services: Add 'fstab-service-type'.

* gnu/services/base.scm (file-system->fstab-entry)
(file-systems->fstab): New procedures.
(fstab-service-type): New variable.
* gnu/services/base.scm (file-system-dmd-service): New procedure, taken
from...
(file-system-service-type): ... here.
* gnu/system.scm (essential-services): Add FSTAB-SERVICE-TYPE instance.

gnu/services/base.scm

@@ -43,7 +43,8 @@ (define-module (gnu services base)
   #:use-module (srfi srfi-26)
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
-  #:export (root-file-system-service
+  #:export (fstab-service-type
+            root-file-system-service
             file-system-service
             user-unmount-service
             device-mapping-service
@@ -105,6 +106,48 @@ (define-module (gnu services base)
 ;;; File systems.
 ;;;
 
+(define (file-system->fstab-entry file-system)
+  "Return a @file{/etc/fstab} entry for @var{file-system}."
+  (string-append (case (file-system-title file-system)
+                   ((label)
+                    (string-append "LABEL=" (file-system-device file-system)))
+                   ((uuid)
+                    (string-append
+                     "UUID="
+                     (uuid->string (file-system-device file-system))))
+                   (else
+                    (file-system-device file-system)))
+                 "\t"
+                 (file-system-mount-point file-system) "\t"
+                 (file-system-type file-system) "\t"
+                 (or (file-system-options file-system) "defaults") "\t"
+
+                 ;; XXX: Omit the 'fs_freq' and 'fs_passno' fields because we
+                 ;; don't have anything sensible to put in there.
+                 ))
+
+(define (file-systems->fstab file-systems)
+  "Return a @file{/etc} entry for an @file{fstab} describing
+@var{file-systems}."
+  `(("fstab" ,(plain-file "fstab"
+                          (string-append
+                           "\
+# This file was generated from your GuixSD configuration.  Any changes
+# will be lost upon reboot or reconfiguration.\n\n"
+                           (string-join (map file-system->fstab-entry
+                                             file-systems)
+                                        "\n")
+                           "\n")))))
+
+(define fstab-service-type
+  ;; The /etc/fstab service.
+  (service-type (name 'fstab)
+                (extensions
+                 (list (service-extension etc-service-type
+                                          file-systems->fstab)))
+                (compose identity)
+                (extend append)))
+
 (define %root-file-system-dmd-service
   (dmd-service
    (documentation "Take care of the root file system.")
@@ -170,12 +213,8 @@ (define dependency->dmd-service-name
     ((? file-system? fs)
      (file-system->dmd-service-name fs))))
 
-(define file-system-service-type
-  ;; TODO(?): Make this an extensible service that takes <file-system> objects
-  ;; and returns a list of <dmd-service>.
-  (dmd-service-type
-   'file-system
-   (lambda (file-system)
+(define (file-system-dmd-service file-system)
+  "Return a list containing the dmd service for @var{file-system}."
   (let ((target  (file-system-mount-point file-system))
         (device  (file-system-device file-system))
         (type    (file-system-type file-system))
@@ -183,7 +222,7 @@ (define file-system-service-type
         (check?  (file-system-check? file-system))
         (create? (file-system-create-mount-point? file-system))
         (dependencies (file-system-dependencies file-system)))
-       (dmd-service
+    (list (dmd-service
            (provision (list (file-system->dmd-service-name file-system)))
            (requirement `(root-file-system
                           ,@(map dependency->dmd-service-name dependencies)))
@@ -233,7 +272,17 @@ (define file-system-service-type
                        #:select (check-file-system canonicalize-device-spec))
                       ,@%default-modules))
            (imported-modules `((gnu build file-systems)
-                            ,@%default-imported-modules)))))))
+                               ,@%default-imported-modules))))))
+
+(define file-system-service-type
+  ;; TODO(?): Make this an extensible service that takes <file-system> objects
+  ;; and returns a list of <dmd-service>.
+  (service-type (name 'file-system)
+                (extensions
+                 (list (service-extension dmd-root-service-type
+                                          file-system-dmd-service)
+                       (service-extension fstab-service-type
+                                          identity)))))
 
 (define* (file-system-service file-system)
   "Return a service that mounts @var{file-system}, a @code{<file-system>}

gnu/system.scm

@@ -299,6 +299,7 @@ (define known-fs
                                     (operating-system-groups os))
                             (operating-system-skeletons os))
            (operating-system-etc-service os)
+           (service fstab-service-type '())
            (session-environment-service
             (operating-system-environment-variables os))
            host-name procs root-fs unmount