mercurial/worker.py

--- a/mercurial/worker.py	Sat Feb 01 01:32:28 2020 -0500
+++ b/mercurial/worker.py	Thu Jan 30 19:16:12 2020 +0100
@@ -226,7 +226,7 @@
     selector = selectors.DefaultSelector()
     for rfd, wfd in pipes:
         os.close(wfd)
-        selector.register(os.fdopen(rfd, 'rb', 0), selectors.EVENT_READ)
+        selector.register(os.fdopen(rfd, 'rb'), selectors.EVENT_READ)
 
     def cleanup():
         signal.signal(signal.SIGINT, oldhandler)