system: Add btrfs file system support.

* gnu/build/file-systems.scm (%btrfs-endianness, btrfs-superblock?,
  read-btrfs-superblock, btrfs-superblock-uuid,
  btrfs-superblock-volume-name, check-btrfs-file-system): New variables.
  (%paritition-label-readers, %partition-uuid-readers): Add btrfs
  readers.
* gnu/system/linux-initrd.scm (linux-modules): Add btrfs modules when a
  btrfs file-system is used.
* gnu/tests/install.scm (%btrfs-root-os %btrfs-root-os-source,
  %btrfs-root-installation-script, %test-btrfs-root-os): New system
  test.
* doc/guix.texi: Adjust accordingly.

Fixes <http://bugs.gnu.org/19280>.

doc/guix.texi

@@ -6919,9 +6919,9 @@ cfdisk
 
 Once you are done partitioning the target hard disk drive, you have to
 create a file system on the relevant partition(s)@footnote{Currently
-GuixSD pretty much assumes an ext4 file system.  In particular, code
+GuixSD only supports ext4 and btrfs file systems.  In particular, code
-that reads partition UUIDs and labels only works with ext4.  This will
+that reads partition UUIDs and labels only works for these file system
-be fixed in the future.}.
+types.}.
 
 Preferably, assign partitions a label so that you can easily and
 reliably refer to them in @code{file-system} declarations (@pxref{File

gnu/build/file-systems.scm

@@ -144,6 +144,43 @@ if DEVICE does not contain an ext2 file system."
     (2 'reboot-required)
     (_ 'fatal-error)))
 
+
+;;;
+;;; Btrfs file systems.
+;;;
+
+;; <https://btrfs.wiki.kernel.org/index.php/On-disk_Format#Superblock>.
+
+(define-syntax %btrfs-endianness
+  ;; Endianness of btrfs file systems.
+  (identifier-syntax (endianness little)))
+
+(define (btrfs-superblock? sblock)
+  "Return #t when SBLOCK is a btrfs superblock."
+  (bytevector=? (sub-bytevector sblock 64 8)
+                (string->utf8 "_BHRfS_M")))
+
+(define (read-btrfs-superblock device)
+  "Return the raw contents of DEVICE's btrfs superblock as a bytevector, or #f
+if DEVICE does not contain a btrfs file system."
+  (read-superblock device 65536 4096 btrfs-superblock?))
+
+(define (btrfs-superblock-uuid sblock)
+  "Return the UUID of a btrfs superblock SBLOCK as a 16-byte bytevector."
+  (sub-bytevector sblock 32 16))
+
+(define (btrfs-superblock-volume-name sblock)
+  "Return the volume name of SBLOCK as a string of at most 256 characters, or
+#f if SBLOCK has no volume name."
+  (null-terminated-latin1->string (sub-bytevector sblock 299 256)))
+
+(define (check-btrfs-file-system device)
+  "Return the health of a btrfs file system on DEVICE."
+  (match (status:exit-val
+          (system* "btrfs" "device" "scan"))
+    (0 'pass)
+    (_ 'fatal-error)))
+
 
 ;;;
 ;;; LUKS encrypted devices.
@@ -257,11 +294,15 @@ partition field reader that returned a value."
 
 (define %partition-label-readers
   (list (partition-field-reader read-ext2-superblock
-                                ext2-superblock-volume-name)))
+                                ext2-superblock-volume-name)
+        (partition-field-reader read-btrfs-superblock
+                                btrfs-superblock-volume-name)))
 
 (define %partition-uuid-readers
   (list (partition-field-reader read-ext2-superblock
-                                ext2-superblock-uuid)))
+                                ext2-superblock-uuid)
+        (partition-field-reader read-btrfs-superblock
+                                btrfs-superblock-uuid)))
 
 (define read-partition-label
   (cut read-partition-field <> %partition-label-readers))
@@ -428,6 +469,7 @@ the following:
   (define check-procedure
     (cond
      ((string-prefix? "ext" type) check-ext2-file-system)
+     ((string-prefix? "btrfs" type) check-btrfs-file-system)
      (else #f)))
 
