cmdserver: wrap 'pipe' mode server by service object

This is the stub for new mode that will listen for connections on unix domain
socket.

Though --daemon option is not banned in 'pipe' mode, it is useless because
the detached 'pipe' mode server exits immediately due to null stdin. Should
it abort if --daemon is specified with --cmdserver pipe or --stdio?

mercurial/commands.py

@@ -5536,8 +5536,8 b' def serve(ui, repo, **opts):'
         s.serve_forever()
 
     if opts["cmdserver"]:
-        s = commandserver.server(ui, repo, opts["cmdserver"])
-        return s.serve()
+        service = commandserver.pipeservice(ui, repo, opts)
+        return cmdutil.service(opts, initfn=service.init, runfn=service.run)
 
     # this way we can check if something was given in the command-line
     if opts.get('port'):

mercurial/commandserver.py

@@ -248,3 +248,13 b' class server(object):'
             return 1
 
         return 0
+
+class pipeservice(object):
+    def __init__(self, ui, repo, opts):
+        self.server = server(ui, repo, opts['cmdserver'])
+
+    def init(self):
+        pass
+
+    def run(self):
+        return self.server.serve()