Question

I have a PHP page that should be accessible only by one user at a time. It’s a kind of “poor man’s cron”: A “Javascript” file, that is requested in the background. At the moment I’m thinking of creating a lock file like this:

if(file_exists($lockfile) && filemtime($lockfile) + EXPIRES_AFTER > time() ) {
    die("// Page is locked.");
}

touch($lockfile);
// do stuff
unlink($lockfile);
echo "// Cron stuff was run";

But I’m not sure if there could be a very short window of opportunity between the file_exists and the touch call where another page request could check for the file existence and see that it isn’t there. We’re probably talking microseconds here so I would like top know at which amount of requests I really need to start worrying.

Answer 1

If you want to do this really precisely, then use a different approach, because there IS some time between the check and the lock.

Two possible implementations:

1. Use flock:
   https://secure.php.net/manual/en/function.flock.php

2. Use something like STM:
   E.g. open the lockfile for append, write something into it, close the handle. Then read the file back, and if it only has what you wrote into it then you have acquired the lock.

Other than that, your original code would probably not cause any problems.