Support lazy initialization of reader to avoid unnecessarily doing it…

… on workers.

examples/synchronizer_ex.py

@@ -12,6 +12,8 @@
 else:
     reader = MongoLRUReader({"env": "test", "tb": False, "lb": True}, "last_login")
 synchronizer.register(reader)
+# optionally replace this with lazy initalization of Reader to avoid unnecessarily doing it on workers:
+# synchronizer.register(None, MongoLRUReader, {"env": "test", "tb": False, "lb": True}, "last_login")
 
 
 class MyUser(HttpUser):

locust_plugins/synchronizer.py

@@ -1,4 +1,4 @@
-from typing import Dict, Iterator
+from typing import Dict, Iterator, Type, Optional
 import logging
 from gevent.event import AsyncResult
 from locust import User, events
@@ -8,19 +8,30 @@
 from locust.runners import MasterRunner, WorkerRunner
 
 test_data: Dict[int, AsyncResult] = {}
-iterator: Iterator[Dict] = None
+iterator: Optional[Iterator[Dict]] = None
 
 
-def register(i: Iterator[dict]):
+def register(i: Optional[Iterator[dict]], reader_class: Optional[Type[Iterator[Dict]]] = None, *args, **kwargs):
+    """Register synchronizer methods and tie them to use the iterator that you pass.
+
+    To avoid unnecessarily instantiating the iterator on workers (where it isnt used),
+    you can pass an iterator class and initialization parameters instead of an object instance.
+    """
     global iterator
     iterator = i
 
     @events.test_start.add_listener
     def test_start(environment, **_kw):
+        global iterator
         runner = environment.runner
+        if not i and not isinstance(runner, WorkerRunner):
+            assert reader_class
+            logging.debug(f"about to initialize reader class {reader_class}")
+            iterator = reader_class(*args, **kwargs)
         if runner:
             # called on master
             def user_request(environment: Environment, msg, **kwargs):
+                assert iterator  # should have been instantiated by now...
                 data = next(iterator)
                 # data["_id"] = str(data["_id"])  # this is an ObjectId, msgpack doesnt know how to serialize it
                 environment.runner.send_message(