system: Add support for swap devices.

* gnu/services/base.scm (swap-service): New procedure.
* gnu/system.scm (<operating-system>)[swap-devices]: New field.
  (swap-services): New procedure.
  (essential-services): Use it.

gnu/services/base.scm

@@ -39,6 +39,7 @@
   #:export (root-file-system-service
             file-system-service
             device-mapping-service
+            swap-service
             user-processes-service
             host-name-service
             console-font-service
@@ -614,6 +615,27 @@ gexp, to open it, and evaluate @var{close} to close it."
              (stop #~(lambda _ (not #$close)))
              (respawn? #f)))))
 
+(define (swap-service device)
+  "Return a service that uses @var{device} as a swap device."
+  (define requirement
+    (if (string-prefix? "/dev/mapper/" device)
+        (list (symbol-append 'device-mapping-
+                             (string->symbol (basename device))))
+        '()))
+
+  (with-monad %store-monad
+    (return (service
+             (provision (list (symbol-append 'swap- (string->symbol device))))
+             (requirement `(udev ,@requirement))
+             (documentation "Enable the given swap device.")
+             (start #~(lambda ()
+                        (swapon #$device)
+                        #t))
+             (stop #~(lambda _
+                       (swapoff #$device)
+                       #f))
+             (respawn? #f)))))
+
 (define %base-services
   ;; Convenience variable holding the basic services.
   (let ((motd (text-file "motd" "

gnu/system.scm

@@ -105,6 +105,8 @@
   (mapped-devices operating-system-mapped-devices ; list of <mapped-device>
                   (default '()))
   (file-systems operating-system-file-systems)    ; list of fs
+  (swap-devices operating-system-swap-devices     ; list of strings
+                (default '()))
 
   (users operating-system-users                   ; list of user accounts
          (default '()))
@@ -228,6 +230,11 @@ as 'needed-for-boot'."
                                              (close source target))))
                  (operating-system-mapped-devices os))))
 
+(define (swap-services os)
+  "Return the list of swap services for OS as a monadic list."
+  (sequence %store-monad
+            (map swap-service (operating-system-swap-devices os))))
+
 (define (essential-services os)
   "Return the list of essential services for OS.  These are special services
 that implement part of what's declared in OS are responsible for low-level
@@ -235,13 +242,14 @@ bookkeeping."
   (mlet* %store-monad ((mappings  (device-mapping-services os))
                        (root-fs   (root-file-system-service))
                        (other-fs  (other-file-system-services os))
+                       (swaps     (swap-services os))
                        (procs     (user-processes-service
                                    (map (compose first service-provision)
                                         other-fs)))
                        (host-name (host-name-service
                                    (operating-system-host-name os))))
     (return (cons* host-name procs root-fs
-                   (append other-fs mappings)))))
+                   (append other-fs mappings swaps)))))
 
 (define (operating-system-services os)
   "Return all the services of OS, including \"internal\" services that do not