# Buggy code
def circuit_dead(self, broken_circuit):
    """
    When a circuit dies, we update the destinations dictionary and remove all peers that are affected.
    """
    counter = 0
    affected_destinations = set()
    for hops, destinations in self.destinations.items():
        new_affected_destinations = set(
            destination
            for destination, tunnel_circuit in destinations.items()
            if tunnel_circuit == broken_circuit
        )
        for destination in new_affected_destinations:
            if destination in self.destinations[hops]:
                del self.destinations[hops][destination]
                counter += 1

        affected_destinations.update(new_affected_destinations)

    if counter > 0:
        self._logger.debug("Deleted %d peers from destination list", counter)

    if broken_circuit.circuit_id in self.circuit_id_to_connection:
        self.circuit_id_to_connection.pop(broken_circuit.circuit_id, None)

    return affected_destinations


# Traceback (RuntimeError)
Traceback (most recent call last):
File "TriblerGUI\event_request_manager.py", line 114, in on_read_data
RuntimeError: [Failure instance: Traceback: <class 'RuntimeError'>: dictionary changed size during iteration
lib\threading.py:926:_bootstrap_inner
lib\threading.py:870:run
lib\site-packages\twisted\_threads\_threadworker.py:46:work
lib\site-packages\twisted\_threads\_team.py:190:doWork
--- <exception caught here> ---
lib\site-packages\twisted\python\threadpool.py:250:inContext
lib\site-packages\twisted\python\threadpool.py:266:<lambda>
lib\site-packages\twisted\python\context.py:122:callWithContext
lib\site-packages\twisted\python\context.py:85:callWithContext
Tribler\Core\APIImplementation\LaunchManyCore.py:572:sesscb_states_callback
Tribler\community\triblertunnel\community.py:510:monitor_downloads
Tribler\pyipv8\ipv8\messaging\anonymization\hidden_services.py:86:join_swarm
Tribler\pyipv8\ipv8\messaging\anonymization\hidden_services.py:108:leave_swarm
Tribler\community\triblertunnel\community.py:406:remove_circuit
Tribler\community\triblertunnel\dispatcher.py:97:circuit_dead
Tribler\community\triblertunnel\dispatcher.py:97:<genexpr>
]

# Fixed code
def circuit_dead(self, broken_circuit):
    """
    When a circuit dies, we update the destinations dictionary and remove all peers that are affected.
    """
    counter = 0
    affected_destinations = set()
    for hops, destinations in list(self.destinations.items()):
        new_affected_destinations = set(
            destination
            for destination, tunnel_circuit in list(destinations.items())
            if tunnel_circuit == broken_circuit
        )
        for destination in new_affected_destinations:
            if destination in self.destinations[hops]:
                del self.destinations[hops][destination]
                counter += 1

        affected_destinations.update(new_affected_destinations)

    if counter > 0:
        self._logger.debug("Deleted %d peers from destination list", counter)

    if broken_circuit.circuit_id in self.circuit_id_to_connection:
        self.circuit_id_to_connection.pop(broken_circuit.circuit_id, None)

    return affected_destinations