   (if check-procedure

gnu/system/linux-initrd.scm

@@ -197,6 +197,9 @@ loaded at boot time in the order in which they appear."
       ,@(if (find (file-system-type-predicate "vfat") file-systems)
             '("nls_iso8859-1")
             '())
+      ,@(if (find (file-system-type-predicate "btrfs") file-systems)
+            '("btrfs")
+            '())
       ,@(if volatile-root?
             '("fuse")
             '())
@@ -214,6 +217,9 @@ loaded at boot time in the order in which they appear."
                   file-systems)
             (list fatfsck/static)
             '())
+      ,@(if (find (file-system-type-predicate "btrfs") file-systems)
+            (list btrfs-progs/static)
+            '())
       ,@(if volatile-root?
             (list unionfs-fuse/static)
             '())))

gnu/tests/install.scm

@@ -36,7 +36,8 @@
   #:export (%test-installed-os
             %test-separate-store-os
             %test-raid-root-os
-            %test-encrypted-os))
+            %test-encrypted-os
+            %test-btrfs-root-os))
 
 ;;; Commentary:
 ;;;
@@ -518,4 +519,78 @@ build (current-guix) and then store a couple of full system images.")
       (run-basic-test %encrypted-root-os command "encrypted-root-os"
                       #:initialization enter-luks-passphrase)))))
 
+
+;;;
+;;; Btrfs root file system.
+;;;
+
+(define-os-with-source (%btrfs-root-os %btrfs-root-os-source)
+  ;; The OS we want to install.
+  (use-modules (gnu) (gnu tests) (srfi srfi-1))
+
+  (operating-system
+    (host-name "liberigilo")
+    (timezone "Europe/Paris")
+    (locale "en_US.UTF-8")
+
+    (bootloader (grub-configuration (device "/dev/vdb")))
+    (kernel-arguments '("console=ttyS0"))
+    (file-systems (cons (file-system
+                          (device "my-root")
+                          (title 'label)
+                          (mount-point "/")
+                          (type "btrfs"))
+                        %base-file-systems))
+    (users (cons (user-account
+                  (name "charlie")
+                  (group "users")
+                  (home-directory "/home/charlie")
+                  (supplementary-groups '("wheel" "audio" "video")))
+                 %base-user-accounts))
+    (services (cons (service marionette-service-type
+                             (marionette-configuration
+                              (imported-modules '((gnu services herd)
+                                                  (guix combinators)))))
+                    %base-services))))
+
+(define %btrfs-root-installation-script
+  ;; Shell script of a simple installation.
+  "\
+. /etc/profile
+set -e -x
+guix --version
+
+export GUIX_BUILD_OPTIONS=--no-grafts
+ls -l /run/current-system/gc-roots
+parted --script /dev/vdb mklabel gpt \\
+  mkpart primary ext2 1M 3M \\
+  mkpart primary ext2 3M 1G \\
+  set 1 boot on \\
+  set 1 bios_grub on
+mkfs.btrfs -L my-root /dev/vdb2
+mount /dev/vdb2 /mnt
+btrfs subvolume create /mnt/home
+herd start cow-store /mnt
+mkdir /mnt/etc
+cp /etc/target-config.scm /mnt/etc/config.scm
+guix system build /mnt/etc/config.scm
+guix system init /mnt/etc/config.scm /mnt --no-substitutes
+sync
+reboot\n")
+
+(define %test-btrfs-root-os
+  (system-test
+   (name "btrfs-root-os")
+   (description
+    "Test basic functionality of an OS installed like one would do by hand.
+This test is expensive in terms of CPU and storage usage since we need to
+build (current-guix) and then store a couple of full system images.")
+   (value
+    (mlet* %store-monad ((image   (run-install %btrfs-root-os
+                                               %btrfs-root-os-source
+                                               #:script
+                                               %btrfs-root-installation-script))
+                         (command (qemu-command/writable-image image)))
+      (run-basic-test %btrfs-root-os command "btrfs-root-os")))))
+
 ;;; install.scm ends here