Josh,

Thank you, I will investigate and report back.

Neil

On Jul 25, 2011, at 1:23 PM, Joshua Ballanco wrote:

> Have you considered the possibility that memcache might be recycling keys on you?
> 
> We had an issue a while back where we were using memcache for both fragment caching and session storage. Occasionally, we would get exceptions in retrieving a session, and looking at the error message it was clear that what was retrieved from memcache was a fragment and not a session. Unfortunately, we moved back to a cookie-based session store before we had a chance to look deeper into the issue. What I can say is that based on the key generation scheme we were using for sessions and cache keys, there was effectively 0 chance that we were duplicating keys. Instead, it seemed like memcache was re-using slots for different keys when we started exhausting free slots.
> 
> Hope that helps!
> 
> - Joshua Ballanco
> On Monday, July 25, 2011 at 2:22 PM, Neil wrote:
> 
>> While it's entirely possible that this issue is caused by some other
>> factor, but we are getting session collisions as well as an issue
>> where one user is getting another user's session. This is clearly
>> bad, but I cannot for the life of me figure out how this could even
>> happen in the first place. The code looks thread safe to me, and a
>> quick discussion on #ruby-lang seems to support that.
>> 
>> Thoughts:
>> 1. Session IDs are being generated in the same sequence (uses
>> securerandom -> openssl which does not have a static seed)
>> 2. Threads. Looks good to me.
>> 3. Maybe memcached is returning something other than "STORED/
>> NOT_STORED" for @pool.add(sid, session), but the operation still
>> succeeded?
>> 4. Gnomes.
>> 
>> Any input is GREATLY appreciated. Please don't say "it's an RC, what
>> do you expect?" :)
>> 
>> 
>> From https://github.com/rack/rack/blob/master/lib/rack/session/memcache.rb
>> def generate_sid
>> loop do
>> sid = super
>> break sid unless @pool.get(sid, true)
>> end
>> end
>> 
>> def get_session(env, sid)
>> with_lock(env, [nil, {}]) do
>> unless sid and session = @pool.get(sid)
>> sid, session = generate_sid, {}
>> unless /^STORED/ =~ @pool.add(sid, session)
>> raise "Session collision on '#{sid.inspect}'"
>> end
>> end
>> [sid, session]
>> end
>> end
>> 
>> def set_session(env, session_id, new_session, options)
>> expiry = options[:expire_after]
>> expiry = expiry.nil? ? 0 : expiry + 1
>> 
>> with_lock(env, false) do
>> @pool.set session_id, new_session, expiry
>> session_id
>> end
>> end
>