Fast session resumption segfault

[Phil Mayers]

On 10/20/2011 05:01 PM, Phil Mayers wrote:
> Is it possible that the following sequence of events is occurring:
>
> 1. thread #1: client does session resumption a split second before
> expiry, gets cached VPs
> 2. thread #1: blocks (e.g. doing SQL)
> 3. thread #2: receives new TLS session, calls SSL_CTX_flush_sessions
> 4. thread #2: calls pairfree() on VPs from session 1, now expired
> 5. thread #1: resumes - boom
>
> It might explain why it happens very rarely, and why we see it but Alex
> doesn't (load-related - Imperial has a few more students that SOAS IIRC)
>

Having gone for a run and thought about this en-route, I'm sure this 
must be the cause. Alex of course doesn't see it because he's not 
running c145c7dabbd4 in which the "free" occurs (why he's not seeing the 
memory leak I don't know - maybe *that* is load-related)

Quite how we solve it I don't know... I'm going to (shudder) look at the 
OpenSSL source to see if it does any locking around session objects; I 
just don't see from the API how you can used the get/set_ex_data 
functions safely in a threaded environment!