MDL-54606 session: Add redis handler with locking.

config-dist.php

@@ -249,6 +249,15 @@
 //      $CFG->session_memcached_acquire_lock_timeout = 120;
 //      $CFG->session_memcached_lock_expire = 7200;       // Ignored if PECL memcached is below version 2.2.0
 //
+//   Redis session handler (requires redis server and redis extension):
+//      $CFG->session_handler_class = '\core\session\redis';
+//      $CFG->session_redis_host = '127.0.0.1';
+//      $CFG->session_redis_port = 6379;  // Optional.
+//      $CFG->session_redis_database = 0;  // Optional, default is db 0.
+//      $CFG->session_redis_prefix = ''; // Optional, default is don't set one.
+//      $CFG->session_redis_acquire_lock_timeout = 120;
+//      $CFG->session_redis_lock_expire = 7200;
+//
 //   Memcache session handler (requires memcached server and memcache extension):
 //      $CFG->session_handler_class = '\core\session\memcache';
 //      $CFG->session_memcache_save_path = '127.0.0.1:11211';

lib/classes/session/redis.php

@@ -0,0 +1,382 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Redis based session handler.
+ *
+ * @package    core
+ * @copyright  2015 Russell Smith <[email protected]>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core\session;
+
+use RedisException;
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * Redis based session handler.
+ *
+ * The default Redis session handler does not handle locking in 2.2.7, so we have written a php session handler
+ * that uses locking.  The places where locking is used was modeled from the memcached code that is used in Moodle
+ * https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached_session.c
+ *
+ * @package    core
+ * @copyright  2016 Russell Smith
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class redis extends handler {
+    /** @var string $host save_path string  */
+    protected $host = '';
+    /** @var int $port The port to connect to */
+    protected $port = 6379;
+    /** @var int $database the Redis database to store sesions in */
+    protected $database = 0;
+    /** @var array $servers list of servers parsed from save_path */
+    protected $prefix = '';
+    /** @var int $acquiretimeout how long to wait for session lock in seconds */
+    protected $acquiretimeout = 120;
+    /**
+     * @var int $lockexpire how long to wait in seconds before expiring the lock automatically
+     * so that other requests may continue execution, ignored if PECL redis is below version 2.2.0.
+     */
+    protected $lockexpire = 7200;
+
+    /** @var Redis Connection */
+    protected $connection = null;
+
+    /** @var array $locks List of currently held locks by this page. */
+    protected $locks = array();
+
+    /**
+     * Create new instance of handler.
+     */
+    public function __construct() {
+        global $CFG;
+
+        if (isset($CFG->session_redis_host)) {
+            $this->host = $CFG->session_redis_host;
+        }
+
+        if (isset($CFG->session_redis_port)) {
+            $this->port = (int)$CFG->session_redis_port;
+        }
+
+        if (isset($CFG->session_redis_database)) {
+            $this->database = (int)$CFG->session_redis_database;
+        }
+
+        if (isset($CFG->session_redis_prefix)) {
+            $this->prefix = $CFG->session_redis_prefix;
+        }
+
+        if (isset($CFG->session_redis_acquire_lock_timeout)) {
+            $this->acquiretimeout = (int)$CFG->session_redis_acquire_lock_timeout;
+        }
+
+        if (isset($CFG->session_redis_lock_expire)) {
+            $this->lockexpire = (int)$CFG->session_redis_lock_expire;
+        }
+    }
+
+    /**
+     * Start the session.
+     *
+     * @return bool success
+     */
+    public function start() {
+        $result = parent::start();
+
+        return $result;
+    }
+
+    /**
+     * Init session handler.
+     */
+    public function init() {
+        if (!extension_loaded('redis')) {
+            throw new exception('sessionhandlerproblem', 'error', '', null, 'redis extension is not loaded');
+        }
+
+        if (empty($this->host)) {
+            throw new exception('sessionhandlerproblem', 'error', '', null,
+                    '$CFG->session_redis_host must be specified in config.php');
+        }
+
+        // The session handler requires a version of Redis with the SETEX command (at least 2.0).
