better vardata caching by implementing pre-check/post-check algo

[moo]

• Update API: xcache_isset($name [, $post_check_ttl])

Example usage:

<?php
define(TMPDIR, '/tmp');

function load_abc_data()
{
  $name = "abc_data";
  $ttl = 120; // cache for 2 minutes
  $post_check_ttl = 60; // reload every 1 minutes, while other reader keep using the stale data
  if (!xcache_isset($name, $post_check_ttl)) {
    // it worth a lock here to avoid useless yet harmful concurrent load from any slow backend (backend=mysql here).
    $fp = fopen(TMPDIR . "/abc_data.lock", "w");
    // only one worker update it and other keep on reading, if it's in post-check time
    $nb = xcache_isset($name) ? 0 : LOCK_NB;
    if (flock($fp, LOCK_EX | $nb)) {
      // check AGAIN after we get the lock
      if (!xcache_isset($name, $post_check_ttl)) {
        mysql_query .... and get $data
        xcache_set($name, $data, 120);
        fclose($fp);
        return $data;
      }
      else {
        fclose($fp);
      }
    }
  }
  return xcache_get($name);
}

?>

See MSDN for Cache-Control pre-check/post-check algo