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 @@ upon boot."
         (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 @@ upon boot."
             (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 @@ upon boot."
                                  (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 @@
             file-system-repair
             file-system-create-mount-point?
             file-system-dependencies
+            file-system-shepherd-requirements
             file-system-location
 
             file-system-type-predicate
@@ -185,6 +186,8 @@ flags are found."
                          (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)))