Add functionality to run listener functions for custom_messages concurrently

docs/running-distributed.rst

@@ -131,7 +131,20 @@ order to coordinate the test. This can be easily accomplished with custom messag
             environment.runner.send_message('test_users', users)  
 
 Note that when running locally (i.e. non-distributed), this functionality will be preserved; 
-the messages will simply be handled by the runner that sends them.  
+the messages will simply be handled by the runner that sends them.
+
+.. note::
+    Using the default options while registering a message handler will run the listener function
+    in a **blocking** way, resulting in the heartbeat and other messages being delayed for the amount
+    of the execution.
+    If it is known that the listener function will handle time-intensive tasks, it is possible to register the
+    function as **concurrent** (as a separate greenlet).
+
+    .. code-block::
+        environment.runner.register_message('test_users', setup_test_users, concurrent=True)
+
+    Please use this feature with care, as otherwise it could result in greenlets running and influencing
+    the running loadtest.
 
 For more details, see the `complete example <https://github.com/locustio/locust/tree/master/examples/custom_messages.py>`_.
 

examples/custom_messages.py

@@ -1,12 +1,18 @@
 from locust import HttpUser, between, events, task
 from locust.runners import MasterRunner, WorkerRunner
 
+import gevent
+
 usernames = []
 
 
 def setup_test_users(environment, msg, **kwargs):
     # Fired when the worker receives a message of type 'test_users'
     usernames.extend(map(lambda u: u["name"], msg.data))
+    # Even though "acknowledge_concurrent_users" was sent first, "acknowledge_users"
+    # will print its statement first, as "acknowledge_concurrent_users" was registered
+    # running concurrently, and therefore not blocking other messages.
+    environment.runner.send_message("concurrent_message", "This is a non blocking message")
     environment.runner.send_message("acknowledge_users", f"Thanks for the {len(msg.data)} users!")
 
 
@@ -15,12 +21,18 @@ def on_acknowledge(msg, **kwargs):
     print(msg.data)
 
 
+def on_concurrent_message(msg, **kwargs):
+    gevent.sleep(10)
+    print(msg.data)
+
+
 @events.init.add_listener
 def on_locust_init(environment, **_kwargs):
     if not isinstance(environment.runner, MasterRunner):
         environment.runner.register_message("test_users", setup_test_users)
     if not isinstance(environment.runner, WorkerRunner):
         environment.runner.register_message("acknowledge_users", on_acknowledge)
+        environment.runner.register_message("concurrent_message", on_concurrent_message, concurrent=True)
 
 
 @events.test_start.add_listener

locust/runners.py

@@ -119,7 +119,7 @@ def __init__(self, environment: Environment) -> None:
         self.target_user_classes_count: dict[str, int] = {}
         # target_user_count is set before the ramp-up/ramp-down occurs.
         self.target_user_count: int = 0
-        self.custom_messages: dict[str, Callable] = {}
+        self.custom_messages: dict[str, tuple[Callable, bool]] = {}
 
         self._users_dispatcher: UsersDispatcher | None = None
 
@@ -420,7 +420,7 @@ def log_exception(self, node_id: str, msg: str, formatted_tb: str) -> None:
         row["nodes"].add(node_id)
         self.exceptions[key] = row
 
-    def register_message(self, msg_type: str, listener: Callable) -> None:
+    def register_message(self, msg_type: str, listener: Callable, concurrent=False) -> None:
         """
         Register a listener for a custom message from another node
 
@@ -429,7 +429,7 @@ def register_message(self, msg_type: str, listener: Callable) -> None:
         """
         if msg_type in self.custom_messages:
             raise Exception(f"Tried to register listener method for {msg_type}, but it already had a listener!")
-        self.custom_messages[msg_type] = listener
+        self.custom_messages[msg_type] = (listener, concurrent)
 
 
 class LocalRunner(Runner):
@@ -568,7 +568,7 @@ def send_message(self, msg_type: str, data: Any | None = None, client_id: str |
         """
         logger.debug("Running locally: sending %s message to self" % msg_type)
         if msg_type in self.custom_messages:
-            listener = self.custom_messages[msg_type]
+            listener, concurrent = self.custom_messages[msg_type]
             msg = Message(msg_type, data, "local")
             listener(environment=self.environment, msg=msg)
         else:
@@ -1139,7 +1139,11 @@ def client_listener(self) -> NoReturn:
                     f"Received {msg.type} message from worker {msg.node_id} (index {self.get_worker_index(msg.node_id)})"
                 )
                 try:
-                    self.custom_messages[msg.type](environment=self.environment, msg=msg)
+                    listener, concurrent = self.custom_messages[msg.type]
+                    if not concurrent:
+                        listener(environment=self.environment, msg=msg)
+                    else:
+                        gevent.spawn(listener, self.environment, msg)
                 except Exception:
                     logging.error(f"Uncaught exception in handler for {msg.type}\n{traceback.format_exc()}")
 
@@ -1393,7 +1397,11 @@ def worker(self) -> NoReturn:
                 self.last_heartbeat_timestamp = time.time()
             elif msg.type in self.custom_messages:
                 logger.debug("Received %s message from master" % msg.type)
-                self.custom_messages[msg.type](environment=self.environment, msg=msg)
+                listener, concurrent = self.custom_messages[msg.type]
+                if not concurrent:
+                    listener(environment=self.environment, msg=msg)
+                else:
+                    gevent.spawn(listener, self.environment, msg)
             else:
                 logger.warning(f"Unknown message type received: {msg.type}")
 

locust/test/test_runners.py

@@ -606,6 +606,31 @@ def on_custom_msg(msg, **kw):
         self.assertTrue(test_custom_msg[0])
         self.assertEqual(123, test_custom_msg_data[0]["test_data"])
 
+    def test_concurrent_custom_message(self):
+        class MyUser(User):
+            wait_time = constant(1)
+
+            @task
+            def my_task(self):
+                pass
+
+        test_custom_msg = [False]
+        test_custom_msg_data = [{}]
+
+        def on_custom_msg(msg, **kw):
+            test_custom_msg[0] = True
+            test_custom_msg_data[0] = msg.data
+
+        environment = Environment(user_classes=[MyUser])
+        runner = LocalRunner(environment)
+
+        runner.register_message("test_custom_msg", on_custom_msg, concurrent=True)
+        runner.send_message("test_custom_msg", {"test_data": 123})
+
+        gevent.sleep(0.5)
+        self.assertTrue(test_custom_msg[0])
+        self.assertEqual(123, test_custom_msg_data[0]["test_data"])
+
     def test_undefined_custom_message(self):
         class MyUser(User):
             wait_time = constant(1)