diff -r ed9170ff791a -r 5d28246b9acc mercurial/worker.py
--- a/mercurial/worker.py	Tue May 17 13:53:14 2022 +0100
+++ b/mercurial/worker.py	Sat May 21 22:22:19 2022 +0200
@@ -280,6 +280,10 @@
     selector = selectors.DefaultSelector()
     for rfd, wfd in pipes:
         os.close(wfd)
+        # The stream has to be unbuffered. Otherwise, if all data is read from
+        # the raw file into the buffer, the selector thinks that the FD is not
+        # ready to read while pickle.load() could read from the buffer. This
+        # would delay the processing of readable items.
         selector.register(os.fdopen(rfd, 'rb', 0), selectors.EVENT_READ)
 
     def cleanup():