#22845: Memcached backend handles infinite (None) timeout defined in default
settings improperly
-------------------------------------+------------------------------
Reporter: Althalus | Owner: nobody
Type: Bug | Status: new
Component: Core (Cache system) | Version: master
Severity: Normal | Keywords: cache, memcached
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+------------------------------
#9595 introduced a way to use `None` as way to set infinite memcached
timeout.
It works fine while you specify this timeout explicitly like
`cache.set('key', 'val', None)`.
But it works wrong if you define it as a default cache timeout in
django.conf.settings.CACHES.
Consider the following parts:
{{{#!python
# base.py
class BaseCache(object):
def __init__(self, params):
timeout = params.get('timeout', params.get('TIMEOUT', 300))
if timeout is not None:
try:
timeout = int(timeout)
except (ValueError, TypeError):
timeout = 300
self.default_timeout = timeout
}}}
{{{#!python
# memcached.py
def get_backend_timeout(self, timeout=DEFAULT_TIMEOUT):
"""
Memcached deals with long (> 30 days) timeouts in a special
way. Call this function to obtain a safe value for your timeout.
"""
if timeout == DEFAULT_TIMEOUT:
return self.default_timeout
if timeout is None:
# Using 0 in memcache sets a non-expiring timeout.
return 0
elif int(timeout) == 0:
# Other cache backends treat 0 as set-and-expire. To achieve
this
# in memcache backends, a negative timeout must be passed.
timeout = -1
if timeout > 2592000: # 60*60*24*30, 30 days
# See http://code.google.com/p/memcached/wiki/FAQ
# "You can set expire times up to 30 days in the future. After
that
# memcached interprets it as a date, and will expire the item
after
# said date. This is a simple (but obscure) mechanic."
#
# This means that we have to switch to absolute timestamps.
timeout += int(time.time())
return int(timeout)
}}}
As you can see special handling of `None`, `0` and long timeouts is
performed in `get_backend_timeout`.
But it only handles explicit timeouts. For default timeout there's no
special processing while it's needed - timeout is just taken from
`BaseCache.__init__`.
So when trying to set `None` as default timeout django just transfers it
to the memcached driver and which leads to errors.
Also when timeout is set to some high value (more then 30 days) it is not
converted to timestamp as memcached requires. In my case it caused all my
cache entries to expire immidiately.
--
Ticket URL: <https://code.djangoproject.com/ticket/22845>
Django <https://code.djangoproject.com/>
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