webots: Emitter/Receiver communication between nodes not working after controller and simulation reset

Description Main Issue: I’m using a supervisor node to reset a robot controller and the simulation. The supervisor node and the robot node communicate with receivers/emitters. They work ok up until i reset the robot controller. In the dumbed down example below the robot is sending a counter to the supervisor, who resets the simulation and the robot controller when it receives the value 10.

After resetting the robot controller and it is restarted and properly broadcasting, it seems that the supervisor receiver stops receiving the messages (the queue length stays at 0). After several steps (12 for me in the example below) the supervisor receiver starts to pick up the old messages (after reset) in order, thus they get badly desynchronized. The send method in the robot emitter properly returns 1.

What i found: The issue seems to happen when the simulation is also reset via the supervisor.simulationReset() method is called (it is irrelevant if its called before or after resetting the robot controller. If removed, the expected behavior happens.

Side issue: Moreover, after the robot controller reset, the supervisor receives an extra value that it probably shouldn’t. According to the docs, the sent messages are received instantly and the emitter/receiver queues are cleared by the simulationReset() method.

Based on the main + side issues my guess is that the receiver/emitter queues are messed up after the simulationReset() method.

Note: In the very first step, the supervisor doesn’t receive a value, probably depending on the order the step functions of the two controllers are called.

Steps to Reproduce World .wbt file:

#VRML_SIM R2020a utf8
WorldInfo {
}
Viewpoint {
  orientation -0.6931032800836722 0.6931032800836722 0.1980295085953349 0.75
  position 1.2 1.6 2.3
}
TexturedBackground {
}
TexturedBackgroundLight {
}
RectangleArena {
}
DEF ROBOT Robot {
  translation 0.16 0.08 0
  children [
    Receiver {
    }
    Emitter {
    }
  ]
  controller "robot_controller"
}
DEF SUPERVISOR Robot {
  translation -0.22 0.08 0.1
  children [
    Receiver {
    }
    Emitter {
    }
  ]
  name "supervisor"
  controller "supervisor_controller"
  supervisor TRUE
}

robot_controller.py

from controller import Robot


class RobotEmitter:
    def __init__(self):
        self.robot = Robot()

        self.timestep = int(self.robot.getBasicTimeStep())

        self.emitter = self.robot.getEmitter("emitter")

        self.data = 0

    def get_timestep(self):
        return self.timestep

    def run(self):
        while self.robot.step(self.timestep) != -1:
            self.data += 1
            self.handle_emitter()

    def handle_emitter(self):
        string_message = str(self.data)
        string_message = string_message.encode("utf-8")
        print("robot handle emitter string message to send:", string_message)
        if self.emitter.send(string_message):
            print("message sent")
        else:
            print("emitter queue full")


robot_controller = RobotEmitter()
robot_controller.run()

supervisor_controller.py

from controller import Supervisor


class SupervisorReceiver:
    def __init__(self):
        self.supervisor = Supervisor()
        self.timestep = int(self.supervisor.getBasicTimeStep())

        self.robot = self.supervisor.getFromDef("ROBOT")
        
        self.receiver = self.supervisor.getReceiver("receiver")
        self.receiver.enable(self.timestep)
        self.receivedData = ""

    def get_timestep(self):
        return self.timestep

    def run(self):
        while self.supervisor.step(self.timestep) != -1:
            self.handle_receiver()

            if self.receivedData == "10":
                print("RESET")
                self.supervisor.simulationReset()  # This causes the issue, if you comment it out it works as expected
                self.robot.restartController()

    def handle_receiver(self):
        if self.receiver.getQueueLength() > 0:
            self.receivedData = self.receiver.getData().decode("utf-8")
            print("supervisor handle receiver data:", self.receivedData)
            self.receiver.nextPacket()
        else:
            print("supervisor receiver q is empty")


supervisor = SupervisorReceiver()
supervisor.run()

Expected behavior The supervisor receives the values the robot is emitting when they are emitted after resetting the robot controller and the simulation

System

• Windows 10
• Webots version R2020a rev 1