linux-initrd: Produce cpio archives with zeroed timestamps, etc.

* guix/cpio.scm (file->cpio-header*): New procedure.
* gnu/build/linux-initrd.scm (write-cpio-archive): Add #:file->header argument
  to 'cpio:write-cpio-archive'.

gnu/build/linux-initrd.scm

@@ -68,7 +68,8 @@ COMPRESS? is true, compress it using GZIP.  On success, return OUTPUT."
 
   (call-with-output-file output
     (lambda (port)
-      (cpio:write-cpio-archive files port)))
+      (cpio:write-cpio-archive files port
+                               #:file->header cpio:file->cpio-header*)))
 
   (or (not compress?)
       (and (zero? (system* gzip "--best" output))

guix/cpio.scm

@@ -26,6 +26,7 @@
   #:export (cpio-header?
             make-cpio-header
             file->cpio-header
+            file->cpio-header*
             write-cpio-header
             read-cpio-header
 
@@ -174,6 +175,18 @@ using FILE-NAME as its file name."
                       #:rdev (stat:rdev st)
                       #:name-size (string-length file-name))))
 
+(define* (file->cpio-header* file
+                             #:optional (file-name file)
+                             #:key (stat lstat))
+  "Similar to 'file->cpio-header', but return a header with a zeroed
+modification time, inode number, UID/GID, etc.  This allows archives to be
+produced in a deterministic fashion."
+  (let ((st (stat file)))
+    (make-cpio-header #:mode (stat:mode st)
+                      #:nlink (stat:nlink st)
+                      #:size (stat:size st)
+                      #:name-size (string-length file-name))))
+
 (define %trailer
   "TRAILER!!!")