How to prevent PHP script running more than once?

Question

Currently, I tried to prevent an onlytask.php script from running more than once:

$fp = fopen(\"/tmp/\".\"onlyme.lock\", \"a+\");
if (flock($fp,         


        

Answer 1

Never use unlink for lock files or other functions like rename. It's break your LOCK_EX on Linux. For example, after unlink or rename lock file, any other script always get true from flock().

Best way to detect previous valid end - write to lock file few bytes on the end lock, before LOCK_UN to handle. And after LOCK_EX read few bytes from lock files and ftruncate handle.

Important note: All tested on PHP 5.4.17 on Linux and 5.4.22 on Windows 7.

Example code:

set semaphore:

$handle = fopen($lockFile, 'c+');
if (!is_resource($handle) || !flock($handle, LOCK_EX | LOCK_NB)) {
    if (is_resource($handle)) {
        fclose($handle);
    }
    $handle = false;
    echo SEMAPHORE_DENY;
    exit;
} else {
    $data = fread($handle, 2);
    if ($data !== 'OK') {
        $timePreviousEnter = fileatime($lockFile);
        echo SEMAPHORE_ALLOW_AFTER_FAIL;
    } else {
        echo SEMAPHORE_ALLOW;
    }
    fseek($handle, 0);
    ftruncate($handle, 0);
}

leave semaphore (better call in shutdown handler):

if (is_resource($handle)) {
    fwrite($handle, 'OK');
    flock($handle, LOCK_UN);
    fclose($handle);
    $handle = false;
}