Undefined offset: 2 in /.../persistent_login.module on line 308

Confirmed -- it magically went away on clearing the site cache. I looked at that line, too, and I'm very confused about how there could be an undefined index.

The error seemed to pop up after I cleared the site cache, viewed a subdomain of the site, and then went back to the primary domain of the site. I have a feeling it has something to do with the cookie array.

I'll post back here if I see the message again...

This issue has happened again on the same site. There are about 7000 users on the site, and a script runs every 30 minutes, activating and/or deactivating user account, and creates new accounts.

That script calls user_save for a lot of users. This might be doing something to trigger an alteration of the cookie.

I ran a few test scripts on the server to see what exactly in line 308 would produce that error.

There shouldn't be any issue with an undefined array index there:

true.. and there's not. The error is not "undefined array index" it's "undefined offset"

There's a difference. The difference is that the $_COOKIE[$cookie_name] is set and not empty, so it passes your check:
if ($user->uid == 0 && isset($_COOKIE[$cookie_name]) && !empty($_COOKIE[$cookie_name]) && !isset($_SESSION['persistent_login_check'])) ...

But somehow, the cookie does not have enough ":" characters, so the error is with the list() function trying to pull 3 values from an array with 1.

Take this example code, which throws the exact same error on the same server:

$test = array('key'=>'a:b:c','foo'=>' ');
list($uid, $series, $token) = explode(':', $test['foo']);
echo "<pre>uid: $uid\n series:$series\n token:$token</pre>";

In this code, $test['foo'] is set and not empty, but when run through your checker it passes and then throws up the exact notices that I'm seeing persistent_login throw.

Somehow the cookie value is not set properly, and then the code hits line 308 and blows up.

By replacing line 308 as done in the following example, the error won't show up, but will get sent to the logs. I can't reproduce this issue, but if I see this in the logs I'll let you know what value the cookie has when it occurs:

line 308 from:

list($uid, $series, $token) = explode(':', $_COOKIE[$cookie_name]);

to:

	
	$cookie_array = explode(':', $_COOKIE[$cookie_name]);
	if(count($cookie_array)==3){
		list($uid, $series, $token) = $cookie_array;
	}else{
		watchdog('peristent_login', 'Mysterious malformed cookie: <pre>'.print_r($_COOKIE[$cookie_name],true).'</pre>.', WATCHDOG_ERROR);
	}