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.