The really hard part of APC is the internal locking code it has - it's not that hard to do, just hard to figure out if you've done it wrong. And I'm just about to really mess around with the assembly spin locks and pthread mutex locks to make them cross-process locks which live in shared memory (remember that "volatile" keyword in C?). The other couple of lock modes are already cross-process and slow (because of the syscall). If these work right, I won't really have to cripple the fast part of the code to implement the features I have in mind.
But before I start to go MIA into the locking code, I'd like to get my testing in place. So I've written a small and tiny test app called lockhammer - read the makefile and please run it on every platform you want APC to work. (make APC_DIR=~/apc link; make)
The code in lockhammer.c should be easily understood - basically it allocates some shared memory, creates a lock in it, forks, re-attaches the memory in each process. Every process is a loop of lock, write PID into shm, sleep, check the PID. In case someone has a better idea of how to test locks, I'll also like modifications to it, in case any of you think there's some corner case I missed (yes, random sleep & random fork-order is also on my list of TODOs).
Fundamentally, the information about locks is privately held within the lock type code in APC. The information needs to be moved into a shared mode (or at least, transparent) for multiple un-related processes to be able to share the cache without collisions. Eventually, you should be able to use APC in a standard FastCGI deployment without allocating a cache per-process.
And if you're a user, I'd like read something other than a bugreport, occasionaly.
--They're gonna lock me up and throw away the key!