Weird PHP error: exec() hangs sometimes on simple script

Question

Heyas,

So this simple exec() script runs fine for the first two times, in trying to generate a PDF file from a webpage (using wkhtmltopdf). It first) deletes the existing file, and second) creates the new PDF file in its place. If I run the script a second time, it deletes the file again, and then creates a new one, as expected. However, if I run it one more time, it deletes the files, creates a new one, but then the script seems to hang until the 30-second 504 timeout error is given. The script, when it works, only takes about 3 seconds to run/return. It also kills the entire server (any other local PHP sites no longer work). If I restart the PHP server, everything still hangs (with no success). Interestingly, if I run the script once, and then restart the PHP server, I can keep doing this without issue (but only generating the PDF up to two times). No PHP errors are logged. Why would it be stalling out subsequent times?

    $filePath = 'C:\\wtserver\\tmp\\order_' . $orderId . '.pdf';

    // delete an existing file
    if(file_exists($filePath)) {
        if(!unlink($filePath)) {
            echo 'Error deleting existing file: ' . $filePath;
            return;
        }
    }

    // generates PDF file at C:\wtserver\tmp\order_ID.pdf
    exec('wkhtmltopdf http://google.com ' . $filePath);

I've tried a simple loop to check for the script's completion (successful output), and then try to exit, but it still hangs:

    while(true) {
        if(file_exists($filePath)) {
            echo 'exit';
            exit(); // have also tried die()
            break;
        }
        //todo: add time check/don't hang
    }

If I can't figure this bit out, for now, is there a way to kill the exec script, wrapping it somehow? The PDF is still generated, so the script is working, but I need to kill it and return a response to the user.

Solution:

Have to redirect standard output AND standard error, to end the process immediately, ie. in Windows:

exec('wkhtmltopdf http://google.com ' . $filePath . ' > NUL 2> NUL');

where did you tried the simple check .your exec call is blocking. so just wondering were probably your while loop is running. — Sachin Sharma
– Sachin Sharma, Commented Jul 14, 2016 at 7:20

Sachin Sharma · Accepted Answer · 2016-07-14 07:14:45Z

1

do you know that you can run the executable in background, like this

exec($cmd . " > /dev/null &");

This way you can immediately come out of it.

answered Jul 14, 2016 at 7:14

Sachin Sharma

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2 Comments

Ryan Weiss Over a year ago

That sounded really promising, but I tried it, and it's still hanging in the same way :( Thanks though...

Ryan Weiss Over a year ago

Oh I think we got it! Looks like I also had to redirect the standard error as well as the standard output, ie. (check solution above). Thank you!!

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