diff -r 580bca200874 -r 5ad37164a8fe mercurial/testing/__init__.py
--- a/mercurial/testing/__init__.py	Wed Aug 04 19:45:13 2021 +0200
+++ b/mercurial/testing/__init__.py	Tue Aug 03 19:26:26 2021 +0200
@@ -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)