add memcached session storage (#135)

* add memcached session storage

* composer update to fix php-cs-fixer failing for itself

* Revert "composer update to fix php-cs-fixer failing for itself"

This reverts commit db548046f863b6648e56c284271d0eb832707427.

* composer update to fix php-cs-fixer failing for itself

backend/src/plugins/Sessionstorage/memcached.php

@@ -0,0 +1,100 @@
+<?php
+
+namespace Plugins\Sessionstorage;
+
+require '../vendor/autoload.php';
+
+/**
+ * Implements a session storage plugin for using PHPs APCu.
+ */
+class memcached implements InterfaceSessionstorage
+{
+    /** @var \Monolog\Logger */
+    private $logger;
+
+    /** @var \Memcached */
+    private $memcached;
+
+    /**
+     * Construct the object
+     * 
+     * @param   $logger Monolog logger instance for error handling
+     * @param   $config The configuration for the Plugin if any was provided
+     */
+    public function __construct(\Monolog\Logger $logger, array $config = null)
+    {
+        $this->logger = $logger;
+
+        if (!class_exists('Memcached')) {
+            $this->logger->critical('PHP Memcached extension is not available but configured as session storage backend exiting now');
+            exit();
+        }
+        $this->memcached = new \Memcached();
+        if (!array_key_exists('host', $config) || !array_key_exists('port', $config)) {
+            $this->logger->critical('Memcached session configuration missing host or port value');
+            exit();
+        }
+        $this->memcached->addServer($config['host'], $config['port']);
+    }
+
+    /**
+     * Save new entry.
+     * 
+     * @param   $key    The key for the entry
+     * @param   $value  The value for the entry
+     * @param   $ttl    The time (in s) for which this item should be available
+     */
+    public function set(string $key, string $value, int $ttl) : void
+    {
+        $this->logger->debug('Storing data to Memcached', ['key' => $key, 'value' => $value, 'ttl' => $ttl]);
+
+        $this->memcached->set($key, $value, $ttl);
+    }
+
+    /**
+     * Queries the existence of some entry.
+     * 
+     * @param   $key    The key to query
+     */
+    public function exists(string $key) : bool
+    {
+        $this->logger->debug('Checking for Memcached key existence', ['key' => $key]);
+
+        $this->memcached->get($key);
+        return \Memcached::RES_NOTFOUND !== $this->memcached->getResultCode();
+    }
+
+    /**
+     * Get the value for a given key. This should also reset the ttl to the given value.
+     * 
+     * @param   $key    The key for the entry to get
+     * @param   $ttl    The new ttl for the entry
+     */
+    public function get(string $key, int $ttl) : string
+    {
+        $this->logger->debug('Getting data from Memcached', ['key' => $key, 'ttl' => $ttl]);
+
+        $value = $this->memcached->get($key);
+
+        if ($value == false) {
+            $this->logger->error('Non existing key was queried from Memcached', ['key' => $key]);
+            throw new \InvalidArgumentException('The requested key was not in the database!');
+        }
+
+        $this->memcached->touch($key, $ttl);
+
+        return $value;
+    }
+
+    /**
+     * Delete the value for a given key.
+     * 
+     * @param   $key    The key to delete
+     */
+    public function delete(string $key) : void
+    {
+        $this->logger->debug('Deleting key from Memcached', ['key' => $key]);
+
+        $this->memcached->delete($key);
+    }
+}