--- a/mercurial/commandserver.py	Thu Feb 20 16:21:00 2020 -0800
+++ b/mercurial/commandserver.py	Mon Feb 24 13:28:49 2020 +0900
@@ -160,7 +160,7 @@
             buf = s
             # keep asking for more until there's either no more or
             # we got a full line
-            while s and s[-1] != b'\n':
+            while s and not s.endswith(b'\n'):
                 s = self._read(size, b'L')
                 buf += s