--- a/mercurial/utils/procutil.py	Thu Nov 12 15:28:06 2020 -0800
+++ b/mercurial/utils/procutil.py	Tue Nov 17 19:29:08 2020 +0900
@@ -80,6 +80,13 @@
     return LineBufferedWrapper(stream)
 
 
+def unwrap_line_buffered(stream):
+    if isinstance(stream, LineBufferedWrapper):
+        assert not isinstance(stream.orig, LineBufferedWrapper)
+        return stream.orig
+    return stream
+
+
 class WriteAllWrapper(object):
     def __init__(self, orig):
         self.orig = orig