Some cache tests are flaky (e.g. cache.tests.BaseMemcachedTests.test_touch)

[Chris Jerdonek]

The following tests occasionally fail in CI:

• cache.tests.PyLibMCCacheTests.test_touch
• cache.tests.PyMemcacheCacheTests.test_touch

Traceback (most recent call last):
  File "/home/jenkins/workspace/pull-requests-bionic/database/sqlite3/label/bionic-pr/python/python3.8/tests/cache/tests.py", line 481, in test_touch
    self.assertIs(cache.touch('expire1'), True)
AssertionError: False is not True

The line that fails is here: ​https://github.com/django/django/blob/ed3af3ff4b3cfb72de598f1b39a1028ba3826efb/tests/cache/tests.py#L481

Both test classes inherit from BaseMemcachedTests, which inherits from BaseCacheTests.

[Mariusz Felisiak]

Yes these tests are flaky (and probably 2-3 more cache tests). We tried to fix them in the past but with no success (see ​PR).

[Chris Jerdonek]

Could this be due to different concrete subclasses of BaseMemcachedTests stepping on each other during a parallel run? I ask because different test classes are still using the same cache key.

[Chris Jerdonek]

Another cache-related flaky test is: cache.tests.CacheMiddlewareTest.test_cache_page_timeout on ​this line.

Traceback (most recent call last):
  File "C:\Jenkins\workspace\pull-requests-windows\database\sqlite3\label\windows\python\Python39\tests\cache\tests.py", line 2449, in test_cache_page_timeout
    self.assertEqual(
AssertionError: b'Hello World 1' != b'Hello World 2'

[Chris Jerdonek]

A possible pattern for fixing issues like this would be to keep retrying the assertion in a while loop, with a small sleep in between each attempt, and a relatively large cumulative amount of time before giving up. That would make it as pass as quickly as possible, without waiting longer than necessary. I'm thinking of something like--

    def assert_with_retries(self, assert_func, *args,  interval_time=0.1, total_time=10, **kwargs):
        start = time.time()
        while True:
            try:
                assert_func(*args, **kwargs)
            except Exception as exc:
                if time.time() - start > total_time:
                    raise RuntimeError(f'Total retry time exceeded.') from exc
            else:
                break
            time.sleep(interval_time)

[Chris Jerdonek]

Something else that occurred to me is that it could only help to start each test by asserting that each key being used isn't set, and to end each test by clearing the key in a finally block. The former would help to show if a test isolation issue is at fault.

[Chris Jerdonek]

Another flaky test failure: cache.tests.MemcachedCacheTests.test_forever_timeout() at this line-- ​https://github.com/django/django/blob/1697098795707bd39501d9ceddd3d9be93f8d549/tests/cache/tests.py#L606
(again with pull-requests-bionic/database=spatialite,label=bionic-pr,python=python3.8).