Cannot spot the issue with PHP Index error-possible multiple threads issue

Question

I'm struggling to find the problem with the following piece of PHP code:

The code is as follows

class WorkerThreads extends Thread
{
  private $from_list;

  public function __construct($x,$host,$users_email,$pass,$inbox)
  {
    $this->from_list = array(); # holds the unique froms extracted from headers
  }

  public function run()
  {
    # Get Froms
    if (preg_match('/From\:\ (.+)/i', $headers, $matches, PREG_OFFSET_CAPTURE)) {
      $from = trim(str_ireplace("From: ", "", $matches[0][0]));
      if (!array_key_exists($from, $this->from_list)) {
        $this->from_list[$from] = 1;
        echo "<br/>FROM: ".$from."-".$this->from_list[$from]."<br/><br/>";
      }
    } else {
      echo "NO FROM <br/><rb/>";
    }

The following error occurs:

Notice: Undefined index: Viva in /var/www/BAMCode/yahoofroms.php on line 198    
FROM: Viva-

The offending line 200 is

echo "<br/>FROM: ".$from."-".$this->from_list[$from]."<br/><br/>";

There seems to be a problem with the array

Answer 1

This will work perfectly fine if you don't use multiple threads.

I think many threads are trying to modify and read the same list. Try synchronizing the modification/read of the from_list. We may use a mutex to synchronize read/write to the from_list.

Check the code below:

<?php
class WorkerThreads extends Thread
{
  private $from_list;
  private $mylock;

  public function __construct($x,$host,$users_email,$pass,$inbox)
  {
    $this->from_list = array(); # holds the unique froms extracted from headers
    $this->mylock = Mutex::create();
  }

  public function __destruct()
  {
    Mutex::destroy($this->mylock);
    $this->mylock = null;
  }

  public function run()
  {
    # Get Froms
    if (preg_match('/From\:\ (.+)/i', $headers, $matches, PREG_OFFSET_CAPTURE)) {
      $from = trim(str_ireplace("From: ", "", $matches[0][0]));
      Mutex::lock($this->mylock);
      if (!array_key_exists($from, $this->from_list)) {
        $this->from_list[$from] = 1;
        echo "<br/>FROM: ".$from."-".$this->from_list[$from]."<br/><br/>";
      }
      Mutex::unlock($this->mylock);
    } else {
      echo "NO FROM <br/><rb/>";
    }
  }
}

?>