system: Add a 'needed-for-boot?' field to 'mapped-device'.

* gnu/system/file-systems.scm (<mapped-device>)[needed-for-boot?]: New
  field.
* gnu/system.scm (operating-system-user-mapped-devices,
  operating-system-boot-mapped-devices): Use it instead of trying to
  guess.  Guessing doesn't work when one refers to a partition using its
  label, for instance.
* doc/guix.texi (Mapped Devices): Document 'needed-for-boot?'.

doc/guix.texi

@@ -3837,19 +3837,27 @@ detailed below.
 Objects of this type represent device mappings that will be made when
 the system boots up.
 
-@table @code
-@item source
+@table @asis
+@item @code{source}
 This string specifies the name of the block device to be mapped, such as
 @code{"/dev/sda3"}.
 
-@item target
+@item @code{target}
 This string specifies the name of the mapping to be established.  For
 example, specifying @code{"my-partition"} will lead to the creation of
 the @code{"/dev/mapper/my-partition"} device.
 
-@item type
+@item @code{type}
 This must be a @code{mapped-device-kind} object, which specifies how
 @var{source} is mapped to @var{target}.
+
+@item @code{needed-for-boot?} (default: @code{#f})
+This Boolean value indicates whether the device mapping must be made at
+boot time---i.e., from the initial RAM disk, before any user file
+systems are mounted.
+
+You would set it to @code{#t} for instance when the mapped device is
+used by the root file system.
 @end table
 @end deftp
 

gnu/system.scm

@@ -250,23 +250,14 @@ as 'needed-for-boot'."
 (define (operating-system-user-mapped-devices os)
   "Return the subset of mapped devices that can be installed in
 user-land--i.e., those not needed during boot."
-  (let ((devices      (operating-system-mapped-devices os))
-        (file-systems (operating-system-file-systems os)))
-   (filter (lambda (md)
-             (let ((user (mapped-device-user md file-systems)))
-               (or (not user)
-                   (not (file-system-needed-for-boot? user)))))
-           devices)))
+  (remove mapped-device-needed-for-boot?
+          (operating-system-mapped-devices os)))
 
 (define (operating-system-boot-mapped-devices os)
   "Return the subset of mapped devices that must be installed during boot,
 from the initrd."
-  (let ((devices      (operating-system-mapped-devices os))
-        (file-systems (operating-system-file-systems os)))
-   (filter (lambda (md)
-             (let ((user (mapped-device-user md file-systems)))
-               (and user (file-system-needed-for-boot? user))))
-           devices)))
+  (filter mapped-device-needed-for-boot?
+          (operating-system-mapped-devices os)))
 
 (define (device-mapping-services os)
   "Return the list of device-mapping services for OS as a monadic list."

gnu/system/file-systems.scm

@@ -45,6 +45,7 @@
             mapped-device-source
             mapped-device-target
             mapped-device-type
+            mapped-device-needed-for-boot?
 
             mapped-device-kind
             mapped-device-kind?
@@ -157,7 +158,9 @@ file system."
   mapped-device?
   (source    mapped-device-source)                ;string
   (target    mapped-device-target)                ;string
-  (type      mapped-device-type))                 ;<mapped-device-kind>
+  (type      mapped-device-type)                  ;<mapped-device-kind>
+  (needed-for-boot? mapped-device-needed-for-boot? ;Boolean
+                    (default #f)))
 
 (define-record-type* <mapped-device-type> mapped-device-kind
   make-mapped-device-kind