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I am working with the following code on a PHP script:

try{
    if(file_exists($dir."pid.txt"))throw new Exception("Process is already running");
}
catch(Exception $e){
    die("Warning: ".$e->getMessage()." in ".$e->getFile()." on line ".$e->getLine());
}

touch($dir."pid.txt");
sleep(20); // Just for a proof of concept

Basically what the script does is:

If the file $dir."pid.txt" - where $dir contains a directory with the right permissions - already exists, kill the script. If not, create the pid.txt file and sleep during 20 seconds.

The intended effect is to prevent this script from being accessed twice.

If I open the script in one tab and try again in another tab within that 20 seconds period, the script on the second tab won't die. However, different tabs in different browsers do work. I have been trying with Chrome, Firefox, and IE, latest stable release versions.

I am 100% sure that the file "pid.txt" has been created correctly by the first script.

I suspect it has something to do with the try/catch statement because if I do

if(file_exists($dir."pid.txt"))die("The process is already running");

it does work. However, this is one of many possible errors and I would like to group them in a try/catch statement. That's why I don't want to proceed with this last approach.

Ideas?

delolmo
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  • where you define `$dir`? – Vytautas Feb 29 '12 at 08:37
  • @Vytautas: $dir is defined on a previous line and looks something like $dir = $_SERVER["DOCUMENT_ROOT"]."/path/to/dir/"; – delolmo Feb 29 '12 at 08:45
  • I can quite confidently assure you that PHP does not care about browser tabs. :) Try to figure out what else is different about the two requests or if there's a problem in your logic. – deceze Feb 29 '12 at 08:49
  • @deceze and yet if fails. Maybe it has something to do with working with HTTP authentication? I know this sounds weird and that this behavior seems to have nothing to do with PHP (server-side language, that, as you say, does not care about browser tabs) but I can assure you that the only code running is PHP. No HTML, Javascript or any of any other kind. – delolmo Feb 29 '12 at 08:55
  • @deceze BTW: I have just checked the HTTP request headers and they are identical. – delolmo Feb 29 '12 at 08:56
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    Could it be that running the script in the same browser the second time doesn't really creates a request to the server but rather pulls the result from the cache? have you tried adding no-cache headers? – Yaniro Feb 29 '12 at 09:27
  • @Yaniro you were right. I added no-cache headers and it worked. I should add that because this script is intended to last long - which is why I need to check if it is already running to prevent parallel execution - headers must be flushed upon definition. – delolmo Feb 29 '12 at 10:01

2 Answers2

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Code should work fine unless $dir."pid.txt" is not what you think is. Or you made mistake somewhere else. provide more code if you want more helpful answer.

Vytautas
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0

As noted by @Yaniro, no-cache headers must be added for the script to work as intended and flushed upon definition. Final code would be:

header("Last-Modified: ".gmdate("D, d M Y H:i:s")." GMT");
header("Expires: Sat, 26 Jul 1970 05:00:00 GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");
flush();

try{
    if(file_exists($dir."pid.txt"))throw new Exception("Process is already running");
}
catch(Exception $e){
    die("Warning: ".$e->getMessage()." in ".$e->getFile()." on line ".$e->getLine());
}

touch($dir."pid.txt");
sleep(20); // Just for a proof of concept

flush() is necessary if the script takes too long to finish (thus preventing parallel execution of the script, which is the purpose of the code).

delolmo
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