MDL-41292 cache: only update identifiers if they change

cache/classes/definition.php

@@ -256,7 +256,7 @@ class cache_definition {
      * An array of identifiers provided to this cache when it was initialised.
      * @var array
      */
-    protected $identifiers = array();
+    protected $identifiers = null;
 
     /**
      * Key prefix for use with single key cache stores
@@ -582,6 +582,9 @@ class cache_definition {
      * @return array
      */
     public function get_identifiers() {
+        if (!isset($this->identifiers)) {
+            return array();
+        }
         return $this->identifiers;
     }
 
@@ -694,11 +697,22 @@ class cache_definition {
      * @throws coding_exception
      */
     public function set_identifiers(array $identifiers = array()) {
+        // If we are setting the exact same identifiers then just return as nothing really changed.
+        // We don't care about order as cache::make will use the same definition order all the time.
+        if ($identifiers === $this->identifiers) {
+            return;
+        }
+
         foreach ($this->requireidentifiers as $identifier) {
             if (!array_key_exists($identifier, $identifiers)) {
                 throw new coding_exception('Identifier required for cache has not been provided: '.$identifier);
             }
         }
+
+        if ($this->identifiers === null) {
+            // Initialize identifiers if they have not been.
+            $this->identifiers = array();
+        }
         foreach ($identifiers as $name => $value) {
             $this->identifiers[$name] = (string)$value;
         }
@@ -820,7 +834,7 @@ class cache_definition {
                 'area' => $this->area,
                 'siteidentifier' => $this->get_cache_identifier()
             );
-            if (!empty($this->identifiers)) {
+            if (isset($this->identifiers) && !empty($this->identifiers)) {
                 $identifiers = array();
                 foreach ($this->identifiers as $key => $value) {
                     $identifiers[] = htmlentities($key, ENT_QUOTES, 'UTF-8').'='.htmlentities($value, ENT_QUOTES, 'UTF-8');