[KAFKA-4673] Python VerifiableConsumer service has thread-safety bug for event_handlers

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.10.1.1
• Fix Version/s: 0.10.1.2, 0.10.2.0, 0.11.0.0
• Component/s: system tests
• Assignee: Ewen Cheslack-Postava
• Reporter: Ewen Cheslack-Postava
• Reviewer: Ewen Cheslack-Postava
• Created: 19/Jan/17 00:03
• Updated: 24/Jan/17 21:20
• Resolved: 24/Jan/17 21:20

Description

From http://confluent-kafka-0-10-1-system-test-results.s3-us-west-2.amazonaws.com/2017-01-17--001.1484653357--apache--0.10.1--d436fa2/report.html

====================================================================================================
test_id: 2017-01-17--001.kafkatest.tests.client.consumer_test.OffsetValidationTest.test_consumer_bounce.clean_shutdown=True.bounce_mode=rolling
status: FAIL
run time: 1 minute 42.663 seconds

dictionary changed size during iteration
Traceback (most recent call last):
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/venv/local/lib/python2.7/site-packages/ducktape-0.5.3-py2.7.egg/ducktape/tests/runner.py", line 106, in run_all_tests
data = self.run_single_test()
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/venv/local/lib/python2.7/site-packages/ducktape-0.5.3-py2.7.egg/ducktape/tests/runner.py", line 162, in run_single_test
return self.current_test_context.function(self.current_test)
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/venv/local/lib/python2.7/site-packages/ducktape-0.5.3-py2.7.egg/ducktape/mark/_mark.py", line 331, in wrapper
return functools.partial(f, *args, **kwargs)(*w_args, **w_kwargs)
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/tests/kafkatest/tests/client/consumer_test.py", line 137, in test_consumer_bounce
self.await_all_members(consumer)
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/tests/kafkatest/tests/verifiable_consumer_test.py", line 83, in await_all_members
self.await_members(consumer, self.num_consumers)
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/tests/kafkatest/tests/verifiable_consumer_test.py", line 80, in await_members
err_msg="Consumers failed to join in a reasonable amount of time")
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/venv/local/lib/python2.7/site-packages/ducktape-0.5.3-py2.7.egg/ducktape/utils/util.py", line 31, in wait_until
if condition():
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/tests/kafkatest/tests/verifiable_consumer_test.py", line 78, in <lambda>
wait_until(lambda: len(consumer.joined_nodes()) == num_consumers,
File "/var/lib/jenkins/workspace/system-test-kafka-0.10.1/kafka/tests/kafkatest/services/verifiable_consumer.py", line 317, in joined_nodes
return [handler.node for handler in self.event_handlers.itervalues()
RuntimeError: dictionary changed size during iteration

It looks like the background thread is incorrectly inserting elements into self.event_handlers during this iteration. This is the first time I've seen this, so I suspect it's rarely hit. Looking at the code, most access is protected by a lock except for the additions at the beginning of the _worker method. Looking through the stacktrace, this looks like the likely culprit since we start the consumer and immediately start calling `await_all_members`, which ultimately iterates over that list. If that call happens faster than the thread gets started we could hit a bad interleaving.

Fix is probably easy – just add the proper locking around use of that object.

Issue Links

• is duplicated by: KAFKA-4036 Unsafe dictionary access in VerifiableConsumer service
• links to: GitHub Pull Request #2427