services: base: Add optional delayed mount of file-systems

Add a mechanism to only require mounting a subset of file-system entries
during early Shepherd initialization. Any file-system with additional Shepherd
service requirements (e.g. networking) is not required to provision
'file-systems.

* gnu/services/base.scm (file-system-shepherd-service): Splice
file-system-requirements into the Shepherd service requirement list.
(file-system-shepherd-services): Provision 'file-system only when file system
services without additional Shepherd requirements are started.
* gnu/system/file-systems.scm (file-system): Add shepherd-requirements field
to the file-system record. This field is used for adding additional Shepherd
requirements to a file-system Shepherd service.
* doc/guix.texi: Add documentation for file-system shepherd-requirements.

Change-Id: If0392db03d48e8820aa53df1df482c12ec72e1a5
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -17862,6 +17862,20 @@ a dependency of @file{/sys/fs/cgroup/cpu} and
 
 Another example is a file system that depends on a mapped device, for
 example for an encrypted partition (@pxref{Mapped Devices}).
+
+@item @code{shepherd-requirements} (default: @code{'()})
+This is a list of symbols denoting Shepherd requirements that must be
+met before mounting the file system.
+
+As an example, an NFS file system would typically have a requirement for
+@code{networking}.
+
+Typically, file systems are mounted before most other Shepherd services
+are started.  However, file systems with a non-empty
+shepherd-requirements field are mounted after Shepherd services have
+begun.  Any Shepherd service that depends on a file system with a
+non-empty shepherd-requirements field must depend on it directly and not
+on the generic symbol @code{file-systems}.
 @end table
 @end deftp
 

gnu/services/base.scm

@@ -404,6 +404,7 @@ (define (file-system-shepherd-service file-system)
         (create? (file-system-create-mount-point? file-system))
         (mount?  (file-system-mount? file-system))
         (dependencies (file-system-dependencies file-system))
+        (requirements (file-system-shepherd-requirements file-system))
         (packages (file-system-packages (list file-system))))
     (and (or mount? create?)
          (with-imported-modules (source-module-closure
@@ -412,7 +413,8 @@ (define (file-system-shepherd-service file-system)
             (provision (list (file-system->shepherd-service-name file-system)))
             (requirement `(root-file-system
                            udev
-                           ,@(map dependency->shepherd-service-name dependencies)))
+                           ,@(map dependency->shepherd-service-name dependencies)
+                           ,@requirements))
             (documentation "Check, mount, and unmount the given file system.")
             (start #~(lambda args
                        #$(if create?
@@ -461,12 +463,20 @@ (define (file-system-shepherd-services file-systems)
                                  (or (file-system-mount? x)
                                      (file-system-create-mount-point? x)))
                                file-systems)))
+
     (define sink
       (shepherd-service
        (provision '(file-systems))
        (requirement (cons* 'root-file-system 'user-file-systems
                            (map file-system->shepherd-service-name
-                                file-systems)))
+                                ;; Do not require file systems with Shepherd
+                                ;; requirements to provision
+                                ;; 'file-systems. Many Shepherd services
+                                ;; require 'file-systems, so we would likely
+                                ;; deadlock.
+                                (filter (lambda (file-system)
+                                          (null? (file-system-shepherd-requirements file-system)))
+                                        file-systems))))
        (documentation "Target for all the initially-mounted file systems")
        (start #~(const #t))
        (stop #~(const #f))))

gnu/system/file-systems.scm

@@ -57,6 +57,7 @@ (define-module (gnu system file-systems)
             file-system-repair
             file-system-create-mount-point?
             file-system-dependencies
+            file-system-shepherd-requirements
             file-system-location
 
             file-system-type-predicate
@@ -161,33 +162,35 @@ (define-syntax validate-file-system-flags
 (define-record-type* <file-system> file-system
   make-file-system
   file-system?
-  (device           file-system-device) ; string | <uuid> | <file-system-label>
-  (mount-point      file-system-mount-point)      ; string
-  (type             file-system-type)             ; string
-  (flags            file-system-flags             ; list of symbols
-                    (default '())
-                    (sanitize validate-file-system-flags))
-  (options          file-system-options           ; string or #f
-                    (default #f))
-  (mount?           file-system-mount?            ; Boolean
-                    (default #t))
-  (mount-may-fail?  file-system-mount-may-fail?   ; Boolean
-                    (default #f))
-  (needed-for-boot? %file-system-needed-for-boot? ; Boolean
-                    (default #f))
-  (check?           file-system-check?            ; Boolean
-                    (default #t))
-  (skip-check-if-clean? file-system-skip-check-if-clean? ; Boolean
-                        (default #t))
-  (repair           file-system-repair            ; symbol or #f
-                    (default 'preen))
-  (create-mount-point? file-system-create-mount-point? ; Boolean
-                       (default #f))
-  (dependencies     file-system-dependencies      ; list of <file-system>
-                    (default '()))                ; or <mapped-device>
-  (location         file-system-location
-                    (default (current-source-location))
-                    (innate)))
+  (device                file-system-device)               ; string | <uuid> | <file-system-label>
+  (mount-point           file-system-mount-point)          ; string
+  (type                  file-system-type)                 ; string
+  (flags                 file-system-flags                 ; list of symbols
+                         (default '())
+                         (sanitize validate-file-system-flags))
+  (options               file-system-options               ; string or #f
+                         (default #f))
+  (mount?                file-system-mount?                ; Boolean
+                         (default #t))
+  (mount-may-fail?       file-system-mount-may-fail?       ; Boolean
+                         (default #f))
+  (needed-for-boot?      %file-system-needed-for-boot?     ; Boolean
+                         (default #f))
+  (check?                file-system-check?                ; Boolean
+                         (default #t))
+  (skip-check-if-clean?  file-system-skip-check-if-clean?  ; Boolean
+                         (default #t))
+  (repair                file-system-repair                ; symbol or #f
+                         (default 'preen))
+  (create-mount-point?   file-system-create-mount-point?   ; Boolean
+                         (default #f))
+  (dependencies          file-system-dependencies          ; list of <file-system>
+                         (default '()))                    ; or <mapped-device>
+  (shepherd-requirements file-system-shepherd-requirements ; list of symbols
+                         (default '()))
+  (location              file-system-location
+                         (default (current-source-location))
+                         (innate)))
 
 ;; A file system label for use in the 'device' field.
 (define-record-type <file-system-label>