MAGETWO-44681: page cache entries not tagged with "FPC" cache type

 - use plugin to add cache tag

Author: Dale Sikkema

app/code/Magento/PageCache/Model/App/PageCachePlugin.php

@@ -0,0 +1,31 @@
+<?php
+/***
+ * Copyright © 2015 Magento. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\PageCache\Model\App;
+
+class PageCachePlugin
+{
+    /**
+     * Attach FPC tag to all saved entries to enable cache type management
+     *
+     * @param \Magento\Framework\App\PageCache\Cache $subject
+     * @param $data
+     * @param $identifier
+     * @param array $tags
+     * @param null $lifeTime
+     * @return array
+     */
+    public function beforeSave(
+        \Magento\Framework\App\PageCache\Cache $subject,
+        $data,
+        $identifier,
+        $tags = [],
+        $lifeTime = null
+    ) {
+        $tags[] = \Magento\PageCache\Model\Cache\Type::CACHE_TAG;
+        return [$data, $identifier, $tags, $lifeTime];
+    }
+}

app/code/Magento/PageCache/Test/Unit/Model/App/PageCachePluginTest.php

@@ -0,0 +1,28 @@
+<?php
+/***
+ * Copyright © 2015 Magento. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
+use Magento\PageCache\Model\Cache\Type;
+
+class PageCachePluginTest extends \PHPUnit_Framework_TestCase
+{
+    public function testBeforeSave()
+    {
+        /** @var \Magento\PageCache\Model\App\PageCachePlugin $plugin */
+        $plugin = (new ObjectManager($this))->getObject('\Magento\PageCache\Model\App\PageCachePlugin');
+        $subjectMock = $this->getMockBuilder('\Magento\Framework\App\PageCache\Cache')
+            ->disableOriginalConstructor()
+            ->getMock();
+        $initTags = ['tag', 'otherTag'];
+        $result = $plugin->beforeSave($subjectMock, 'data', 'identifier', $initTags);
+        $tags = isset($result[2]) ? $result[2] : null;
+        $expectedTags = array_merge($initTags, [Type::CACHE_TAG]);
+        $this->assertNotNull($tags);
+        foreach ($expectedTags as $expected) {
+            $this->assertContains($expected, $tags);
+        }
+    }
+}

app/code/Magento/PageCache/etc/di.xml

@@ -10,4 +10,7 @@
         <plugin name="core-app-area-design-exception-plugin"
                 type="Magento\PageCache\Model\App\CacheIdentifierPlugin" sortOrder="10"/>
     </type>
+    <type name="Magento\Framework\App\PageCache\Cache">
+        <plugin name="fpc-tag-addition-plugin" type="Magento\PageCache\Model\App\PageCachePlugin"/>
+    </type>
 </config>