build/file-systems.scm (canonicalize-device-spec): Extend to support NFS
directly, and...
* gnu/build/linux-boot.scm (boot-system): ...remove NFS special casing from
here.  Remove nested definitions for root-fs-type, root-fs-flags and
root-fs-options, and bind those inside the let* instead.  Make "--root" take
precedence over the device field string representation of the root file
system.
* doc/guix.texi (Initial RAM Disk): Document that "--root" can be left
unspecified.

doc/guix.texi

@@ -26534,9 +26534,10 @@ service activation programs and then spawns the GNU@tie{}Shepherd, the
 initialization system.
 
 @item --root=@var{root}
-Mount @var{root} as the root file system.  @var{root} can be a
+Mount @var{root} as the root file system.  @var{root} can be a device
-device name like @code{/dev/sda1}, a file system label, or a file system
+name like @code{/dev/sda1}, a file system label, or a file system UUID.
-UUID.
+When unspecified, the device name from the root file system of the
+operating system declaration is used.
 
 @item --system=@var{system}
 Have @file{/run/booted-system} and @file{/run/current-system} point to

gnu/build/file-systems.scm

@@ -661,8 +661,10 @@ (define (resolve find-partition spec fmt)
 
   (match spec
     ((? string?)
-     ;; Nothing to do, but wait until SPEC shows up.
+     (if (string-contains spec ":/")
-     (resolve identity spec identity))
+         spec                  ; do not resolve NFS devices
+         ;; Nothing to do, but wait until SPEC shows up.
+         (resolve identity spec identity)))
     ((? file-system-label?)
      ;; Resolve the label.
      (resolve find-partition-by-label

gnu/build/linux-boot.scm

@@ -498,25 +498,13 @@ (define* (boot-system #:key
   (define (root-mount-point? fs)
     (string=? (file-system-mount-point fs) "/"))
 
-  (define root-fs-type
+  (define (device-string->file-system-device device-string)
-    (or (any (lambda (fs)
+    ;; The "--root=SPEC" kernel command-line option always provides a
-               (and (root-mount-point? fs)
+    ;; string, but the string can represent a device, a UUID, or a
-                    (file-system-type fs)))
+    ;; label.  So check for all three.
-             mounts)
+    (cond ((string-prefix? "/" device-string) device-string)
-        "ext4"))
+          ((uuid device-string) => identity)
-
+          (else (file-system-label device-string))))
-  (define root-fs-flags
-    (mount-flags->bit-mask (or (any (lambda (fs)
-                                      (and (root-mount-point? fs)
-                                           (file-system-flags fs)))
-                                    mounts)
-                               '())))
-
-  (define root-fs-options
-    (any (lambda (fs)
-           (and (root-mount-point? fs)
-                (file-system-options fs)))
-         mounts))
 
   (display "Welcome, this is GNU's early boot Guile.\n")
   (display "Use '--repl' for an initrd REPL.\n\n")
@@ -526,7 +514,21 @@ (define root-fs-options
       (mount-essential-file-systems)
       (let* ((args    (linux-command-line))
              (to-load (find-long-option "--load" args))
-             (root    (find-long-option "--root" args)))
+             (root-fs (find root-mount-point? mounts))
+             (root-fs-type (or (and=> root-fs file-system-type)
+                               "ext4"))
+             (root-fs-device (and=> root-fs file-system-device))
+             (root-fs-flags (mount-flags->bit-mask
+                             (or (and=> root-fs file-system-flags)
+                                 '())))
+             (root-options (if root-fs
+                               (file-system-options root-fs)
+                               #f))
+             ;; --root takes precedence over the 'device' field of the root
+             ;; <file-system> record.
+             (root-device (or (and=> (find-long-option "--root" args)
+                                     device-string->file-system-device)
+                              root-fs-device)))
 
         (when (member "--repl" args)
           (start-repl))
@@ -561,21 +563,12 @@ (define root-fs-options
 
         (setenv "EXT2FS_NO_MTAB_OK" "1")
 
-        (if root
+        (if root-device
-            ;; The "--root=SPEC" kernel command-line option always provides a
+            (mount-root-file-system (canonicalize-device-spec root-device)
-            ;; string, but the string can represent a device, a UUID, or a
+                                    root-fs-type
-            ;; label.  So check for all three.
+                                    #:volatile-root? volatile-root?
-            (let ((device-spec (cond ((string-prefix? "/" root) root)
+                                    #:flags root-fs-flags
-                                     ((uuid root) => identity)
+                                    #:options root-options)
-                                     ((string-contains root ":/") #f) ; nfs
-                                     (else (file-system-label root)))))
-              (mount-root-file-system (if device-spec
-                                          (canonicalize-device-spec device-spec)
-                                          root)
-                                      root-fs-type
-                                      #:volatile-root? volatile-root?
-                                      #:flags root-fs-flags
-                                      #:options root-fs-options))
             (mount "none" "/root" "tmpfs"))
 
         ;; Mount the specified file systems.