system: Add '%devtmpfs-file-system' for udev, and '%base-file-systems'.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>.

* gnu/services/base.scm (udev-service)[requirement]: Add
  'file-system-/dev'.
* gnu/system/file-systems.scm (%devtmpfs-file-system,
  %base-file-systems): New variables.
* gnu/system/install.scm (installation-services)[file-systems]: Use
  %base-file-systems.
* build-aux/hydra/demo-os.scm (file-systems): Likewise.
* doc/guix.texi (System Installation): Show %BASE-FILE-SYSTEMS in the
  example.
  (Using the Configuration System): Likewise.
  (File Systems): Document %base-file-systems, %devtmpfs-file-system,
  %binary-format-file-system, and %fuse-control-file-system.

build-aux/hydra/demo-os.scm

@@ -44,13 +44,13 @@
  (file-systems
   ;; We provide a dummy file system for /, but that's OK because the VM build
   ;; code will automatically declare the / file system for us.
-  (list (file-system
+  (cons* (file-system
            (mount-point "/")
            (device "dummy")
            (type "dummy"))
          ;; %fuse-control-file-system   ; needs fuse.ko
          ;; %binary-format-file-system  ; needs binfmt.ko
-        ))
+         %base-file-systems))
 
  (users (list (user-account
                (name "guest")

doc/guix.texi

@@ -2826,10 +2826,11 @@ only a root account would look like this:
   ;; Assuming /dev/sdX is the target hard disk, and /dev/sdX1 the
   ;; target root file system.
   (bootloader (grub-configuration (device "/dev/sdX")))
-  (file-systems (list (file-system
+  (file-systems (cons (file-system
                         (device "/dev/sdX1")
                         (mount-point "/")
-                        (type "ext4")))))
+                        (type "ext4"))
+                      %base-file-systems)))
 @end example
 
 @noindent
@@ -2925,10 +2926,11 @@ kernel, initial RAM disk, and boot loader looks like this:
   (locale "fr_FR.UTF-8")
   (bootloader (grub-configuration
                 (device "/dev/sda")))
-  (file-systems (list (file-system
+  (file-systems (cons (file-system
                         (device "/dev/sda1") ; or partition label
                         (mount-point "/")
-                        (type "ext3"))))
+                        (type "ext3"))
+                      %base-file-systems))
   (users (list (user-account
                 (name "alice")
                 (password "")
@@ -3055,6 +3057,32 @@ errors before being mounted.
 @end table
 @end deftp
 
+The @code{(gnu system file-systems)} exports the following useful
+variables.
+
+@defvr {Scheme Variable} %base-file-systems
+These are essential file systems that are required on normal systems,
+such as @var{%devtmpfs-file-system} (see below.)  Operating system
+declarations should always contain at least these.
+@end defvr
+
+@defvr {Scheme Variable} %devtmpfs-file-system
+The @code{devtmpfs} file system to be mounted on @file{/dev}.  This is a
+requirement for udev (@pxref{Base Services, @code{udev-service}}).
+@end defvr
+
+@defvr {Scheme Variable} %binary-format-file-system
+The @code{binfmt_misc} file system, which allows handling of arbitrary
+executable file types to be delegated to user space.  This requires the
+@code{binfmt.ko} kernel module to be loaded.
+@end defvr
+
+@defvr {Scheme Variable} %fuse-control-file-system
+The @code{fusectl} file system, which allows unprivileged users to mount
+and unmount user-space FUSE file systems.  This requires the
+@code{fuse.ko} kernel module to be loaded.
+@end defvr
+
 @node User Accounts
 @subsection User Accounts
 
@@ -3245,6 +3273,7 @@ passed to @command{guix-daemon}.
 Run @var{udev}, which populates the @file{/dev} directory dynamically.
 @end deffn
 
+
 @node Networking Services
 @subsubsection Networking Services
 

gnu/services/base.scm

@@ -473,8 +473,13 @@ (define* (udev-service #:key (udev udev))
   (with-monad %store-monad
     (return (service
              (provision '(udev))
-             (requirement '(root-file-system))
-             (documentation "Populate the /dev directory.")
+
+             ;; Udev needs /dev to be a 'devtmpfs' mount so that new device
+             ;; nodes can be added: see
+             ;; <http://www.linuxfromscratch.org/lfs/view/development/chapter07/udev.html>.
+             (requirement '(root-file-system file-system-/dev))
+
+             (documentation "Populate the /dev directory, dynamically.")
              (start #~(lambda ()
                         (define udevd
                           (string-append #$udev "/libexec/udev/udevd"))

gnu/system/file-systems.scm

@@ -30,7 +30,10 @@ (define-module (gnu system file-systems)
             file-system-options
 
             %fuse-control-file-system
-            %binary-format-file-system))
+            %binary-format-file-system
+            %devtmpfs-file-system
+
+            %base-file-systems))
 
 ;;; Commentary:
 ;;;
@@ -72,4 +75,17 @@ (define %binary-format-file-system
     (type "binfmt_misc")
     (check? #f)))
 
+(define %devtmpfs-file-system
+  ;; /dev as a 'devtmpfs' file system, needed for udev.
+  (file-system
+    (device "none")
+    (mount-point "/dev")
+    (type "devtmpfs")
+    (check? #f)))
+
+(define %base-file-systems
+  ;; List of basic file systems to be mounted.  Note that /proc and /sys are
+  ;; currently mounted by the initrd.
+  (list %devtmpfs-file-system))
+
 ;;; file-systems.scm ends here

gnu/system/install.scm

@@ -117,10 +117,11 @@ (define installation-os
     (file-systems
      ;; Note: the disk image build code overrides this root file system with
      ;; the appropriate one.
-     (list (file-system
+     (cons (file-system
              (mount-point "/")
              (device "gnu-disk-image")
-             (type "ext4"))))
+             (type "ext4"))
+           %base-file-systems))
 
     (users (list (user-account
                   (name "guest")