如何锁定文件以防止读取/写入同一文件的多个ajax请求？

Question

I am currently trying to prevent a file from being read if it already exists and has content in it. I am using the flock method to try and prevent other instances of the script running from accessing it, but it is not working. This is only an issue with servers with multiple cores.

while ($aEvent = current($aEvents)){
    $sLockFile = '/var/tmp/'.md5($aEvent['id']).'.LCK';
    $oFile = fopen($sLockFile, 'c+');
    if (flock($oFile, LOCK_EX) && fgets($oFile) == '') {
        fwrite($oFile, 'locked');
        fclose($oFile);
    } else {
        fclose($oFile);
        next($aEvents);
        continue;
    }
    /*
     * Do stuff here.
     */
    next($aEvents);
}

So from this code, I expect that a file is created, locked, and written to, and any request that tries to subsequently read/write the file is either stopped at the flock call or the empty check. When in reality it seems like if the code is hit with enough ajax requests at the same time, the same file is being created/overwritten multiple times. Is there any way to keep PHP thread safe?

Answer 1

It sounds like you want to add the LOCK_NB bit to your flock call. From the flock docs :

By default, this function will block until the requested lock is acquired; this may be controlled with the LOCK_NB option

Your code will block on each call to flock until a lock can be acquired. It sounds like you want to skip to the else block if a lock exists. If so, you want to do this in your code:

<?php
if (flock($oFile, LOCK_EX | LOCK_NB) && fgets($oFile) == '')
...