guix: build: utils: Add delete-all-but.

* guix/build/utils.scm (delete-all-but): New variable.

Change-Id: I15e1d632a97236b8d18cec3be993c81e138dff04
Reviewed-by: Maxim Cournoyer <maxim@guixotic.coop>
Reviewed-by: Cayetano Santos <csantosb@inventati.org>
Reviewed-by: Gabriel Wicki <gabriel@erlikon.ch>
Signed-off-by: Nguyễn Gia Phong <cnx@loang.net>

guix/build/utils.scm

@@ -78,6 +78,7 @@
             make-file-writable
             copy-recursively
             delete-file-recursively
+            delete-all-but
             file-name-predicate
             find-files
             false-if-file-not-found
@@ -523,6 +524,14 @@ errors."
                       ;; Don't follow symlinks.
                       lstat)))
 
+(define (delete-all-but directory . preserve)
+  "Delete DIRECTORY recursively except for the files in PRESERVE immediately
+found under it."
+  (with-directory-excursion directory
+    (let* ((pred (negate (cut member <> (cons* "." ".." preserve))))
+           (items (scandir "." pred)))
+      (for-each (cut delete-file-recursively <>) items))))
+
 (define (file-name-predicate regexp)
   "Return a predicate that returns true when passed a file name whose base
 name matches REGEXP."