utils: 'with-atomic-file-output' calls 'fdatasync'.

Suggested by Danny Milosavljevic <dannym@scratchpost.org>
at <https://lists.gnu.org/archive/html/guix-devel/2016-06/msg00456.html>.

* guix/build/syscalls.scm (fdatasync): New procedure.
* guix/utils.scm (with-atomic-file-output): Use it.  Use 'close-port'
instead of 'close'.

guix/build/syscalls.scm

@@ -64,6 +64,7 @@ (define-module (guix build syscalls)
 
             processes
             mkdtemp!
+            fdatasync
             pivot-root
             fcntl-flock
 
@@ -506,6 +507,20 @@ (define mkdtemp!
                  (list err)))
         (pointer->string result)))))
 
+(define fdatasync
+  (let ((proc (syscall->procedure int "fdatasync" (list int))))
+    (lambda (port)
+      "Flush buffered output of PORT, an output file port, and then call
+fdatasync(2) on the underlying file descriptor."
+      (force-output port)
+      (let* ((fd  (fileno port))
+             (ret (proc fd))
+             (err (errno)))
+        (unless (zero? ret)
+          (throw 'system-error "fdatasync" "~S: ~A"
+                 (list fd (strerror err))
+                 (list err)))))))
+
 
 (define-record-type <file-system>
   (file-system type block-size blocks blocks-free

guix/utils.scm

@@ -34,7 +34,7 @@ (define-module (guix utils)
   #:use-module ((rnrs bytevectors) #:select (bytevector-u8-set!))
   #:use-module (guix combinators)
   #:use-module ((guix build utils) #:select (dump-port))
-  #:use-module ((guix build syscalls) #:select (mkdtemp!))
+  #:use-module ((guix build syscalls) #:select (mkdtemp! fdatasync))
   #:use-module (ice-9 vlist)
   #:use-module (ice-9 format)
   #:autoload   (ice-9 popen)  (open-pipe*)
@@ -625,7 +625,8 @@ (define (with-atomic-file-output file proc)
     (with-throw-handler #t
       (lambda ()
         (let ((result (proc out)))
-          (close out)
+          (fdatasync out)
+          (close-port out)
           (rename-file template file)
           result))
       (lambda (key . args)