diff -r 1527f40de3b3 -r 5bc7ff103081 mercurial/commandserver.py
--- a/mercurial/commandserver.py	Sat Mar 10 15:50:09 2018 +0900
+++ b/mercurial/commandserver.py	Sat Mar 10 15:57:16 2018 +0900
@@ -303,8 +303,8 @@
     ui.flush()
     newfiles = []
     nullfd = os.open(os.devnull, os.O_RDWR)
-    for f, sysf, mode in [(ui.fin, util.stdin, pycompat.sysstr('rb')),
-                          (ui.fout, util.stdout, pycompat.sysstr('wb'))]:
+    for f, sysf, mode in [(ui.fin, util.stdin, r'rb'),
+                          (ui.fout, util.stdout, r'wb')]:
         if f is sysf:
             newfd = os.dup(f.fileno())
             os.dup2(nullfd, f.fileno())