diff -r afcad425a0b6 -r 95c672c07116 mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py	Wed Jun 24 04:25:34 2020 +0200
+++ b/mercurial/utils/procutil.py	Thu Jun 25 00:06:23 2020 -0700
@@ -615,7 +615,9 @@
                     def _do_wait():
                         os.waitpid(pid, 0)
 
-                    threading.Thread(target=_do_wait, daemon=True).start()
+                    t = threading.Thread(target=_do_wait)
+                    t.daemon = True
+                    t.start()
                     return
                 # Parent process
                 (_pid, status) = os.waitpid(pid, 0)