Don't disable FPC for maintenance, instead send "no cache" headers

app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php

@@ -20,22 +20,31 @@ class LayoutPlugin
      */
     protected $response;
 
+    /**
+     * @var \Magento\Framework\App\MaintenanceMode
+     */
+    private $maintenanceMode;
+
     /**
      * Constructor
      *
      * @param \Magento\Framework\App\ResponseInterface $response
-     * @param \Magento\PageCache\Model\Config $config
+     * @param \Magento\PageCache\Model\Config          $config
+     * @param \Magento\Framework\App\MaintenanceMode   $maintenanceMode
      */
     public function __construct(
         \Magento\Framework\App\ResponseInterface $response,
-        \Magento\PageCache\Model\Config $config
+        \Magento\PageCache\Model\Config $config,
+        \Magento\Framework\App\MaintenanceMode $maintenanceMode
     ) {
         $this->response = $response;
         $this->config = $config;
+        $this->maintenanceMode = $maintenanceMode;
     }
 
     /**
      * Set appropriate Cache-Control headers
+     *
      * We have to set public headers in order to tell Varnish and Builtin app that page should be cached
      *
      * @param \Magento\Framework\View\Layout $subject
@@ -44,7 +53,7 @@ public function __construct(
      */
     public function afterGenerateXml(\Magento\Framework\View\Layout $subject, $result)
     {
-        if ($subject->isCacheable() && $this->config->isEnabled()) {
+        if ($subject->isCacheable() && !$this->maintenanceMode->isOn() && $this->config->isEnabled()) {
             $this->response->setPublicHeaders($this->config->getTtl());
         }
         return $result;
@@ -68,6 +77,7 @@ public function afterGetOutput(\Magento\Framework\View\Layout $subject, $result)
                     if ($isVarnish && $isEsiBlock) {
                         continue;
                     }
+                    // phpcs:ignore
                     $tags = array_merge($tags, $block->getIdentities());
                 }
             }

app/code/Magento/PageCache/Observer/SwitchPageCacheOnMaintenance/PageCacheState.php

@@ -14,6 +14,8 @@
 
 /**
  * Page Cache state.
+ *
+ * @deprecated Originally used by now removed observer SwitchPageCacheOnMaintenance
  */
 class PageCacheState
 {

app/code/Magento/PageCache/Test/Unit/Model/Layout/LayoutPluginTest.php

@@ -27,6 +27,11 @@ class LayoutPluginTest extends \PHPUnit\Framework\TestCase
      */
     protected $configMock;
 
+    /**
+     * @var \Magento\Framework\App\MaintenanceMode|\PHPUnit\Framework\MockObject\MockObject
+     */
+    private $maintenanceModeMock;
+
     protected function setUp()
     {
         $this->layoutMock = $this->getMockForAbstractClass(
@@ -40,27 +45,33 @@ protected function setUp()
         );
         $this->responseMock = $this->createMock(\Magento\Framework\App\Response\Http::class);
         $this->configMock = $this->createMock(\Magento\PageCache\Model\Config::class);
+        $this->maintenanceModeMock = $this->createMock(\Magento\Framework\App\MaintenanceMode::class);
 
         $this->model = new \Magento\PageCache\Model\Layout\LayoutPlugin(
             $this->responseMock,
-            $this->configMock
+            $this->configMock,
+            $this->maintenanceModeMock
         );
     }
 
     /**
      * @param $cacheState
      * @param $layoutIsCacheable
+     * @param $maintenanceModeIsEnabled
+     *
      * @dataProvider afterGenerateXmlDataProvider
      */
-    public function testAfterGenerateXml($cacheState, $layoutIsCacheable)
+    public function testAfterGenerateXml($cacheState, $layoutIsCacheable, $maintenanceModeIsEnabled)
     {
         $maxAge = 180;
         $result = 'test';
 
         $this->layoutMock->expects($this->once())->method('isCacheable')->will($this->returnValue($layoutIsCacheable));
         $this->configMock->expects($this->any())->method('isEnabled')->will($this->returnValue($cacheState));
+        $this->maintenanceModeMock->expects($this->any())->method('isOn')
+            ->will($this->returnValue($maintenanceModeIsEnabled));
 
-        if ($layoutIsCacheable && $cacheState) {
+        if ($layoutIsCacheable && $cacheState && !$maintenanceModeIsEnabled) {
             $this->configMock->expects($this->once())->method('getTtl')->will($this->returnValue($maxAge));
             $this->responseMock->expects($this->once())->method('setPublicHeaders')->with($maxAge);
         } else {
@@ -76,10 +87,11 @@ public function testAfterGenerateXml($cacheState, $layoutIsCacheable)
     public function afterGenerateXmlDataProvider()
     {
         return [
-            'Full_cache state is true, Layout is cache-able' => [true, true],
-            'Full_cache state is true, Layout is not cache-able' => [true, false],
-            'Full_cache state is false, Layout is not cache-able' => [false, false],
-            'Full_cache state is false, Layout is cache-able' => [false, true]
+            'Full_cache state is true, Layout is cache-able' => [true, true, false],
+            'Full_cache state is true, Layout is not cache-able' => [true, false, false],
+            'Full_cache state is false, Layout is not cache-able' => [false, false, false],
+            'Full_cache state is false, Layout is cache-able' => [false, true, false],
+            'Full_cache state is true, Layout is cache-able, Maintenance mode is enabled' => [true, true, true],
         ];
     }