--- a/python/mozbuild/mozbuild/backend/base.py
+++ b/python/mozbuild/mozbuild/backend/base.py
@@ -190,31 +190,32 @@ class BuildBackend(LoggingMixin):
         This is the main method used by child classes to react to build
         metadata.
         """
 
     def consume_finished(self):
         """Called when consume() has completed handling all objects."""
 
     @contextmanager
-    def _write_file(self, path=None, fh=None):
+    def _write_file(self, path=None, fh=None, mode='rU'):
         """Context manager to write a file.
 
         This is a glorified wrapper around FileAvoidWrite with integration to
         update the summary data on this instance.
 
         Example usage:
 
             with self._write_file('foo.txt') as fh:
                 fh.write('hello world')
         """
 
         if path is not None:
             assert fh is None
-            fh = FileAvoidWrite(path, capture_diff=True, dry_run=self.dry_run)
+            fh = FileAvoidWrite(path, capture_diff=True, dry_run=self.dry_run,
+                                mode=mode)
         else:
             assert fh is not None
 
         dirname = mozpath.dirname(fh.name)
         try:
             os.makedirs(dirname)
         except OSError as error:
             if error.errno != errno.EEXIST: