Use thread local storage for caching instances of ServerProxy (#1732)

xmlrpc.client.ServerProxy is not thread safe. See
https://bugs.python.org/issue6907

The symptom of this bug is exceptions in the publisherUpdate
logged in the master.log. For example:

[rosmaster.threadpool][ERROR] : Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosmaster/threadpool.py", line 218, in run
    result = cmd(*args)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosmaster/master_api.py", line 210, in publisher_update_task
    ret = xmlrpcapi(api).publisherUpdate('/master', topic, pub_uris)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosmaster/util.py", line 68, in xmlrpcapi
    close_half_closed_sockets()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosmaster/util.py", line 79, in close_half_closed_sockets
    state = transport._connection[1].sock.getsockopt(socket.SOL_TCP, socket.TCP_INFO)
  File "/usr/lib/python2.7/socket.py", line 228, in meth
    return getattr(self._sock,name)(*args)
  File "/usr/lib/python2.7/socket.py", line 174, in _dummy
    raise error(EBADF, 'Bad file descriptor')
error: [Errno 9] Bad file descriptor

Some subscribers get the update but some do not. For example, the topic
is recorded in a rosbag but not received by nodes that depend on it.

Issue: #1523

tools/rosmaster/src/rosmaster/util.py

@@ -51,8 +51,9 @@
 
 import errno
 import socket
+import threading
 
-_proxies = {} #cache ServerProxys
+_proxies = threading.local() #cache ServerProxys
 def xmlrpcapi(uri):
     """
     @return: instance for calling remote server or None if not a valid URI
@@ -63,16 +64,16 @@ def xmlrpcapi(uri):
     uriValidate = urlparse(uri)
     if not uriValidate[0] or not uriValidate[1]:
         return None
-    if not uri in _proxies:
-        _proxies[uri] = ServerProxy(uri)
+    if not uri in _proxies.__dict__:
+        _proxies.__dict__[uri] = ServerProxy(uri)
     close_half_closed_sockets()
-    return _proxies[uri]
+    return _proxies.__dict__[uri]
 
 
 def close_half_closed_sockets():
     if not hasattr(socket, 'TCP_INFO'):
         return
-    for proxy in _proxies.values():
+    for proxy in _proxies.__dict__.values():
         transport = proxy("transport")
         if transport._connection and transport._connection[1] is not None and transport._connection[1].sock is not None:
             try:
@@ -86,5 +87,5 @@ def close_half_closed_sockets():
 
 
 def remove_server_proxy(uri):
-    if uri in _proxies:
-        del _proxies[uri]
+    if uri in _proxies.__dict__:
+        del _proxies.__dict__[uri]