MDL-41106 Cleaning up expired elements in session cache store

- Always make sure the elements in cache are sorted so we need to remove only elements in the beginning of array
- Remove expired elements from session store to free memory
- Minor bug fixes

Conflicts:
	cache/stores/session/lib.php
	cache/stores/session/tests/session_test.php

cache/stores/session/lib.php

@@ -192,6 +192,7 @@ public function initialise(cache_definition $definition) {
         $this->storeid = $definition->generate_definition_hash();
         $this->store = &self::register_store_id($definition->get_id());
         $this->ttl = $definition->get_ttl();
+        $this->check_ttl();
     }
 
     /**
@@ -223,6 +224,9 @@ public function get($key) {
                 return $this->store[$key][0];
             } else if ($this->store[$key][1] >= (cache::now() - $this->ttl)) {
                 return $this->store[$key][0];
+            } else {
+                // Element is present but has expired.
+                $this->check_ttl();
             }
         }
         return false;
@@ -244,16 +248,23 @@ public function get_many($keys) {
             $maxtime = cache::now() - $this->ttl;
         }
 
+        $hasexpiredelements = false;
         foreach ($keys as $key) {
             $return[$key] = false;
             if (isset($this->store[$key])) {
                 if ($this->ttl == 0) {
                     $return[$key] = $this->store[$key][0];
                 } else if ($this->store[$key][1] >= $maxtime) {
                     $return[$key] = $this->store[$key][0];
+                } else {
+                    $hasexpiredelements = true;
                 }
             }
         }
+        if ($hasexpiredelements) {
+            // There are some elements that are present but have expired.
+            $this->check_ttl();
+        }
         return $return;
     }
 
@@ -264,7 +275,7 @@ public function get_many($keys) {
      * @param mixed $data The data to set.
      * @return bool True if the operation was a success false otherwise.
      */
-    public function set($key, $data, $testmaxsize = true) {
+    public function set($key, $data) {
         if ($this->ttl === 0) {
             $this->store[$key] = array($data, 0);
         } else {
@@ -432,12 +443,37 @@ public function my_name() {
         return $this->name;
     }
 
+    /**
+     * Removes expired elements.
+     * @return int number of removed elements
+     */
+    protected function check_ttl() {
+        if ($this->ttl == 0) {
+            return 0;
+        }
+        $maxtime = cache::now() - $this->ttl;
+        $c = 0;
+        for ($value = reset($this->store); $value !== false; $value = next($this->store)) {
+            if ($value[1] >= $maxtime) {
+                // We know that elements are sorted by ttl so no need to continue;
+                break;
+            }
+            $c++;
+        }
+        if ($c) {
+            // Remove first $c elements as they are expired.
+            $this->store = array_slice($this->store, $c, null, true);
+        }
+        return $c;
+    }
+
     /**
      * Finds all of the keys being stored in the cache store instance.
      *
      * @return array
      */
     public function find_all() {
+        $this->check_ttl();
         return array_keys($this->store);
     }