diff --git a/mercurial/testing/__init__.py b/mercurial/testing/__init__.py
--- a/mercurial/testing/__init__.py
+++ b/mercurial/testing/__init__.py
@@ -33,5 +33,11 @@
 
 
 def write_file(path, content=b''):
-    with open(path, 'wb') as f:
+    if content:
+        write_path = b'%s.tmp' % path
+    else:
+        write_path = path
+    with open(write_path, 'wb') as f:
         f.write(content)
+    if path != write_path:
+        os.rename(write_path, path)