linux-initrd: Mount / as a unionfs when asking for a volatile root.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Make
  /dev/fuse.
  (boot-system): Add #:unionfs parameter.  Invoke UNIONFS instead of
  copying files over when VOLATILE-ROOT? is true.
* gnu/system/linux-initrd.scm (expression->initrd): Add #:inputs
  parameter.
  [files-to-copy]: New procedure.
  [builder]: Add 'to-copy' parameter; honor it.
  (qemu-initrd)[linux-modules]: Add 'fuse.ko' when VOLATILE-ROOT?.
  Pass UNIONFS-FUSE/STATIC as #:inputs; change builder to pass #:unionfs
  to 'boot-system'.

gnu/system/linux-initrd.scm

@@ -21,12 +21,15 @@
   #:use-module (guix utils)
   #:use-module ((guix store)
                 #:select (%store-prefix))
+  #:use-module ((guix derivations)
+                #:select (derivation->output-path))
   #:use-module (gnu packages cpio)
   #:use-module (gnu packages compression)
   #:use-module (gnu packages linux)
   #:use-module (gnu packages guile)
   #:use-module ((gnu packages make-bootstrap)
                 #:select (%guile-static-stripped))
+  #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:export (expression->initrd
             qemu-initrd
@@ -49,12 +52,14 @@
                              (name "guile-initrd")
                              (system (%current-system))
                              (modules '())
+                             (inputs '())
                              (linux #f)
                              (linux-modules '()))
   "Return a package that contains a Linux initrd (a gzipped cpio archive)
 containing GUILE and that evaluates EXP upon booting.  LINUX-MODULES is a list
-of `.ko' file names to be copied from LINUX into the initrd.  MODULES is a
+of `.ko' file names to be copied from LINUX into the initrd.  INPUTS is a list
-list of Guile module names to be embedded in the initrd."
+of additional inputs to be copied in the initrd.  MODULES is a list of Guile
+module names to be embedded in the initrd."
 
   ;; General Linux overview in `Documentation/early-userspace/README' and
   ;; `Documentation/filesystems/ramfs-rootfs-initramfs.txt'.
@@ -63,7 +68,16 @@ list of Guile module names to be embedded in the initrd."
     ;; Return a regexp that matches STR exactly.
     (string-append "^" (regexp-quote str) "$"))
 
-  (define builder
+  (define (files-to-copy)
+    (mlet %store-monad ((inputs (lower-inputs inputs)))
+      (return (map (match-lambda
+                    ((_ drv)
+                     (derivation->output-path drv))
+                    ((_ drv sub-drv)
+                     (derivation->output-path drv sub-drv)))
+                   inputs))))
+
+  (define (builder to-copy)
     `(begin
        (use-modules (guix build utils)
                     (ice-9 pretty-print)
@@ -137,6 +151,18 @@ list of Guile module names to be embedded in the initrd."
                                  ,module module-dir))))
                     linux-modules))
 
+           ,@(if (null? to-copy)
+                 '()
+                 `((let ((store ,(string-append "." (%store-prefix))))
+                     (mkdir-p store)
+                     ;; XXX: Should we do export-references-graph?
+                     (for-each (lambda (input)
+                                 (let ((target
+                                        (string-append store "/"
+                                                       (basename input))))
+                                  (copy-recursively input target)))
+                               ',to-copy))))
+
            ;; Reset the timestamps of all the files that will make it in the
            ;; initrd.
            (for-each (cut utime <> 0 0 0 0)
@@ -184,8 +210,10 @@ list of Guile module names to be embedded in the initrd."
                     ("modules/compiled" ,compiled)
                     ,@(if linux
                           `(("linux" ,linux))
-                          '())))))
+                          '())
-   (derivation-expression name builder
+                    ,@inputs)))
+       (to-copy  (files-to-copy)))
+   (derivation-expression name (builder to-copy)
                           #:modules '((guix build utils))
                           #:inputs inputs)))
 
@@ -224,8 +252,13 @@ to it are lost."
             '())
       ,@(if (assoc-ref mounts '9p)
             virtio-9p-modules
+            '())
+      ,@(if volatile-root?
+            '("fuse.ko")
             '())))
 
+  (mlet %store-monad
+      ((unionfs (package-file unionfs-fuse/static "bin/unionfs")))
     (expression->initrd
      `(begin
         (use-modules (guix build linux-initrd))
@@ -234,12 +267,16 @@ to it are lost."
                      #:linux-modules ',linux-modules
                      #:qemu-guest-networking? #t
                      #:guile-modules-in-chroot? ',guile-modules-in-chroot?
+                     #:unionfs ,unionfs
                      #:volatile-root? ',volatile-root?))
      #:name "qemu-initrd"
      #:modules '((guix build utils)
                  (guix build linux-initrd))
      #:linux linux-libre
-   #:linux-modules linux-modules))
+     #:linux-modules linux-modules
+     #:inputs (if volatile-root?
+                  `(("unionfs" ,unionfs-fuse/static))
+                  '()))))
 
 (define (gnu-system-initrd)
   "Initrd for the GNU system itself, with nothing QEMU-specific."

guix/build/linux-initrd.scm

@@ -143,7 +143,10 @@
   (symlink "/proc/self/fd" (scope "dev/fd"))
   (symlink "/proc/self/fd/0" (scope "dev/stdin"))
   (symlink "/proc/self/fd/1" (scope "dev/stdout"))
-  (symlink "/proc/self/fd/2" (scope "dev/stderr")))
+  (symlink "/proc/self/fd/2" (scope "dev/stderr"))
+
+  ;; File systems in user space (FUSE).
+  (mknod (scope "dev/fuse") 'char-special #o666 (device-number 10 229)))
 
 (define %host-qemu-ipv4-address
   (inet-pton AF_INET "10.0.2.10"))
@@ -212,7 +215,7 @@ the last argument of `mknod'."
                       (linux-modules '())
                       qemu-guest-networking?
                       guile-modules-in-chroot?
-                      volatile-root?
+                      volatile-root? unionfs
                       (mounts '()))
   "This procedure is meant to be called from an initrd.  Boot a system by
 first loading LINUX-MODULES, then setting up QEMU guest networking if
@@ -277,27 +280,20 @@ to it are lost."
           (lambda ()
             (if volatile-root?
                 (begin
-                  ;; XXX: For lack of a union file system...
                   (mkdir-p "/real-root")
                   (mount root "/real-root" "ext3" MS_RDONLY)
-                  (mount "none" "/root" "tmpfs")
+                  (mkdir-p "/rw-root")
+                  (mount "none" "/rw-root" "tmpfs")
 
-                  ;; XXX: 'copy-recursively' cannot deal with device nodes, so
+                  ;; We want read-write /dev nodes.
-                  ;; explicitly avoid /dev.
+                  (make-essential-device-nodes #:root "/rw-root")
-                  (for-each (lambda (file)
-                              (unless (string=? "dev" file)
-                                (copy-recursively (string-append "/real-root/"
-                                                                 file)
-                                                  (string-append "/root/"
-                                                                 file)
-                                                  #:log (%make-void-port
-                                                         "w"))))
-                            (scandir "/real-root"
-                                     (lambda (file)
-                                       (not (member file '("." ".."))))))
 
-                  ;; TODO: Unmount /real-root.
+                  ;; Make /root a union of the tmpfs and the actual root.
-                  )
+                  (unless (zero? (system* unionfs "-o"
+                                          "cow,allow_other,use_ino,dev"
+                                          "/rw-root=RW:/real-root=RO"
+                                          "/root"))
+                    (error "unionfs failed")))
                 (mount root "/root" "ext3")))
           (lambda args
             (format (current-error-port) "exception while mounting '~a': ~s~%"