+        $version = phpversion('Redis');
+        if (!$version or version_compare($version, '2.0') <= 0) {
+            throw new exception('sessionhandlerproblem', 'error', '', null, 'redis extension version must be at least 2.0');
+        }
+
+        $this->connection = new \Redis();
+
+        $result = session_set_save_handler(array($this, 'handler_open'),
+            array($this, 'handler_close'),
+            array($this, 'handler_read'),
+            array($this, 'handler_write'),
+            array($this, 'handler_destroy'),
+            array($this, 'handler_gc'));
+        if (!$result) {
+            throw new exception('redissessionhandlerproblem', 'error');
+        }
+
+        try {
+            // One second timeout was chosen as it is long for connection, but short enough for a user to be patient.
+            if (!$this->connection->connect($this->host, $this->port, 1)) {
+                throw new RedisException('Unable to connect to host.');
+            }
+            if (!$this->connection->setOption(\Redis::OPT_SERIALIZER, \Redis::SERIALIZER_PHP)) {
+                throw new RedisException('Unable to set Redis PHP Serializer option.');
+            }
+
+            if ($this->prefix !== '') {
+                // Use custom prefix on sessions.
+                if (!$this->connection->setOption(\Redis::OPT_PREFIX, $this->prefix)) {
+                    throw new RedisException('Unable to set Redis Prefix option.');
+                }
+            }
+            if ($this->database !== 0) {
+                if (!$this->connection->select($this->database)) {
+                    throw new RedisException('Unable to select Redis database '.$this->database.'.');
+                }
+            }
+            $this->connection->ping();
+            return true;
+        } catch (RedisException $e) {
+            error_log('Failed to connect to redis at '.$this->host.':'.$this->port.', error returned was: '.$e->getMessage());
+            return false;
+        }
+    }
+
+    /**
+     * Update our session search path to include session name when opened.
+     *
+     * @param string $savepath  unused session save path. (ignored)
+     * @param string $sessionname Session name for this session. (ignored)
+     * @return bool true always as we will succeed.
+     */
+    public function handler_open($savepath, $sessionname) {
+        return true;
+    }
+
+    /**
+     * Close the session completely. We also remove all locks we may have obtained that aren't expired.
+     *
+     * @return bool true on success.  false on unable to unlock sessions.
+     */
+    public function handler_close() {
+        try {
+            foreach ($this->locks as $id => $expirytime) {
+                if ($expirytime > $this->time()) {
+                    $this->unlock_session($id);
+                }
+                unset($this->locks[$id]);
+            }
+        } catch (RedisException $e) {
+            error_log('Failed talking to redis: '.$e->getMessage());
+            return false;
+        }
+
+        return true;
+    }
+    /**
+     * Read the session data from storage
+     *
+     * @param string $id The session id to read from storage.
+     * @return string The session data for PHP to process.
+     *
+     * @throws RedisException when we are unable to talk to the Redis server.
+     */
+    public function handler_read($id) {
+        try {
+            $this->lock_session($id);
+            $sessiondata = $this->connection->get($id);
+            if ($sessiondata === false) {
+                $this->unlock_session($id);
+                return '';
+            }
+            $this->connection->expire($id, $this->lockexpire);
+        } catch (RedisException $e) {
+            error_log('Failed talking to redis: '.$e->getMessage());
+            throw $e;
+        }
+        return $sessiondata;
+    }
+
+    /**
+     * Write the serialized session data to our session store.
+     *
+     * @param string $id session id to write.
+     * @param string $data session data
+     * @return bool true on write success, false on failure
+     */
+    public function handler_write($id, $data) {
+        if (is_null($this->connection)) {
+            // The session has already been closed, don't attempt another write.
+            error_log('Tried to write session: '.$id.' before open or after close.');
+            return false;
+        }
+
+        // We do not do locking here because memcached doesn't.  Also
+        // PHP does open, read, destroy, write, close. When a session doesn't exist.
+        // There can be race conditions on new sessions racing each other but we can
+        // address that in the future.
