services: swap: Allow for UUIDs and file system labels.

* gnu/services/base.scm (swap-service-type)[device-lookup, device-name]:
New variables.
Add 'modules' field to 'shepherd-service'.  In 'start' and 'stop', use
'device-lookup' to resolve UUIDs and labels.
* doc/guix.texi (operating-system Reference): Adjust accordingly.

doc/guix.texi

@@ -13075,14 +13075,38 @@ A list of mapped devices.  @xref{Mapped Devices}.
 @item @code{file-systems}
 A list of file systems.  @xref{File Systems}.
 
-@item @code{swap-devices} (default: @code{'()})
 @cindex swap devices
-A list of strings identifying devices or files to be used for ``swap
+@cindex swap space
+@item @code{swap-devices} (default: @code{'()})
+A list of UUIDs, file system labels, or strings identifying devices or
+files to be used for ``swap
 space'' (@pxref{Memory Concepts,,, libc, The GNU C Library Reference
-Manual}).  For example, @code{'("/dev/sda3")} or @code{'("/swapfile")}.
+Manual}).  Here are some examples:
+
+@table @code
+@item (list (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb"))
+Use the swap partition with the given UUID.  You can learn the UUID of a
+Linux swap partition by running @command{swaplabel @var{device}}, where
+@var{device} is the @file{/dev} file name of that partition.
+
+@item (list (file-system-label "swap"))
+Use the partition with label @code{swap}.  Again, the
+@command{swaplabel} command allows you to view and change the label of a
+Linux swap partition.
+
+@item (list "/swapfile")
+Use the file @file{/swapfile} as swap space.
+
+@item (list "/dev/sda3" "/dev/sdb2")
+Use the @file{/dev/sda3} and @file{/dev/sdb2} partitions as swap space.
+We recommend referring to swap devices by UUIDs or labels as shown above
+instead.
+@end table
+
 It is possible to specify a swap file in a file system on a mapped
-device, provided that the necessary device mapping and file system are
-also specified.  @xref{Mapped Devices} and @ref{File Systems}.
+device (under @file{/dev/mapper}), provided that the necessary device
+mapping and file system are also specified.  @xref{Mapped Devices} and
+@ref{File Systems}.
 
 @item @code{users} (default: @code{%base-user-accounts})
 @itemx @code{groups} (default: @code{%base-groups})

gnu/services/base.scm

@@ -2104,22 +2104,52 @@ instance."
    'swap
    (lambda (device)
      (define requirement
-       (if (string-prefix? "/dev/mapper/" device)
+       (if (and (string? device)
+                (string-prefix? "/dev/mapper/" device))
            (list (symbol-append 'device-mapping-
                                 (string->symbol (basename device))))
            '()))
 
+     (define (device-lookup device)
+       ;; The generic 'find-partition' procedures could return a partition
+       ;; that's not swap space, but that's unlikely.
+       (cond ((uuid? device)
+              #~(find-partition-by-uuid #$(uuid-bytevector device)))
+             ((file-system-label? device)
+              #~(find-partition-by-label
+                 #$(file-system-label->string device)))
+             (else
+              device)))
+
+     (define service-name
+       (symbol-append 'swap-
+                      (string->symbol
+                       (cond ((uuid? device)
+                              (string-take (uuid->string device) 6))
+                             ((file-system-label? device)
+                              (file-system-label->string device))
+                             (else
+                              device)))))
+
+     (with-imported-modules (source-module-closure '((gnu build file-systems)))
        (shepherd-service
-      (provision (list (symbol-append 'swap- (string->symbol device))))
+        (provision (list service-name))
         (requirement `(udev ,@requirement))
         (documentation "Enable the given swap device.")
+        (modules `((gnu build file-systems)
+                   ,@%default-modules))
         (start #~(lambda ()
-                 (restart-on-EINTR (swapon #$device))
-                 #t))
+                   (let ((device #$(device-lookup device)))
+                     (and device
+                          (begin
+                            (restart-on-EINTR (swapon device))
+                            #t)))))
         (stop #~(lambda _
-                (restart-on-EINTR (swapoff #$device))
-                #f))
-      (respawn? #f)))))
+                  (let ((device #$(device-lookup device)))
+                    (when device
+                      (restart-on-EINTR (swapoff device)))
+                    #f)))
+        (respawn? #f))))))
 
 (define (swap-service device)
   "Return a service that uses @var{device} as a swap device."