如何在ROS-SMACH FSM中正确传递可变对象并解决状态转换错误？

Hey there, let's break down your SMACH object-passing issue and those frustrating errors you're seeing.

Short answer: Yes, in principle—SMACH's userdata is designed to handle mutable objects like your Talk instance. But the errors you're running into point to thread-safety and serialization issues that come with passing objects that manage active ROS interfaces (publishers/subscribers).

Let's unpack the two main issues you mentioned:

1. Exception in thread sm_introViewer:status_publisher: #

The SMACH introspection server runs in a separate thread, and it tries to serialize and publish the state of your userdata to visualize state transitions. Your Talk instance contains ROS Publisher/Subscriber objects, which aren't serializable (they have low-level handles to ROS's communication infrastructure). When the introspection thread tries to serialize talker_obj, it hits an error that crashes the thread—this is why you don't see state transitions showing up.

2. Could not execute transition callback: #

This error happens when SMACH tries to run transition callbacks while there's a race condition with your Talk object. ROS subscriber callbacks run in their own threads, and SMACH state execution/transition logic runs in another. If your Talk class doesn't use thread synchronization (like locks), multiple threads could be modifying or reading the object's state at the same time. This leads to inconsistent state that breaks the transition callback logic.

Absolutely. When the introspection thread crashes, it can leave SMACH's internal state tracking in a broken state. Worse, race conditions with your Talk object can lead to unexpected behavior in state transitions—like getting stuck in a state, or triggering the wrong transition. As your FSM grows with more states, the chance of these race conditions hitting critical paths increases, which explains why your small example works but larger FSMs fail.

Here are actionable steps to resolve these issues:

1. Make your Talk class thread-safe #

Wrap all access to mutable state in your Talk class with a lock to prevent race conditions. For example:

import threading

class Talk:
    def __init__(self):
        self._lock = threading.Lock()
        self._message_count = 0  # Example mutable state
        self.pub = None
        self.sub = None

    def init_publish(self):
        with self._lock:
            self.pub = rospy.Publisher('/topic', String, queue_size=10)

    def init_subscribe(self):
        with self._lock:
            self.sub = rospy.Subscriber('/topic', String, self._callback)

    def _callback(self, msg):
        with self._lock:
            self._message_count += 1
            # Handle incoming message safely

2. Avoid passing full ROS interface objects in userdata #

Instead of passing the entire Talk instance, extract only the data you need to share between states (like message content, counters, or flags) and pass those via userdata. Keep the ROS publisher/subscriber management inside individual states or use a thread-safe global singleton for the Talk instance. This avoids serialization issues with the introspection server entirely.

3. Verify userdata key declarations #

Double-check that every state that accesses talker_obj declares it in the appropriate keys. For example, your SENDMSG state should have:

class startMSG(smach.State):
    def __init__(self):
        smach.State.__init__(self, outcomes=['SENT', 'ENDING'], 
                             input_keys=['talker_obj'],
                             output_keys=['talker_obj'])  # Include this if you modify the object

Your core approach to passing mutable objects isn't wrong, but SMACH interacts with multiple threads (state execution, ROS callbacks, introspection) that require careful handling of shared state. Fixing thread safety and avoiding serializing ROS interface objects should resolve the errors and prevent your simulation failures.

内容的提问来源于stack exchange，提问作者TIZ012