publishers.py

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from time import time
from copy import copy
from threading import Lock
from rospy import Publisher, SubscribeListener
from rostopic import get_topic_type
from rosbridge_library.internal import ros_loader, message_conversion
from rosbridge_library.internal.topics import TopicNotEstablishedException, TypeConflictException


class PublisherConsistencyListener(SubscribeListener):
    """ This class is used to solve the problem that sometimes we create a
    publisher and then immediately publish a message, before the subscribers
    have set up their connections.

    Call attach() to attach the listener to a publisher.  It sets up a buffer
    of outgoing messages, then when a new connection occurs, sends the messages
    in the buffer.

    Call detach() to detach the listener from the publisher and restore the
    original publish methods.

    After some particular timeout (default to 1 second), the listener stops
    buffering messages as it is assumed by this point all subscribers will have
    successfully set up their connections."""

    timeout = 1  # Timeout in seconds to wait for new subscribers
    attached = False

    def attach(self, publisher):
        """ Overrides the publisher's publish method, and attaches a subscribe
        listener to the publisher, effectively routing incoming connections
        and outgoing publish requests through this class instance """
        # Do the attaching
        self.publisher = publisher
        publisher.impl.add_subscriber_listener(self)
        self.publish = publisher.publish
        publisher.publish = self.publish_override

        # Set state variables
        self.lock = Lock()
        self.established_time = time()
        self.msg_buffer = []
        self.attached = True

    def detach(self):
        """ Restores the publisher's original publish method and unhooks the
        subscribe listeners, effectively finishing with this object """
        self.publisher.publish = self.publish
        if self in self.publisher.impl.subscriber_listeners:
            self.publisher.impl.subscriber_listeners.remove(self)
        self.attached = False

    def peer_subscribe(self, topic_name, topic_publish, peer_publish):
        """ Called whenever there's a new subscription.

        If we're still inside the subscription setup window, then we publish
        any buffered messages to the peer.

        We also check if we're timed out, but if we are we don't detach (due
        to threading complications), we just don't propagate buffered messages
        """
        if not self.timed_out():
            self.lock.acquire()
            msgs = copy(self.msg_buffer)
            self.lock.release()
            for msg in msgs:
                peer_publish(msg)

    def timed_out(self):
        """ Checks to see how much time has elapsed since the publisher was
        created """
        return time() - self.established_time > self.timeout

    def publish_override(self, message):
        """ The publisher's publish method is replaced with this publish method
        which checks for timeout and if we haven't timed out, buffers outgoing
        messages in preparation for new subscriptions """
        if not self.timed_out():
            self.lock.acquire()
            self.msg_buffer.append(message)
            self.lock.release()
        self.publish(message)


class MultiPublisher():
    """ Keeps track of the clients that are using a particular publisher.

    Provides an API to publish messages and register clients that are using
    this publisher """

    def __init__(self, topic, msg_type=None):
        """ Register a publisher on the specified topic.

        Keyword arguments:
        topic    -- the name of the topic to register the publisher to
        msg_type -- (optional) the type to register the publisher as.  If not
        provided, an attempt will be made to infer the topic type

        Throws:
        TopicNotEstablishedException -- if no msg_type was specified by the
        caller and the topic is not yet established, so a topic type cannot
        be inferred
        TypeConflictException        -- if the msg_type was specified by the
        caller and the topic is established, and the established type is
        different to the user-specified msg_type

        """
        # First check to see if the topic is already established
        topic_type = get_topic_type(topic)[0]

        # If it's not established and no type was specified, exception
        if msg_type is None and topic_type is None:
            raise TopicNotEstablishedException(topic)

        # Use the established topic type if none was specified
        if msg_type is None:
            msg_type = topic_type

        # Load the message class, propagating any exceptions from bad msg types
        msg_class = ros_loader.get_message_class(msg_type)

        # Make sure the specified msg type and established msg type are same
        if topic_type is not None and topic_type != msg_class._type:
            raise TypeConflictException(topic, topic_type, msg_class.type)

        # Create the publisher and associated member variables
        self.clients = {}
        self.topic = topic
        self.msg_class = msg_class
        self.publisher = Publisher(topic, msg_class)
        self.listener = PublisherConsistencyListener()
        self.listener.attach(self.publisher)

    def unregister(self):
        """ Unregisters the publisher and clears the clients """
        # Rospy's unregister is unreliable.
        # self.publisher.unregister()
        self.clients.clear()

    def verify_type(self, msg_type):
        """ Verify that the publisher publishes messages of the specified type.

