Opened 4 years ago

Closed 8 weeks ago

Last modified 8 weeks ago

#19914 closed Bug (fixed)

MemcachedCacheTests failing on pylibmc

Reported by: Bas Peschier Owned by: Ed Morley
Component: Core (Cache system) Version: master
Severity: Normal Keywords:
Cc: emorley@… Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


Tests fail due to a bug in either pylibmc or libmemcached (detailts at pylibmc-issue:

(sprint)mbdev001:tests basp$ python --settings=test_sqlite_cache cache.MemcachedCacheTests
Creating test database for alias 'default'...
Creating test database for alias 'other'...
ERROR: test_invalid_keys (regressiontests.cache.tests.MemcachedCacheTests)
Traceback (most recent call last):
  File "/Users/basp/dev/repos/django/tests/regressiontests/cache/", line 963, in tearDown
  File "/Users/basp/dev/envs/sprint/lib/python2.7/site-packages/django/core/cache/backends/", line 140, in clear
SomeErrors: error 19 from flush_all: SUCCESS

Ran 35 tests in 4.085s

FAILED (errors=1)
Destroying test database for alias 'default'...
Destroying test database for alias 'other'...


    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.PyLibMCCache',
        'LOCATION': '',

Change History (10)

comment:1 Changed 4 years ago by Jacob

Needs documentation: unset
Needs tests: unset
Patch needs improvement: unset
Resolution: invalid
Status: newclosed

It seems to me that this is a bug with pylibmc, right? I'm not sure Django could do anything here. I'm going to close this, please feel free to reopen if the bug's actually in Django.

comment:2 Changed 2 months ago by Ed Morley

Cc: emorley@… added

This still reproduces with pylibmc 1.5.1 and libmemcached 1.0.8 (the latest for Ubuntu 14.04).

The pylibmc issue was closed as "expected" - perhaps we should just skip these tests on pylibmc?

test_incr_version (cache.tests.MemcachedCacheTests) ... ok
test_invalid_keys (cache.tests.MemcachedCacheTests) ... ERROR
test_long_timeout (cache.tests.MemcachedCacheTests) ... ERROR
test_memcached_deletes_key_on_failed_set (cache.tests.MemcachedCacheTests) ... python: libmemcached/ memcached_return_t memcached_send_ascii(memcached_st*, memcached_server_write_instance_st, const char*, size_t, const char*, size_t, time_t, uint32_t, uint64_t, bool, bool, memcached_storage_action_t): Assertion `memcached_failed(rc)' failed.

If not, then I think the ticket's resolution is best 'wontfix' rather than 'invalid' - though I mainly just wanted to update this ticket now to have something to point at when my PR fails some of the tests, to help prove that the failures were pre-existing.

comment:3 Changed 2 months ago by Tim Graham

Maybe the warning could become an exception on memcached?

comment:4 Changed 8 weeks ago by Ed Morley

So I've dug into this a bit more, there are actually two tests that are broken under pylibmc, and the above only covers the first.

Issues with test_invalid_keys:

This test tries to set two different types of invalid keys:

  1. a key that contains space characters
  2. another that exceeds the max allowed length

Both python-memcached and pylibmc handle (2) fine. However (1) is problematic with pylibmc, since it causes subsequent requests to the server (ie teardown and later tests in the suite) to fail for the next few seconds.

The cause of this is roughly:

  • python-memcached validates key names before sending, whereas by pylibmc doesn't by default (or more precisely, libmemcached doesn't by default)
  • therefore for pylibmc the invalid key name gets sent to the server, rather than being rejected client-side
  • this appears to expose some memcached server/libmemcache bug that isn't pylibmc's fault (though I'm hoping for clarification, see:

Possible workarounds are:

  1. not set keys names containing spaces (ie skip the test)
  2. enable pylibmc's client side key validation passing verify_keys within behaviors (just for this specific test)
  3. use the binary protocol (since it's not affected)

In addition, since the bug only affects half of test_invalid_keys(), we could split the test in two, to reduce the amount we skip/test in a non-standard configuration.

Note: The issue presents itself in two forms depending on libmemcached/memcached server versions. On Ubuntu 14.04 an assertion occurs in libmemcached, which causes the whole test run to abort (example in comment 2 above). Whereas on Ubuntu 16.04 a handful of exceptions are seen and the rest of the tests complete fine. I'm pretty sure the assertion *is* something pylibmc should be handling better, for which I've filed:
(though even with that fixed, the underlying ~server bug still needs to be worked around)

Issues with test_memcached_deletes_key_on_failed_set:

With the problematic test above skipped, the one remaining failure under pylibmc is:

ERROR: test_memcached_deletes_key_on_failed_set (cache.tests.MemcachedCacheTests)
Traceback (most recent call last):
  File "/home/vagrant/src/_todo/django/tests/cache/", line 1233, in test_memcached_deletes_key_on_failed_set
    cache.set('small_value', large_value)
  File "/home/vagrant/src/_todo/django/django/core/cache/backends/", line 83, in set
    if not self._cache.set(key, value, self.get_backend_timeout(timeout)):
Error: error 37 from memcached_set: SUCCESS

This is due to difference in behaviour between clients when the *value* is too long.


  • When the *key* is too long:
    • python-memcached: memcache.MemcachedKeyLengthError: Key length is > 250
    • pylibmc: ValueError: key length 251 too long, max is 250
  • When the *value* is too long:
    • python-memcached: Returns successfully but didn't set the key-value (this is by design)
    • pylibmc 1.5.1: pylibmc.Error: error 37 from memcached_set: SUCCESS
    • pylibmc master: pylibmc.TooBig

Possible options:

  1. just add a try-except around the .set() for both python-memcached and pylibmc
  2. differentiate between the two, and only expect exceptions for the pylibmc case (either by just catching them, or by enforcing using assertRaises)

comment:5 Changed 8 weeks ago by Ed Morley

Resolution: invalid
Status: closednew
Triage Stage: UnreviewedAccepted

Reopening, since a test suite should be green on master, even if the underlying library is at fault.

comment:6 Changed 8 weeks ago by Ed Morley

Owner: changed from nobody to Ed Morley
Status: newassigned

This is fixed by the PR in #27132.

For the first failing test in comment 4, I went with option (a) after splitting the test into two.
For the second test, I also went with its option (a).

For more details, see the comments added to the tests in the PR:

comment:7 Changed 8 weeks ago by Tim Graham <timograham@…>

In cfd1f93d:

Refs #19914 -- Split the test_invalid_keys cache test into two.

The first half of the test fails when using pylibmc (so will need
to be skipped).

comment:8 Changed 8 weeks ago by Tim Graham <timograham@…>

In 5756edd4:

[1.10.x] Refs #19914 -- Split the test_invalid_keys cache test into two.

The first half of the test fails when using pylibmc (so will need
to be skipped).

Backport of cfd1f93d55d3b9317bdf26b426fe21d935ab3399 from master

comment:9 Changed 8 weeks ago by Tim Graham <timograham@…>

Resolution: fixed
Status: assignedclosed

In 674e3fe1:

Fixed #19914 -- Fixed test failures with pylibmc.

comment:10 Changed 8 weeks ago by Ed Morley <emorley@…>

In 255456be:

[1.10.x] Fixed #19914 -- Fixed test failures with pylibmc.

Backport of 674e3fe13e5156344bfafbea59018b8837eb3044 from master

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