MDL-70233 cache: Disabled factory now gets writer from parent

cache/classes/factory.php

@@ -658,4 +658,36 @@ public static function get_administration_display_helper() : core_cache\administ
         }
         return self::$displayhelper;
     }
+
+    /**
+     * Gets the cache_config_writer to use when caching is disabled.
+     * This should only be called from cache_factory_disabled.
+     *
+     * @return cache_config_writer
+     */
+    public static function get_disabled_writer(): cache_config_writer {
+        global $CFG;
+
+        // Figure out if we are in a recursive loop using late static binding.
+        // This happens when get_disabled_writer is not overridden. We just want the default.
+        $loop = false;
+        if (!empty($CFG->alternative_cache_factory_class)) {
+            $loop = get_called_class() === $CFG->alternative_cache_factory_class;
+        }
+
+        if (!$loop && !empty($CFG->alternative_cache_factory_class)) {
+            // Get the class to use from the alternative factory.
+            $factoryinstance = new $CFG->alternative_cache_factory_class();
+            return $factoryinstance::get_disabled_writer();
+        } else {
+            // We got here from cache_factory_disabled.
+            // We should use the default writer here.
+            // Make sure we have a default config if needed.
+            if (!cache_config::config_file_exists()) {
+                cache_config_writer::create_default_configuration(true);
+            }
+
+            return new cache_config_writer();
+        }
+    }
 }

cache/disabledlib.php

@@ -313,13 +313,13 @@ public function create_config_instance($writer = false) {
             self::set_state(self::STATE_INITIALISING);
             if ($class === 'cache_config_disabled') {
                 $configuration = $class::create_default_configuration();
+                $this->configs[$class] = new $class;
             } else {
                 $configuration = false;
-                if (!cache_config::config_file_exists()) {
-                    cache_config_writer::create_default_configuration(true);
-                }
+                // If we need a writer, we should get the classname from the generic factory.
+                // This is so alternative classes can be used if a different writer is required.
+                $this->configs[$class] = parent::get_disabled_writer();
             }
-            $this->configs[$class] = new $class;
             $this->configs[$class]->load($configuration);
         }
         self::set_state(self::STATE_READY);