From a39389f9deb1c59321ddd9ff5ef67c71176abfab Mon Sep 17 00:00:00 2001
From: Lars Holmberg <lars.holmberg@svenskaspel.se>
Date: Tue, 6 Sep 2022 23:37:26 +0200
Subject: [PATCH 1/3] test_main.py/test_distributed_tags: actually detect an
 error that was happening

---
 locust/test/test_main.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/locust/test/test_main.py b/locust/test/test_main.py
index 680af8a607..7742f76656 100644
--- a/locust/test/test_main.py
+++ b/locust/test/test_main.py
@@ -1228,6 +1228,7 @@ def task2(self):
             )
             stdout, stderr = proc.communicate()
             stdout_worker, stderr_worker = proc_worker.communicate()
+            self.assertNotIn("ERROR", stderr_worker)
             self.assertIn("task1", stdout_worker)
             self.assertNotIn("task2", stdout_worker)
             self.assertNotIn("Traceback", stderr)

From eb2018c9584cfa70f60cce2d13b8f4f410d91d34 Mon Sep 17 00:00:00 2001
From: Lars Holmberg <lars.holmberg@svenskaspel.se>
Date: Tue, 6 Sep 2022 23:38:33 +0200
Subject: [PATCH 2/3] test_main.py/test_distributed_tags: fix error in test

---
 locust/test/test_main.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/locust/test/test_main.py b/locust/test/test_main.py
index 7742f76656..cab3b2c416 100644
--- a/locust/test/test_main.py
+++ b/locust/test/test_main.py
@@ -1170,10 +1170,8 @@ def on_test_stop(environment, **kwargs):
             self.assertEqual(0, proc_worker.returncode)
 
     def test_distributed_tags(self):
-        content = (
-            MOCK_LOCUSTFILE_CONTENT
-            + """
-from locust import tag
+        content = """
+from locust import HttpUser, TaskSet, task, between, LoadTestShape, tag
 class SecondUser(HttpUser):
     host = "http://127.0.0.1:8089"
     wait_time = between(0, 0.1)
@@ -1187,7 +1185,6 @@ def task1(self):
     def task2(self):
         print("task2")
 """
-        )
         with mock_locustfile(content=content) as mocked:
             proc = subprocess.Popen(
                 [

From 3a7a478961449969c6f25b42f68cdf8702dcdb52 Mon Sep 17 00:00:00 2001
From: Lars Holmberg <lars.holmberg@svenskaspel.se>
Date: Tue, 6 Sep 2022 23:39:47 +0200
Subject: [PATCH 3/3] --tags: Log a warning if no tasks remain on a
 User/TaskSet after filtering

---
 locust/test/test_main.py | 32 ++++++++++++++++++++++++++++++++
 locust/user/task.py      |  2 ++
 2 files changed, 34 insertions(+)

diff --git a/locust/test/test_main.py b/locust/test/test_main.py
index cab3b2c416..7310262004 100644
--- a/locust/test/test_main.py
+++ b/locust/test/test_main.py
@@ -1077,6 +1077,38 @@ def test_error_when_locustfiles_directory_is_empty(self):
             self.assertIn(f"Could not find any locustfiles in directory '{temp_dir}'", stderr)
             self.assertEqual(1, proc.returncode)
 
+    def test_error_when_no_tasks_match_tags(self):
+        content = """
+from locust import HttpUser, TaskSet, task, constant, LoadTestShape, tag
+class MyUser(HttpUser):
+    host = "http://127.0.0.1:8089"
+    wait_time = constant(1)
+    @tag("tag1")
+    @task
+    def task1(self):
+        print("task1")
+    """
+        with mock_locustfile(content=content) as mocked:
+            proc = subprocess.Popen(
+                [
+                    "locust",
+                    "-f",
+                    mocked.file_path,
+                    "--headless",
+                    "-t",
+                    "1",
+                    "--tags",
+                    "tag2",
+                ],
+                stdout=PIPE,
+                stderr=PIPE,
+                text=True,
+            )
+            stdout, stderr = proc.communicate()
+            self.assertIn("MyUser had no tasks left after filtering", stderr)
+            self.assertIn("No tasks defined on MyUser", stderr)
+            self.assertEqual(1, proc.returncode)
+
 
 class DistributedIntegrationTests(ProcessIntegrationTest):
     def test_expect_workers(self):
diff --git a/locust/user/task.py b/locust/user/task.py
index 4ce76ba977..40906f96ba 100644
--- a/locust/user/task.py
+++ b/locust/user/task.py
@@ -208,6 +208,8 @@ def filter_tasks_by_tags(
         checked[task] = passing
 
     task_holder.tasks = new_tasks
+    if not new_tasks:
+        logging.warning(f"{task_holder.__name__} had no tasks left after filtering, instantiating it will fail!")
 
 
 class TaskSetMeta(type):