Break out our chmod while loop into fielutils.py

src/allmydata/frontends/magic_folder.py

@@ -593,18 +593,8 @@ class WriteFileMixin(object):
         if now is None:
             now = time.time()
 
-        # ensure parent directory exists
-        head, tail = os.path.split(abspath_u)
-
-        fileutil.make_dirs(head)
-        os.chmod(head, (~ self._umask) & 0777)
-        while head and head != os.path.abspath(local_path_u):
-            head, tail = os.path.split(head)
-            if head == os.path.abspath(local_path_u):
-                break
-            else:
-                os.chmod(head, (~ self._umask) & 0777)
-
+        initial, last = os.path.split(abspath_u)
+        fileutil.make_dirs_with_absolute_mode(local_path_u, initial, (~ self._umask) & 0777)
         fileutil.write(replacement_path_u, file_contents)
         os.chmod(replacement_path_u, (~ self._umask) & 0777)
 

src/allmydata/util/fileutil.py

@@ -170,6 +170,27 @@ def is_ancestor_path(parent, dirname):
             return False
     return True
 
+def make_dirs_with_absolute_mode(parent, dirname, mode,):
+    """
+    Make directory `dirname` and chmod it to `mode` afterwards.
+    We chmod all parent directories of `dirname` until we reach
+    `parent`.
+    """
+    make_dirs(dirname)
+    os.chmod(dirname, mode)
+    initial, last = os.path.split(dirname)
+    if initial == os.path.abspath(parent):
+        os.chmod(initial, mode)
+    else:
+        return
+    while initial and initial != os.path.abspath(parent):
+        initial, last = os.path.split(initial)
+        if initial == os.path.abspath(parent):
+            break
+        else:
+            os.chmod(initial, mode)
+
+
 def make_dirs(dirname, mode=0777):
     """
     An idempotent version of os.makedirs().  If the dir already exists, do