ajax long polling crashes at page refresh\links to another page

Question

i have already posted a similar question here, but failed to get a response that will fix my problem, also the problem has changed a bit so i'm re-posting and desperate to get some help!

link to previous question:

ajax long polling with mysql

CURRENT CODE:

JS(I run it from php):

$oldIDq = mysql_query("SELECT * FROM messages ORDER BY id DESC LIMIT 1");
while($oldrow = mysql_fetch_array($oldIDq)){
$oldID = $oldrow['id'];    
}

$func = '
var oldID = '.$oldID.';

function wait() {
$.ajax({
    type: "GET",
    url: "../scripts/msg_scripts/msg.php?oldid=" + oldID,
    async: true,
    cache: false,

    success: function (data){

        if(data != \'1\'){
            var json = eval(\'(\' + data + \')\');
             if (json[\'msg_content\'] != "") {
                  alert("new meassage added"); 
                  } 
                  oldID = json[\'oldID\']; 
                  setTimeout(\'wait()\',1000); }

    },

    disconnect: function()
    {
        return false;
        setTimeout(\'wait()\',1000);
    },

    error: function(XMLHttpRequest, textStatus, errorThrown){
      alert("error: " + textStatus + "(" + errorThrown + ")");  
      setTimeout(\'wait()\',1000);
    }

});
}

$(document).ready(function(){

    wait();
});
';

SERVER:

    $connect = mysql_connect ("localhost", "root", "")

or die ("couldnt connect");
mysql_select_db ("***") or die ("not found"); //if db was not found die
mysql_query("SET NAMES 'utf8'");

$oldID = $_GET['oldid']; 

if($oldID == "") {

die('timeout');
}
else{

$result = mysql_query("SELECT id FROM messages ORDER BY id DESC LIMIT 1");
while($row = mysql_fetch_array($result))
{
    $last_msg_id = $row['id']; 
}
while($last_msg_id <= $oldID)
{
    usleep(10000);
    clearstatcache();
    $result = mysql_query("SELECT id FROM messages ORDER BY id DESC LIMIT 1");
    while($row = mysql_fetch_array($result))
    {
        $last_msg_id = $row['id'];
    }
}



$response = array();
$response['msg_content'] = 'new';
$response['oldID'] = $last_msg_id;
echo json_encode($response);
}

now, i had a session running on the server side of the process and i removed it for now because i understood that long polling has a problem with sessions i also have sessions running on the page which sends the ajax request, since i removed the session my problem has improved in a way, what happens now is that i can basically click on a link on my website and exit the page and get an error, but if i do it more than 4-5 times, the browser freezes an every click on any link just reruns the ajax function and i get a different error. if i refresh the page of the request i imidetly get the second error and the browser freezes. also if that's helpful information if i close the browser and try to reopen any page of my site it doesn't load at all unless i rerun my server(working on localhost right now) also tried it with chrome and ff.

can some one please point me towards the solution?

Answer 1

-- Updated again to retrieve all new messages

Reading your code, you only wish to return the last message, if so then the while loops are in this case pretty useless. Do keep in mind that there might be more 'new' messages between the oldID and the last ID inserted into your database which you skip, so does this code I provide

$connect = mysql_connect ("localhost", "root", "")
or die ("couldnt connect");
mysql_select_db ("***") or die ("not found"); //if db was not found die
mysql_query("SET NAMES 'utf8'");

$oldID = trim($_GET['oldid']);

// empty response, you may fill it with a default msg
$response = array(
  'msg_content' => 'none',
  'oldID' => $oldID
);

// this if statement will prevent to return a valid 
// JSON string to the ajax request
if(empty($oldID)) {
 die('timeout');
}
else {
  $result = mysql_query("SELECT id FROM messages WHERE id > ".addslashes($oldID)." ORDER BY id DESC");
  $index = 1;
  // check if results have new messages
  if(($num_rows = mysql_num_rows($result) > 0) {
    $response['msg_content'] = 'new';
    while($row = mysql_fetch_array($result)) {
      $response['new_msgs'][] = $row['id']
      if($index == $num_rows)
        $response['oldID'] = $row['id']; // set oldID to last record
  }
}
echo json_encode($response);  

--

To your comment on how to use session_write_close properly.

session_start(); 
  $var_id = $_SESSION['id']; 
  $var_fn = $_SESSION['firstname']; 
  $var_ln = $_SESSION['lastname']; 
  $var_mail = $_SESSION['email']; 
// close write to session here to release it for other sources 
session_write_close();

if (!loggedin()){
  header ("Location: index.php");} 
  if ($_GET['id']) {
    $id = mysql_real_escape_string($_GET['id']);} 
  // you are using session here, use the localized $var_id
  else if (isset($var_id)) {
    $id = mysql_real_escape_string($var_id);
}

When session_start() is called, the session at that point is locked for writing to any other source, except the current scope (.php file of execution) it's in. This is to make sure that no values can be changed during the readout of session values.

From the documentation

Session data is usually stored after your script terminated without the need to call session_write_close(), but as session data is locked to prevent concurrent writes only one script may operate on a session at any time. When using framesets together with sessions you will experience the frames loading one by one due to this locking. You can reduce the time needed to load all the frames by ending the session as soon as all changes to session variables are done.

To the getting stuck problem, I think the while loop is endless, request the page from your browser http://example.com/pathto/scripts/msg_scripts/msg.php and see what happens

$counter = 0;
while($last_msg_id <= $oldID)
{
    usleep(10); // changing to be a bit faster
    clearstatcache();
    $result = mysql_query("SELECT id FROM messages ORDER BY id DESC LIMIT 1");
    $row = mysql_fetch_array($result);
    $last_msg_id = $row['id'];
    $counter++;

    if($counter > 100)
      break;
}
echo "counted: {$counter}";
exit();