Log unhandled exceptions thrown in User.on_start (#2442)

Log unhandled exceptions thrown in User.on_start (instead of just relying on default console logging of unhandled exceptions)

locust/test/test_log.py

@@ -204,3 +204,33 @@ def my_task(self):
             f"{socket.gethostname()}/INFO/root: custom log message",
             log_content,
         )
+
+    def test_user_broken_on_start(self):
+        with temporary_file(
+            textwrap.dedent(
+                """
+            from locust import HttpUser, task
+
+            class TestUser(HttpUser):
+                host = "invalidhost"
+
+                def on_start(self):
+                    self.client.get("/")
+                """
+            )
+        ) as file_path:
+            output = subprocess.check_output(
+                [
+                    "locust",
+                    "-f",
+                    file_path,
+                    "-t",
+                    "1",
+                    "--headless",
+                ],
+                stderr=subprocess.STDOUT,
+                timeout=10,
+                text=True,
+            )
+
+        self.assertIn("ERROR/locust.user.users: Invalid URL", output)

locust/user/users.py

@@ -5,7 +5,10 @@
 from gevent import GreenletExit, greenlet
 from gevent.pool import Group
 from urllib3 import PoolManager
+import logging
+import traceback
 
+logger = logging.getLogger(__name__)
 from locust.clients import HttpSession
 from locust.exception import LocustError, StopUser
 from locust.user.wait_time import constant
@@ -20,6 +23,8 @@
     get_tasks_from_base_classes,
 )
 
+logger = logging.getLogger(__name__)
+
 
 class UserMeta(type):
     """
@@ -140,7 +145,12 @@ def run(self):
         self._taskset_instance = DefaultTaskSet(self)
         try:
             # run the TaskSet on_start method, if it has one
-            self.on_start()
+            try:
+                self.on_start()
+            except Exception as e:
+                # unhandled exceptions inside tasks are logged in TaskSet.run, but since we're not yet there...
+                logger.error("%s\n%s", e, traceback.format_exc())
+                raise
 
             self._taskset_instance.run()
         except (GreenletExit, StopUser):