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  1. Moodle
  2. MDL-67075

Extra session lock debugging for redis session handler

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    • MOODLE_39_STABLE
    • MOODLE_39_STABLE
    • MDL-67075-redis-session-locks
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      1. Ensure debugging is turned on on your site.
      2. Setup redis
      3. Setup redis as the session handler (notice the session_redis_acquire_lock_timeout value): 

        $CFG->session_redis_host = '127.0.0.1';
        		 
        $CFG->session_redis_port = 6379; // Optional.
        		 
        $CFG->session_redis_acquire_lock_timeout = 5;
        		 
        $CFG->session_handler_class = '\core\session\redis';

      4. Write a dud script that hangs onto the session for ages called long-session.php 

        <?php
        		 
        require('config.php');
        		 
        require_login();
        		 
        sleep(10);

      5. Go to <yoursite>/long-session.php and log in
      6. Tail the logs with debugging on (eg. tail -f /usr/local/var/log/httpd/error_log in your terminal)
      7. Open in two tabs at the same time in fast succession within 5 seconds
        1. <yoursite>/long-session.php
        2. <yoursite>/long-session.php
      8. You should see an error log like: 

        Cannot obtain session lock for sid: a5u30psl4he2t3sqpkroghucbj within 5 seconds. It is likely another page (zebrafish:25101 /long-session.php?id=1) has a long session lock, or the session lock was never released.

      Show
      Ensure debugging is turned on on your site. Setup redis Setup redis as the session handler (notice the session_redis_acquire_lock_timeout value): $CFG->session_redis_host = '127.0.0.1' ; $CFG->session_redis_port = 6379 ; // Optional. $CFG->session_redis_acquire_lock_timeout = 5 ; $CFG->session_handler_class = '\core\session\redis' ; Write a dud script that hangs onto the session for ages called long-session.php <?php require( 'config.php' ); require_login(); sleep( 10 ); Go to <yoursite>/long-session.php and log in Tail the logs with debugging on (eg. tail -f /usr/local/var/log/httpd/error_log in your terminal) Open in two tabs at the same time in fast succession within 5 seconds <yoursite>/long-session.php <yoursite>/long-session.php You should see an error log like: Cannot obtain session lock for sid: a5u30psl4he2t3sqpkroghucbj within 5 seconds. It is likely another page (zebrafish:25101 /long-session.php?id=1) has a long session lock, or the session lock was never released.

      This adds more debugging to make it easier to track down open session locks.

      Note this is superficially similar to MDL-64449 but with a couple key differences. Firstly that one mutates the session which means more churning. Secondly it only works after the fact, if you have a blocked session you can't see what's blocking you right now. More importantly it also doesn't work if the session lock times out, for it to work you must eventually gain the session lock to get access to what's inside the session data.

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            brendanheywood Brendan Heywood
            brendanheywood Brendan Heywood
            Mark Nelson Mark Nelson
            Andrew Lyons Andrew Lyons
            Janelle Barcega Janelle Barcega
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            1 Vote for this issue
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