--- a/mercurial/utils/procutil.py	Sun Mar 25 12:07:18 2018 +0900
+++ b/mercurial/utils/procutil.py	Sun Mar 25 12:15:33 2018 +0900
@@ -213,25 +213,27 @@
     return _testfileno(f, sys.__stdout__)
 
 def protectstdio(uin, uout):
-    """Duplicate streams and redirect original to null if (uin, uout) are
-    stdio
+    """Duplicate streams and redirect original if (uin, uout) are stdio
+
+    If uin is stdin, it's redirected to /dev/null. If uout is stdout, it's
+    redirected to stderr so the output is still readable.
 
     Returns (fin, fout) which point to the original (uin, uout) fds, but
     may be copy of (uin, uout). The returned streams can be considered
     "owned" in that print(), exec(), etc. never reach to them.
     """
     uout.flush()
-    nullfd = os.open(os.devnull, os.O_RDWR)
     fin, fout = uin, uout
     if uin is stdin:
         newfd = os.dup(uin.fileno())
+        nullfd = os.open(os.devnull, os.O_RDONLY)
         os.dup2(nullfd, uin.fileno())
+        os.close(nullfd)
         fin = os.fdopen(newfd, r'rb')
     if uout is stdout:
         newfd = os.dup(uout.fileno())
-        os.dup2(nullfd, uout.fileno())
+        os.dup2(stderr.fileno(), uout.fileno())
         fout = os.fdopen(newfd, r'wb')
-    os.close(nullfd)
     return fin, fout
 
 def restorestdio(uin, uout, fin, fout):

--- a/tests/test-commandserver.t	Sun Mar 25 12:07:18 2018 +0900
+++ b/tests/test-commandserver.t	Sun Mar 25 12:15:33 2018 +0900
@@ -249,6 +249,8 @@
   ...                input=stringio('some input'))
   *** runcommand --config hooks.pre-identify=python:hook.hook id
   eff892de26ec tip
+  hook talking
+  now try to read something: ''
 
 Clean hook cached version
   $ rm hook.py*
@@ -619,7 +621,7 @@
   > @command(b"debugwritestdout", norepo=True)
   > def debugwritestdout(ui):
   >     os.write(1, "low-level stdout fd and\n")
-  >     sys.stdout.write("stdout should be redirected to /dev/null\n")
+  >     sys.stdout.write("stdout should be redirected to stderr\n")
   >     sys.stdout.flush()
   > EOF
   $ cat <<EOF >> .hg/hgrc
@@ -657,6 +659,8 @@
   *** runcommand debugreadstdin
   read: ''
   *** runcommand debugwritestdout
+  low-level stdout fd and
+  stdout should be redirected to stderr
 
 
 run commandserver in commandserver, which is silly but should work: