[rospy] made get_published_topics threadsafe

[jabbenseth]

With multiple threads trying to get the currently published topics at least one of them crashes with

Exception in thread Thread-4:
Traceback (most recent call last):
    topics = rospy.get_published_topics()
  File "/ros_comm/clients/rospy/src/rospy/client.py", line 376, in get_published_topics
    code, msg, val = get_master().getPublishedTopics(namespace)
  File "/ros_comm/clients/rospy/src/rospy/msproxy.py", line 106, in wrappedF
    return f(*args, **kwds)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1233, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1587, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.7/xmlrpclib.py", line 1273, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1303, in single_request
    response = h.getresponse(buffering=True)
  File "/usr/lib/python2.7/httplib.py", line 1077, in getresponse
    raise ResponseNotReady()
ResponseNotReady

Exception in thread Thread-5:
Traceback (most recent call last):

    topics = rospy.get_published_topics()
  File "/ros_comm/clients/rospy/src/rospy/client.py", line 376, in get_published_topics
    code, msg, val = get_master().getPublishedTopics(namespace)
  File "/ros_comm/clients/rospy/src/rospy/msproxy.py", line 106, in wrappedF
    return f(*args, **kwds)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1233, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1587, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.7/xmlrpclib.py", line 1273, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1298, in single_request
    self.send_request(h, handler, request_body)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1400, in send_request
    connection.putrequest("POST", handler, skip_accept_encoding=True)
  File "/usr/lib/python2.7/httplib.py", line 906, in putrequest
    raise CannotSendRequest()
CannotSendRequest

the acquisition of f is thread safe, but the final call to f seems not.

[jspricke]

HI @ipa-jba thanks for the pull request, do you have a simple example to play around?
Looking good at first sight, @dirk-thomas any comments, as you did the implementation?

[jabbenseth]

on my machine this script crashes (without modifications) ten out of ten times:

#!/usr/bin/env python

import rospy

rospy.init_node("test_threadsafeness")

for i in range(2):
    rospy.Timer(rospy.Duration(1), rospy.get_published_topics)

rospy.spin()

[dirk-thomas]

I can reproduce the problem as well as confirm that the patch makes this case pass. Since the proxy class claims that all methods are thread-safe (

ros_comm/clients/rospy/src/rospy/msproxy.py

Line 80 in 6a0672c

All methods are thread-safe.

) it makes sense to lock for the actual call.

To avoid the need for two separate locks the operation to get the function from target could be delayed until the actual call which then could be done together in a single lock:

with self._lock:
    f = getattr(self.target, key)
    return f(*args, **kwds)

[jabbenseth]

I updated the code to implement your suggestions @dirk-thomas. One critical information for your review: I tested it on indigo and did an "untested" port to kinetic for this

[dirk-thomas]

I tested it in Kinetic and CI also runs for Kinetic on this PR. I will wait with the merge until the new CI build passes. Thank you for the patch!