MDL-57477 core: If memcached 3.x mimic 2.2.0 lock acquisition behavior

config-dist.php

@@ -259,6 +259,8 @@ $CFG->admin = 'admin';
 //      $CFG->session_memcached_prefix = 'memc.sess.key.';
 //      $CFG->session_memcached_acquire_lock_timeout = 120;
 //      $CFG->session_memcached_lock_expire = 7200;       // Ignored if PECL memcached is below version 2.2.0
+//      $CFG->session_memcached_lock_retry_sleep = 150;   // Spin-lock retry sleeptime (msec). Only effective
+//                                                        // for tuning php-memcached 3.0.x (PHP 7)
 //
 //   Redis session handler (requires redis server and redis extension):
 //      $CFG->session_handler_class = '\core\session\redis';

lib/classes/session/memcached.php

@@ -47,6 +47,13 @@ class memcached extends handler {
      * may continue execution, ignored if PECL memcached is below version 2.2.0.
      */
     protected $lockexpire = 7200;
+    /**
+     *
+     * @var integer $lockretrysleep Used for memcached 3.x (PHP7), the amount of time to
+     * sleep between attempts to acquire the session lock. Mimics the deprecated config
+     * memcached.sess_lock_wait.
+     */
+    protected $lockretrysleep = 150;
 
     /**
      * Create new instance of handler.
@@ -79,6 +86,10 @@ class memcached extends handler {
         if (!empty($CFG->session_memcached_lock_expire)) {
             $this->lockexpire = (int)$CFG->session_memcached_lock_expire;
         }
+
+        if (!empty($CFG->session_memcached_lock_retry_sleep)) {
+            $this->lockretrysleep = (int)$CFG->session_memcached_lock_retry_sleep;
+        }
     }
 
     /**
@@ -98,6 +109,7 @@ class memcached extends handler {
 
         $result = parent::start();
 
+        // MDL-53713...
         // If session_start returned TRUE, but it took as long
         // as the timeout value, and the $_SESSION returned is
         // empty when should not have been (isnewsession false)
@@ -134,15 +146,39 @@ class memcached extends handler {
         ini_set('session.save_path', $this->savepath);
         ini_set('memcached.sess_prefix', $this->prefix);
         ini_set('memcached.sess_locking', '1'); // Locking is required!
+        ini_set('memcached.sess_lock_expire', $this->lockexpire);
 
-        // Try to configure lock and expire timeouts - ignored if memcached is before version 2.2.0.
+        // MDL-57477, MDL-53713...
         if (version_compare($version, '3.0.0-dev') >= 0) {
-            ini_set('memcached.sess_lock_wait_max', $this->acquiretimeout * 1000);
+            // With memcached 3.x (PHP 7) we configure the max retries to make
+            // and the time to sleep between each retry. There are two sleep
+            // config values, an initial and a max value. After each attempt
+            // the memcached module adjusts the sleep value to be the lesser of
+            // the configured max value, or 2X the previous value. With default
+            // memcached.ini configs (5, 1s, 2s) the result is only 5 attempts
+            // to lock over 9 sec. To mimic the behavior of the 2.2.x module so
+            // we get more attempts and much more frequently, config both sleep
+            // values to the old default value of 150 msec (making it constant)
+            // and calculate number of retries using the existing Moodle config
+            // $CFG->session_memcached_acquire_lock_timeout. Doing this so
+            // admins configure session lock attempt timeout in familiar terms,
+            // and more straight-forward to detect if lock attempt timeout has
+            // occurred in start(). If _min and _max values are not equal, the
+            // actual lock acquire timeout will not be the expected configured
+            // value in $CFG->session_memcached_acquire_lock_timeout; this will
+            // cause session data loss when failure to acquire the lock is not
+            // detected.
+            ini_set('memcached.sess_lock_wait_min', $this->lockretrysleep);
+            ini_set('memcached.sess_lock_wait_max', $this->lockretrysleep);
+            ini_set('memcached.sess_lock_retries', (int)(($this->acquiretimeout * 1000) / $this->lockretrysleep) + 1);
         } else {
+            // With memcached 2.2.x we configure max time to attempt lock, and
+            // accept default value (in memcached.ini) for sleep time between
+            // each attempt (usually 150 msec), then memcached calculates the
+            // max number of retries to make.
             ini_set('memcached.sess_lock_max_wait', $this->acquiretimeout);
         }
 
-        ini_set('memcached.sess_lock_expire', $this->lockexpire);
     }
 
     /**