PHP MySQL get_lock

Question

In a script I'm trying to check whether the same script is already running using MySQL GET_LOCK. The problem is, when a script tries to get lock, which isn't free, it blocks forever regardless of the parameter I provide.

<?php

class Controller_Refresher extends Controller {
    public function action_run($key) {
        echo date('H:i:s'."
", time());
        $this->die_if_running();
        $this->run();
        echo date('H:i:s'."
", time());
    }

    private function die_if_running() {
        $result = DB::query(Database::SELECT, "SELECT IS_FREE_LOCK('refresher_running') AS free")->execute();
        if (! intval($result[0]['free'])) die('Running already');
        $result = DB::query(Database::SELECT, "SELECT GET_LOCK('refresher_running', 1)")->execute();
    }

    private function run() {
        echo "Starting
";
        ob_flush();
        sleep(10);

        DB::query(Database::SELECT, "SELECT RELEASE_LOCK('refresher_running')")->execute();
    }
}

When I run this in 2 tabs in browser, I get e.g.:

-tab 1-
20:48:16
Starting
20:48:26
-tab 2-
20:48:27
Starting
20:48:37

While what I want to do is to make the second tab die('Running already');.

Answer 1

Watch out - this problem might actually be caused by php locking the session file:

https://stackoverflow.com/a/5487811/539149

So you should call session_write_close() before any code that needs to run concurrently. I discovered this after trying this Mutex class:

http://code.google.com/p/mutex-for-php/

The class worked great but my php scripts were still running one by one!

Also, you don't need IS_FREE_LOCK(). Just call GET_LOCK('refresher_running', 0) and it will either return 1 if it gives you the lock or 0 if the lock is taken. It's more atomic that way. Of course, lock timeouts can still be useful in certain situations, like when you want to queue up tasks, but watch out for the script timing out if you get too many simultaneous requests.

Zack Morris