MDL-36322 cache: implemented cache_store::create_clone

cache/classes/factory.php

@@ -263,7 +263,13 @@ class cache_factory {
         if (!$store->is_ready() || !$store->is_supported_mode($definition->get_mode())) {
             return false;
         }
-        $store = clone($this->stores[$name]);
+        // We always create a clone of the original store.
+        // If we were to clone a store that had already been initialised with a definition then
+        // we'd run into a myriad of issues.
+        // We use a method of the store to create a clone rather than just creating it ourselves
+        // so that if any store out there doesn't handle cloning they can override this method in
+        // order to address the issues.
+        $store = $this->stores[$name]->create_clone($details);
         $store->initialise($definition);
         return $store;
     }

cache/classes/store.php

@@ -306,4 +306,22 @@ abstract class cache_store implements cache_store_interface {
     public function supports_native_ttl() {
         return $this::get_supported_features() & self::SUPPORTS_NATIVE_TTL;
     }
+
+    /**
+     * Creates a clone of this store instance ready to be initialised.
+     *
+     * This method is used so that a cache store needs only be constructed once.
+     * Future requests for an instance of the store will be given a cloned instance.
+     *
+     * If you are writing a cache store that isn't compatible with the clone operation
+     * you can override this method to handle any situations you want before cloning.
+     *
+     * @param array $details An array containing the details of the store from the cache config.
+     * @return cache_store
+     */
+    public function create_clone(array $details = array()) {
+        // By default we just run clone.
+        // Any stores that have an issue with this will need to override the create_clone method.
+        return clone($this);
+    }
 }