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.
* gnu/services/base.scm (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 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 requirements.

Change-Id: If0392db03d48e8820aa53df1df482c12ec72e1a5

3 files changed, 28 insertions, 2 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 3ee9f54773..5c89e2110f 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -17750,6 +17750,19 @@ 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{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 requirements field
+are mounted after Shepherd services have begun. Any Shepherd service
+that depends on a file system with a non-empty requirements field must
+depend on it directly and not on the generic symbol @code{file-systems}.
 @end table
 @end deftp
 
diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 3f912225a0..4fd946c4aa 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -403,6 +403,7 @@ 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-requirements file-system))
         (packages (file-system-packages (list file-system))))
     (and (or mount? create?)
          (with-imported-modules (source-module-closure
@@ -411,7 +412,8 @@ 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?
@@ -460,12 +462,20 @@ 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-requirements file-system)))
+                                        file-systems))))
        (documentation "Target for all the initially-mounted file systems")
        (start #~(const #t))
        (stop #~(const #f))))
diff --git a/gnu/system/file-systems.scm b/gnu/system/file-systems.scm
index af0567bd3e..76a51a2b69 100644
--- a/gnu/system/file-systems.scm
+++ b/gnu/system/file-systems.scm
@@ -57,6 +57,7 @@
             file-system-repair
             file-system-create-mount-point?
             file-system-dependencies
+            file-system-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>
+  (requirements     file-system-requirements      ; list of symbols
+                    (default '()))
   (location         file-system-location
                     (default (current-source-location))
                     (innate)))