Invoking an ajax request causes php to block access

Question

I have a difficult problem to debug. I have a web page that makes several ajax requests to our server (apache/php - as simple as it gets). We have implemented a php function which uses shell_exec to execute a small c++ program and read the output. While testing on my local, I have had no problems. However, on our actual server, there is a big problem.

Every time I invoke the ajax request which invokes the php function that calls shell_exec, I get a good response, but afterwards I can no longer make successful ajax calls to that php file. I have tested this on different browsers and different computers and the behavior persists every time.

If I delete my cookies, I can reconnect to the php file.

I have inspected the network connections with chrome developer tools and there is nothing telling given. I receive a good response from the ajax request that invokes shell_exec. But afterwards, its blocked. I get a failed to connect error - but only on the device/session that invoked that function. If I open a different browser, it will still connect.

I have googled php session blocking, but I do not understand how this could be the problem. This is a simple ajax request, and only one at a time is being made. Moreover, I have invoked other ajax request several times to reproduce the behavior and I cannot. The only time this happens is when I invoke the php function that invokes shell_exec.

If something went wrong with shell_exec, I wouldn't expect a good response back, but I am getting a good response.

I know this is vague, but its a big problem and I have no ideas how to debug such a problem. Any ideas on what is going on or how I can debug this?

Example code:

function applyForCreditCards() {
  if (!creditCardAppForm.isValid()) return;
    $.ajax({
      type: 'POST',
      url: '../troystest/MobileServices/MobileValidationService.php',
      data: { 
          'action': 'applyForCreditCard',
          'ssn' : ssn, 
          ... more variables...
          'creditCardIds' : selectedCreditCards
      },
      success: function(msg){
        results = JSON.parse(msg);
        // Do something with response
        });
      }
  });
}

Example server side code

if (!$_SESSION)
    session_start();
switch($_POST['action'] {
    case 'applyForCreditCard': $this->applyForCreditCards($vars...);
              break;
}

applyForCreditCards($vars...) {
    $output = explode("\n", shell_exec($pathToExec." ".$commandLineArgs));
    echo json_encode($output);
    exit();
}

The real code is really long, I tried to include relevant parts. The c++ program is small and spits out 10 or so lines of output to std out.

Answer 1

Depending on what the actual command that is run within shell_exec, you might need to kill the process before trying to run again, or run each process in the background.

Have a look here for some classes that will help to do this: http://php.net/manual/en/function.shell-exec.php