        Keyword arguments:
        msg_type -- the type to check this publisher against

        Throws:
        Exception -- if ros_loader cannot load the specified msg type
        TypeConflictException -- if the msg_type is different than the type of
        this publisher

        """
        if not ros_loader.get_message_class(msg_type) is self.msg_class:
            raise TypeConflictException(self.topic,
                                        self.msg_class._type, msg_type)
        return

    def publish(self, msg):
        """ Publish a message using this publisher.

        Keyword arguments:
        msg -- the dict (json) message to publish

        Throws:
        Exception -- propagates exceptions from message conversion if the
        provided msg does not properly conform to the message type of this
        publisher

        """
        # First, check the publisher consistency listener to see if it's done
        if self.listener.attached and self.listener.timed_out():
            self.listener.detach()

        # Create a message instance
        inst = self.msg_class()

        # Populate the instance, propagating any exceptions that may be thrown
        message_conversion.populate_instance(msg, inst)

        # Publish the message
        self.publisher.publish(inst)

    def register_client(self, client_id):
        """ Register the specified client as a client of this publisher.

        Keyword arguments:
        client_id -- the ID of the client using the publisher

        """
        self.clients[client_id] = True

    def unregister_client(self, client_id):
        """ Unregister the specified client from this publisher.

        If the specified client_id is not a client of this publisher, nothing
        happens.

        Keyword arguments:
        client_id -- the ID of the client to remove

        """
        if client_id in self.clients:
            del self.clients[client_id]

    def has_clients(self):
        """ Return true if there are clients to this publisher. """
        return len(self.clients) != 0


class PublisherManager():
    """ The PublisherManager keeps track of ROS publishers

    It maintains a MultiPublisher instance for each registered topic

    When unregistering a client, if there are no more clients for a publisher,
    then that publisher is unregistered from the ROS Master
    """

    def __init__(self):
        self._publishers = {}

    def register(self, client_id, topic, msg_type=None):
        """ Register a publisher on the specified topic.

        Publishers are shared between clients, so a single MultiPublisher
        instance is created per topic, even if multiple clients register.

        Keyword arguments:
        client_id -- the ID of the client making this request
        topic     -- the name of the topic to publish on
        msg_type  -- (optional) the type to publish

        Throws:
        Exception -- exceptions are propagated from the MultiPublisher if
        there is a problem loading the specified msg class or establishing
        the publisher

        """
        if not topic in self._publishers:
            self._publishers[topic] = MultiPublisher(topic, msg_type)

        if msg_type is not None:
            self._publishers[topic].verify_type(msg_type)

        self._publishers[topic].register_client(client_id)

    def unregister(self, client_id, topic):
        """ Unregister a client from the publisher for the given topic.

        If there are no clients remaining for that publisher, then the
        publisher is unregistered from the ROS Master

        Keyword arguments:
        client_id -- the ID of the client making this request
        topic     -- the topic to unregister the publisher for

        """
        if not topic in self._publishers:
            return

        self._publishers[topic].unregister_client(client_id)

        if not self._publishers[topic].has_clients():
            self._publishers[topic].unregister()
            del self._publishers[topic]

    def unregister_all(self, client_id):
        """ Unregisters a client from all publishers that they are registered
        to.

        Keyword arguments:
        client_id -- the ID of the client making this request """
        for topic in self._publishers.keys():
            self.unregister(client_id, topic)

    def publish(self, client_id, topic, msg):
        """ Publish a message on the given topic.

        Tries to create a publisher on the topic if one does not already exist.

        Keyword arguments:
        client_id -- the ID of the client making this request
        topic     -- the topic to publish the message on
        msg       -- a JSON-like dict of fields and values

        Throws:
        Exception -- a variety of exceptions are propagated.  They can be
        thrown if there is a problem setting up or getting the publisher,
        or if the provided msg does not map to the msg class of the publisher.

        """
        self.register(client_id, topic)

        self._publishers[topic].publish(msg)


manager = PublisherManager()