python: simple dbus example- os.fork() in service routine?

Question

I am trying to write dbus server where I want to run some external shell program ( grep here) to do the job.

when I do:

prompt$ server.py

then:

prompt$ client.py # works fine, ie. runs grep command in child process.

prompt$ client.py # ..., but second invocation produces following error message:

DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name org.example.ExampleService was not provided by any .service files

I am stuck. Are You able to help me?

here is server.py (client.py thereafter):

import gtk, glib
import os
import dbus
import dbus.service
import dbus.mainloop.glib
import subprocess

messages_queue=list()
grep_pid=0


def queue_msg(message):
    global messages_queue
    messages_queue.append(message)
    return

def dequeue_msg():
    global messages_queue,grep_pid
    if grep_pid != 0:
        try:
            pid=os.waitpid(grep_pid,os.P_NOWAIT)
        except:
            return True
        if pid[0] == 0:
            return True
        grep_pid=0

    if len(messages_queue) == 0:
            return True
    else:
            tekst=messages_queue.pop(0)

        cmd="grep 'pp'"

        print cmd
        #works fine, when I do return here
        #return True

    grep_pid=os.fork()
    if grep_pid != 0:
        return True
    os.setpgid(0,0)
    pop=subprocess.Popen(cmd,shell=True,stdin=subprocess.PIPE)
    pop.stdin.write(tekst)
    pop.stdin.close()
    pop.wait()
    exit(0)

class DemoException(dbus.DBusException):
    _dbus_error_name = 'org.example.Exception'

class MyServer(dbus.service.Object):

    @dbus.service.method("org.example.ExampleInterface",
                         in_signature='', out_signature='')
    def QueueMsg(self):
            queue_msg("ppppp")

    @dbus.service.method("org.example.ExampleInterface",
                         in_signature='', out_signature='')
    def Exit(self):
        mainloop.quit()

from dbus.mainloop.glib import threads_init

if __name__ == '__main__':
        glib.threads_init()

        threads_init()

        dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)

        session_bus = dbus.SessionBus()
        name = dbus.service.BusName("org.example.ExampleService", session_bus)
        object = MyServer(session_bus, '/My')

        glib.timeout_add_seconds(1, dequeue_msg)
        mainloop = glib.MainLoop()
        print "Running example service."
        mainloop.run()

now client.py:

import sys
from traceback import print_exc
import dbus

def main():
    bus = dbus.SessionBus()

    try:
        remote_object = bus.get_object("org.example.ExampleService",
                                       "/My")

    except dbus.DBusException:
        print_exc()
        sys.exit(1)

    iface = dbus.Interface(remote_object, "org.example.ExampleInterface")

    iface.QueueMsg()

    if sys.argv[1:] == ['--exit-service']:
        iface.Exit()

if __name__ == '__main__':
    main()

Answer 1

You usually get this error message when you try to access a service that is no longer available. Check if your server is still running.

You can use d-feet to debug your dbus connections.

Answer 2

The error message about the missing .service file means that you need to create a service file in dbus-1/services.

For example:

# /usr/local/share/dbus-1/services/org.example.ExampleService.service
[D-BUS Service]
Name=org.example.ExampleService
Exec=/home/user1401567/service.py

A lot of tutorials don't include this detail (maybe .service files didn't use to be required?) But, at least on Ubuntu 12.04, dbus services can't be connected to without it.