Cachalot breaks unit tests --parallel

[kunalgrover05]

Running parallel unit tests in Django breaks when using Cachalot 2.1.0.

Traceback (most recent call last):
  File "/usr/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/lib/python3.6/unittest/case.py", line 601, in run
    self.setUp()
  File "/home/kunal/leanagri/protect/tests.py", line 26, in setUp
    nursery_area_factor=0.29)
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/query.py", line 394, in create
    obj.save(force_insert=True, using=self.db)
  File "/home/kunal/leanagri/core/models.py", line 111, in save
    self.full_clean()
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/base.py", line 1245, in full_clean
    self.validate_unique(exclude=exclude)
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/base.py", line 1034, in validate_unique
    errors = self._perform_unique_checks(unique_checks)
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/base.py", line 1131, in _perform_unique_checks
    if qs.exists():
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/query.py", line 670, in exists
    return self.query.has_results(using=self.db)
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/sql/query.py", line 517, in has_results
    return compiler.has_results()
  File "/home/kunal/env3/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 858, in has_results
    return bool(self.execute_sql(SINGLE))
  File "/home/kunal/env3/lib/python3.6/site-packages/cachalot/monkey_patch.py", line 32, in inner
    return original(compiler, *args, **kwargs)
  File "/home/kunal/env3/lib/python3.6/site-packages/cachalot/monkey_patch.py", line 86, in inner
    cache_key, table_cache_keys)
  File "/home/kunal/env3/lib/python3.6/site-packages/cachalot/monkey_patch.py", line 40, in _get_result_or_execute_query
    data = cache.get_many(table_cache_keys + [cache_key])
  File "/home/kunal/env3/lib/python3.6/site-packages/cachalot/transaction.py", line 22, in get_many
    data.update(self.parent_cache.get_many(missing_keys))
  File "/home/kunal/env3/lib/python3.6/site-packages/cachalot/transaction.py", line 22, in get_many
    data.update(self.parent_cache.get_many(missing_keys))
  File "/home/kunal/env3/lib/python3.6/site-packages/django/core/cache/backends/memcached.py", line 96, in get_many
    ret = self._cache.get_multi(new_keys)
  File "/home/kunal/env3/lib/python3.6/site-packages/memcache.py", line 1216, in get_multi
    retvals[prefixed_to_orig_key[rkey]] = val
KeyError: b'TEST:1:cc5377d1567fb2cc4c59ba5a48b7354d7c987aca'

[Andrew-Chen-Wang]

I'd like to hear if anyone's still having issues with this with --parallel. Would be helpful to know exactly wtf is going on!

[dbartenstein]

I'd like to hear if anyone's still having issues with this with --parallel. Would be helpful to know exactly wtf is going on!

Hi @Andrew-Chen-Wang: we are using cachalot and running our tests with --parallel and we’re not experiencing any issues. We are using Django 3.2 LTS and cachalot 2.4.2.

Edit:
We deactivate the caches when running unit tests. So this might be the reason why it works for us.

@override_settings(
    CACHES={
        'default': {
            'BACKEND': 'django.core.cache.backends.dummy.DummyCache'
        }
    }
)

With Django it’s possible to enforce that selected unit tests run in sequence: https://docs.djangoproject.com/en/3.2/topics/testing/advanced/#enforce-running-test-classes-sequentially