Re: [APC-DEV] Re: [PECL-CVS] svn: /pecl/apc/trunk/ apc_cache.c apc_cache.h php_apc.c php_apc.h

Gopal V wrote:
> Rasmus Lerdorf wrote:
>> shire wrote:
>>> hrm, yeah I don't think we can get away without the lock here, simply
>>> because of the (*slot)->next/key type pointer usage.  I believe the
>>> structure would allow that to be an invalid pointer if it's still in the
>>> process of being updated, inserted, removed etc.
>>
>> I guess that is true.  I was thinking even if a slot is deleted, it
>> would be on the pending list and the *next would still be valid and take
>> us back into the cache entries, but that is only true if the ref_count
>> is non-zero, otherwise the entry is destroyed right away.  With the lock
>> though, apc_exists is only going to be faster than apc_fetch() by a
>> memcpy (or many memcpys for an array).
> 
> The issue with such functions is that it sets up an easy race condition
> in user land.
> 
> if(!apc_exists(...)) {
>     apc_store();
> }
> 
> Which is nearly identical to the user cache slam I keep talking
> about. They're nice in some api sense of the word, but I fail to
> see how someone is going to use this one in any better way than
> the old fetch or store/add.

No, that wouldn't be the use case because that is exactly what apc_add()
is for.  apc_exists() is a slightly faster way to check if something is
in the cache and it doesn't suffer from the value=false problem using
apc_fetch() has when used for this.

-Rasmus