+        try {
+            $this->connection->setex($id, $this->lockexpire, $data);
+        } catch (RedisException $e) {
+            error_log('Failed talking to redis: '.$e->getMessage());
+            return false;
+        }
+        return true;
+    }
+
+    /**
+     * Handle destroying a session.
+     *
+     * @param string $id the session id to destroy.
+     * @return bool true if the session was deleted, false otherwise.
+     */
+    public function handler_destroy($id) {
+        try {
+            $this->connection->del($id);
+            $this->unlock_session($id);
+        } catch (RedisException $e) {
+            error_log('Failed talking to redis: '.$e->getMessage());
+            return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Garbage collect sessions.  We don't we any as Redis does it for us.
+     *
+     * @param integer $maxlifetime All sessions older than this should be removed.
+     * @return bool true, as Redis handles expiry for us.
+     */
+    public function handler_gc($maxlifetime) {
+        return true;
+    }
+
+    /**
+     * Unlock a session.
+     *
+     * @param string $id Session id to be unlocked.
+     */
+    protected function unlock_session($id) {
+        if (isset($this->locks[$id])) {
+            $this->connection->del($id.".lock");
+            unset($this->locks[$id]);
+        }
+    }
+
+    /**
+     * Obtain a session lock so we are the only one using it at the moent.
+     *
+     * @param string $id The session id to lock.
+     * @return bool true when session was locked, exception otherwise.
+     * @throws exception When we are unable to obtain a session lock.
+     */
+    protected function lock_session($id) {
+        $lockkey = $id.".lock";
+
+        $haslock = isset($this->locks[$id]) && $this->time() < $this->locks[$id];
+        $startlocktime = $this->time();
+
+        /* To be able to ensure sessions don't write out of order we must obtain an exclusive lock
+         * on the session for the entire time it is open.  If another AJAX call, or page is using
+         * the session then we just wait until it finishes before we can open the session.
+         */
+        while (!$haslock) {
+            $haslock = $this->connection->setnx($lockkey, '1');
+            if (!$haslock) {
+                usleep(rand(100000, 1000000));
+                if ($this->time() > $startlocktime + $this->acquiretimeout) {
+                    // This is a fatal error, better inform users.
+                    // It should not happen very often - all pages that need long time to execute
+                    // should close session immediately after access control checks.
+                    error_log('Cannot obtain session lock for sid: '.$id.' within '.$this->acquiretimeout.
+                            '. It is likely another page has a long session lock, or the session lock was never released.');
+                    throw new exception("Unable to obtain session lock");
+                }
+            } else {
+                $this->locks[$id] = $this->time() + $this->lockexpire;
+                $this->connection->expire($lockkey, $this->lockexpire);
+                return true;
+            }
+        }
+    }
+
+    /**
+     * Return the current time.
+     *
+     * @return int the current time as a unixtimestamp.
+     */
+    protected function time() {
+        return time();
+    }
+
+    /**
+     * Check the backend contains data for this session id.
+     *
+     * Note: this is intended to be called from manager::session_exists() only.
+     *
+     * @param string $sid
+     * @return bool true if session found.
+     */
+    public function session_exists($sid) {
+        if (!$this->connection) {
+            return false;
+        }
+
+        try {
+            return $this->connection->exists($sid);
+        } catch (RedisException $e) {
+            return false;
+        }
+    }
+
+    /**
+     * Kill all active sessions, the core sessions table is purged afterwards.
+     */
+    public function kill_all_sessions() {
+        global $DB;
+        if (!$this->connection) {
+            return;
+        }
+
+        $rs = $DB->get_recordset('sessions', array(), 'id DESC', 'id, sid');
+        foreach ($rs as $record) {
+            $this->handler_destroy($record->sid);
+        }
+        $rs->close();
+    }
+
+    /**
+     * Kill one session, the session record is removed afterwards.
+     *
+     * @param string $sid
+     */
+    public function kill_session($sid) {
+        if (!$this->connection) {
+            return;
+        }
+
+        $this->handler_destroy($sid);
+    }
+}