services: nfs: Allow pipefs-service-type to be extended.

* gnu/services/nfs.scm (pipefs-service-type): Rewrite using SERVICE-TYPE to
add ability to extend the service.

gnu/services/nfs.scm

@@ -24,6 +24,7 @@
   #:use-module (gnu packages linux)
   #:use-module (guix)
   #:use-module (guix records)
+  #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
   #:use-module (gnu build file-systems)
   #:export (rpcbind-service-type
@@ -96,8 +97,7 @@
                          (default default-pipefs-directory)))
 
 (define pipefs-service-type
-  (shepherd-service-type
-   'pipefs
+  (let ((proc
          (lambda (config)
            (define pipefs-directory (pipefs-configuration-mount-point config))
 
@@ -113,6 +113,17 @@
             (stop #~(lambda (pid . args)
                       (umount #$pipefs-directory MNT_DETACH)
                       (not (member #$pipefs-directory (mount-points)))))))))
+    (service-type
+     (name 'pipefs)
+     (extensions
+      (list (service-extension shepherd-root-service-type
+                               (compose list proc))))
+     ;; We use the extensions feature to allow other services to automatically
+     ;; configure and start this service.  Only one value can be provided.  We
+     ;; override it with the value returned by the extending service.
+     (compose identity)
+     (extend (lambda (config values) (first values)))
+     (default-value (pipefs-configuration)))))