file-systems: Preserve UUID types when serializing.

Reported by Roel Janssen <roel@gnu.org>
at <https://lists.gnu.org/archive/html/help-guix/2017-09/msg00094.html>.

* gnu/system/file-systems.scm (file-system->spec): When DEVICE is a
UUID, serialize it in a way that preserves its type.
(spec->file-system): Adjust accordingly.
* gnu/build/file-systems.scm (canonicalize-device-spec): Add case for
when SPEC is 'uuid?'.

gnu/build/file-systems.scm

@@ -450,8 +450,7 @@ the following:
      \"/dev/sda1\";
   • 'label', in which case SPEC is known to designate a partition label--e.g.,
      \"my-root-part\";
-  • 'uuid', in which case SPEC must be a UUID (a 16-byte bytevector)
-     designating a partition;
+  • 'uuid', in which case SPEC must be a UUID designating a partition;
   • 'any', in which case SPEC can be anything.
 "
   (define max-trials
@@ -497,9 +496,11 @@ the following:
      (resolve find-partition-by-label spec identity))
     ((uuid)
      (resolve find-partition-by-uuid
-              (if (string? spec)
-                  (string->uuid spec)
-                  spec)
+              (cond ((string? spec)
+                     (string->uuid spec))
+                    ((uuid? spec)
+                     (uuid-bytevector spec))
+                    (else spec))
               uuid->string))
     (else
      (error "unknown device title" title))))

gnu/system/file-systems.scm

@@ -18,6 +18,7 @@
 
 (define-module (gnu system file-systems)
   #:use-module (ice-9 match)
+  #:use-module (rnrs bytevectors)
   #:use-module (srfi srfi-1)
   #:use-module (guix records)
   #:use-module (gnu system uuid)
@@ -161,7 +162,7 @@ initrd code."
   (match fs
     (($ <file-system> device title mount-point type flags options _ _ check?)
      (list (if (uuid? device)
-               (uuid-bytevector device)
+               `(uuid ,(uuid-type device) ,(uuid-bytevector device))
                device)
            title mount-point type flags options check?))))
 
@@ -170,7 +171,12 @@ initrd code."
   (match sexp
     ((device title mount-point type flags options check?)
      (file-system
-       (device device) (title title)
+       (device (match device
+                 (('uuid (? symbol? type) (? bytevector? bv))
+                  (bytevector->uuid bv type))
+                 (_
+                  device)))
+       (title title)
        (mount-point mount-point) (type type)
        (flags flags) (options options)
        (check? check?)))))