MDL-48506 cachestore_memcached: added upgrade notice

cache/stores/memcached/addinstanceform.php

@@ -86,6 +86,13 @@ protected function configuration_definition() {
         $form->addHelpButton('setservers', 'setservers', 'cachestore_memcached');
         $form->disabledIf('setservers', 'clustered');
         $form->setType('setservers', PARAM_RAW);
+
+        $version = phpversion('memcached');
+        if (!$version || !version_compare($version, cachestore_memcached::REQUIRED_VERSION, '>=')) {
+            $form->addElement('header', 'upgradenotice', get_string('notice', 'cachestore_memcached'));
+            $form->setExpanded('upgradenotice');
+            $form->addElement('html', nl2br(get_string('upgrade200recommended', 'cachestore_memcached')));
+        }
     }
 
     /**

cache/stores/memcached/lang/en/cachestore_memcached.php

@@ -46,6 +46,7 @@
 $string['hash_fnv1a_32'] = 'FNV1A_32';
 $string['hash_hsieh'] = 'Hsieh';
 $string['hash_murmur'] = 'Murmur';
+$string['notice'] = 'Notice';
 $string['pluginname'] = 'Memcached';
 $string['prefix'] = 'Prefix key';
 $string['prefix_help'] = 'This can be used to create a "domain" for your item keys allowing you to create multiple memcached stores on a single memcached installation. It cannot be longer than 16 characters in order to ensure key length issues are not encountered.';
@@ -88,3 +89,5 @@
 $string['useserialiser_help'] = 'Specifies the serializer to use for serializing non-scalar values.
 The valid serializers are Memcached::SERIALIZER_PHP or Memcached::SERIALIZER_IGBINARY.
 The latter is supported only when memcached is configured with --enable-memcached-igbinary option and the igbinary extension is loaded.';
+$string['upgrade200recommended'] = 'We recommend you upgrade your Memcached PHP extension to version 2.0.0 or greater.
+The version of the Memcached PHP extension you are currently using does not provide the functionality Moodle uses to ensure a sandboxed cache. Until you upgrade we recommend you do not configure any other applications to use the same Memcached servers as Moodle is configured to use.';

cache/upgrade.txt

@@ -6,6 +6,8 @@ Information provided here is intended especially for developers.
   This allows the cache loader to decide if it needs to handle dereferencing or whether the data
   coming directly to it has already had references resolved.
   - see supports_dereferencing_objects in store.php.
+* The Memcached cache store no longer flushes the memcache servers it is connected to providing your memcached php extension is version 2.0.0 or greater.
+  There is a notice to this effect when adding or editing a memcached cache store instance.
 
 === 2.9 ===
 * Cache data source aggregation functionality has been removed. This functionality was found to be broken and unused.