Description:
------------
A couple of months ago i had to write a server-local RAM cache with PHP to store and cache information within one web server (one apache process). I did a lot of benchmarks and tests on different kind of caches available for php and APC seemed to fit the best for my needs. One of the functionalities i had to have is the cache locking by concurrent requests, so that only one request processing thread at a time could access cache. This can be done (according to documentation at http://www.php.net/manual/en/function.apc-add.php) using apc_add() function to create entries exclusively.

See the following code:

function TryLock() {
  return apc_add( '_lock', 1, 0);
}
funciton Unlock() {
 apc_delete( '_lock');
}
while( !TryLock())
 usleep( 10);
... // do something with cache
Unlock();

Imagine two threads executing this code. The thread, which first gets apc_add() returned true, acquires lock and gains control over the cache, until that thread calls Unlock() (deletes the '_lock' key).
The problem is that on my configuration - WINDOWS XP, Apache 2.2.4.0, PHP 5.2.5.5, APC 5.2.5.5, this mechanism does not work, because the second call to apc_add with key '_lock' forces this entry to be deleted from cache. The bug is: the second call of two calls to apc_add() with equal key parameter force the entry to be deleted from cache.. I have tested this code on Linux, and it works as expected.

Reproduce code:
---------------
var_dump( apc_add( '_lock', 1, 0));
$info =apc_cache_info( 'user'); var_dump( $info['cache_list']);
var_dump( apc_add( '_lock', 1, 0));
$info =apc_cache_info( 'user'); var_dump( $info['cache_list']);
var_dump( apc_add( '_lock', 1, 0));
$info =apc_cache_info( 'user'); var_dump( $info['cache_list']);
var_dump( apc_add( '_lock', 1, 0));
$info =apc_cache_info( 'user'); var_dump( $info['cache_list']);

Expected result:
----------------
bool(true)
array(1) {
  [0]=>
  array(10) {
    ["info"]=>
    string(5) "_lock"
    ["ttl"]=>
    int(0)
    ["type"]=>
    string(4) "user"

    ["num_hits"]=>
    int(0)
    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>
    int(1208725222)
    ["deletion_time"]=>
    int(0)

    ["access_time"]=>
    int(1208725222)
    ["ref_count"]=>
    int(0)
    ["mem_size"]=>
    int(96)
  }
}
bool(false)
array(1) {
  [0]=>
  array(10) {

    ["info"]=>
    string(5) "_lock"
    ["ttl"]=>
    int(0)
    ["type"]=>
    string(4) "user"
    ["num_hits"]=>
    int(0)

    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>
    int(1208725222)
    ["deletion_time"]=>
    int(0)
    ["access_time"]=>
    int(1208725222)

    ["ref_count"]=>
    int(0)
    ["mem_size"]=>
    int(96)
  }
}
bool(false)
array(1) {
  [0]=>
  array(10) {
    ["info"]=>
    string(5) "_lock"

    ["ttl"]=>
    int(0)
    ["type"]=>
    string(4) "user"
    ["num_hits"]=>
    int(0)
    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>

    int(1208725222)
    ["deletion_time"]=>
    int(0)
    ["access_time"]=>
    int(1208725222)
    ["ref_count"]=>
    int(0)
    ["mem_size"]=>

    int(96)
  }
}
bool(false)
array(1) {
  [0]=>
  array(10) {
    ["info"]=>
    string(5) "_lock"
    ["ttl"]=>
    int(0)
    ["type"]=>

    string(4) "user"
    ["num_hits"]=>
    int(0)
    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>
    int(1208725222)
    ["deletion_time"]=>

    int(0)
    ["access_time"]=>
    int(1208725222)
    ["ref_count"]=>
    int(0)
    ["mem_size"]=>
    int(96)
  }
}

Actual result:
--------------
bool(true)
array(1) {
  [0]=>
  array(10) {
    ["info"]=>
    string(5) "_lock"
    ["ttl"]=>
    int(0)
    ["type"]=>

    string(4) "user"
    ["num_hits"]=>
    int(0)
    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>
    int(1208725222)
    ["deletion_time"]=>

    int(0)
    ["access_time"]=>
    int(1208725222)
    ["ref_count"]=>
    int(0)
    ["mem_size"]=>
    int(96)
  }
}
bool(false)
array(0) {
}

bool(true)
array(1) {
  [0]=>
  array(10) {
    ["info"]=>
    string(5) "_lock"
    ["ttl"]=>
    int(0)
    ["type"]=>
    string(4) "user"

    ["num_hits"]=>
    int(0)
    ["mtime"]=>
    int(1208725222)
    ["creation_time"]=>
    int(1208725222)
    ["deletion_time"]=>
    int(0)

    ["access_time"]=>
    int(1208725222)
    ["ref_count"]=>
    int(0)
    ["mem_size"]=>
    int(96)
  }
}
bool(false)
array(0) {
}