bootloader: Factorize write-file-on-device.

* gnu/bootloader/extlinux.scm (install-extlinux): Factorize bootloader
writing in a new procedure write-file-on-device defined in (gnu build
bootloader).
* gnu/build/bootloader.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add new file.
* gnu/system/vm.scm (qemu-img): Adapt to import and use (gnu build bootloader)
module during derivation building.
* gnu/scripts/system.scm (bootloader-installer-derivation): Ditto.

gnu/bootloader/extlinux.scm

@@ -20,6 +20,7 @@
 (define-module (gnu bootloader extlinux)
   #:use-module (gnu bootloader)
   #:use-module (gnu system)
+  #:use-module (gnu build bootloader)
   #:use-module (gnu packages bootloaders)
   #:use-module (guix gexp)
   #:use-module (guix monads)
@@ -95,13 +96,8 @@ TIMEOUT ~a~%"
                   (find-files syslinux-dir "\\.c32$"))
         (unless
             (and (zero? (system* extlinux "--install" install-dir))
-                 (call-with-input-file (string-append syslinux-dir "/" #$mbr)
-                   (lambda (input)
-                     (let ((bv (get-bytevector-n input 440)))
-                       (call-with-output-file device
-                         (lambda (output)
-                           (put-bytevector output bv))
-                         #:binary #t)))))
+                 (write-file-on-device
+                  (string-append syslinux-dir "/" #$mbr) 440 device 0))
           (error "failed to install SYSLINUX")))))
 
 (define install-extlinux-mbr

gnu/build/bootloader.scm

@@ -0,0 +1,37 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu build bootloader)
+  #:use-module (ice-9 binary-ports)
+  #:export (write-file-on-device))
+
+
+;;;
+;;; Writing utils.
+;;;
+
+(define (write-file-on-device file size device offset)
+  "Write SIZE bytes from FILE to DEVICE starting at OFFSET."
+  (call-with-input-file file
+    (lambda (input)
+      (let ((bv (get-bytevector-n input size)))
+        (call-with-output-file device
+          (lambda (output)
+            (seek output offset SEEK_SET)
+            (put-bytevector output bv))
+          #:binary #t)))))

gnu/local.mk

@@ -489,6 +489,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/system/vm.scm				\
 						\
   %D%/build/activation.scm			\
+  %D%/build/bootloader.scm			\
   %D%/build/cross-toolchain.scm			\
   %D%/build/file-systems.scm			\
   %D%/build/install.scm				\

gnu/system/vm.scm

@@ -277,10 +277,12 @@ register INPUTS in the store database of the image so that Guix can be used in
 the image."
   (expression->derivation-in-linux-vm
    name
-   (with-imported-modules (source-module-closure '((gnu build vm)
+   (with-imported-modules (source-module-closure '((gnu build bootloader)
+                                                   (gnu build vm)
                                                    (guix build utils)))
      #~(begin
-         (use-modules (gnu build vm)
+         (use-modules (gnu build bootloader)
+                      (gnu build vm)
                       (guix build utils)
                       (srfi srfi-26)
                       (ice-9 binary-ports))

guix/scripts/system.scm

@@ -674,9 +674,11 @@ any, are available.  Raise an error if they're not."
 and TARGET arguments."
   (with-monad %store-monad
     (gexp->file "bootloader-installer"
-                (with-imported-modules '((guix build utils))
+                (with-imported-modules '((gnu build bootloader)
+                                         (guix build utils))
                   #~(begin
-                      (use-modules (guix build utils)
+                      (use-modules (gnu build bootloader)
+                                   (guix build utils)
                                    (ice-9 binary-ports))
                       (#$installer #$bootloader #$device #$target))))))