diff --git a/mercurial/commandserver.py b/mercurial/commandserver.py
--- a/mercurial/commandserver.py
+++ b/mercurial/commandserver.py
@@ -208,15 +208,10 @@ class server(object):
     def __init__(self, ui, repo, fin, fout):
         self.cwd = encoding.getcwd()
 
-        # developer config: cmdserver.log
-        logpath = ui.config("cmdserver", "log")
-        if logpath:
+        if ui.config("cmdserver", "log") == '-':
             global logfile
-            if logpath == '-':
-                # write log on a special 'd' (debug) channel
-                logfile = channeledoutput(fout, 'd')
-            else:
-                logfile = open(logpath, 'a')
+            # switch log stream to the 'd' (debug) channel
+            logfile = channeledoutput(fout, 'd')
 
         if repo:
             # the ui here is really the repo ui so take its baseui so we don't
@@ -361,6 +356,24 @@ class server(object):
 
         return 0
 
+def setuplogging(ui):
+    """Set up server logging facility
+
+    If cmdserver.log is '-', log messages will be sent to the 'd' channel
+    while a client is connected. Otherwise, messages will be written to
+    the stderr of the server process.
+    """
+    # developer config: cmdserver.log
+    logpath = ui.config(b'cmdserver', b'log')
+    if not logpath:
+        return
+
+    global logfile
+    if logpath == b'-':
+        logfile = ui.ferr
+    else:
+        logfile = open(logpath, 'ab')
+
 class pipeservice(object):
     def __init__(self, ui, repo, opts):
         self.ui = ui
diff --git a/mercurial/server.py b/mercurial/server.py
--- a/mercurial/server.py
+++ b/mercurial/server.py
@@ -155,9 +155,11 @@ def runservice(opts, parentfn=None, init
 def _createcmdservice(ui, repo, opts):
     mode = opts['cmdserver']
     try:
-        return _cmdservicemap[mode](ui, repo, opts)
+        servicefn = _cmdservicemap[mode]
     except KeyError:
         raise error.Abort(_('unknown mode %s') % mode)
+    commandserver.setuplogging(ui)
+    return servicefn(ui, repo, opts)
 
 def _createhgwebservice(ui, repo, opts):
     # this way we can check if something was given in the command-line