AC-14999: FPC not work when login by @rogerdz

app/code/Magento/PageCache/Model/App/Request/Http/IdentifierForSave.php

@@ -49,7 +49,8 @@ public function getValue()
         $data = [
             $this->request->isSecure(),
             preg_replace($pattern, $replace, (string)$this->request->getUriString()),
-            $this->context->getVaryString()
+            $this->request->get(\Magento\Framework\App\Response\Http::COOKIE_VARY_STRING)
+                ?: $this->context->getVaryString()
         ];
 
         $data = $this->identifierStoreReader->getPageTagsWithStoreCacheTags($data);

dev/tests/integration/testsuite/Magento/PageCache/Model/App/Request/Http/IdentifierForSaveTest.php

@@ -0,0 +1,130 @@
+<?php
+/**
+ * Copyright 2025 Adobe
+ * All rights reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\PageCache\Model\App\Request\Http;
+
+use Magento\Framework\ObjectManagerInterface;
+use Magento\TestFramework\Fixture\Config as ConfigFixture;
+use Magento\TestFramework\Fixture\DataFixture;
+use Magento\TestFramework\Fixture\DataFixtureStorage;
+use Magento\TestFramework\Fixture\DataFixtureStorageManager;
+use Magento\TestFramework\Helper\Bootstrap;
+use Magento\Customer\Model\Session;
+use Magento\Framework\App\Http\Context;
+use Magento\Framework\Stdlib\CookieManagerInterface;
+use Magento\Framework\Stdlib\Cookie\CookieMetadataFactory;
+use Magento\Customer\Test\Fixture\Customer as CustomerFixture;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Integration test for \Magento\PageCache\Model\App\Request\Http\IdentifierForSave
+ *
+ * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
+ */
+class IdentifierForSaveTest extends TestCase
+{
+    /**
+     * @var ObjectManagerInterface
+     */
+    private $objectManager;
+
+    /**
+     * @var IdentifierForSave
+     */
+    private $identifierForSave;
+
+    /**
+     * @var DataFixtureStorage
+     */
+    private $fixtures;
+
+    /**
+     * @var Context
+     */
+    private $context;
+
+    /**
+     * @var CookieManagerInterface
+     */
+    private $cookieManager;
+
+    /**
+     * @var CookieMetadataFactory
+     */
+    private $cookieMetadataFactory;
+
+    /**
+     * @var string
+     */
+    private const COOKIE_VARY_STRING = 'X-Magento-Vary';
+
+    protected function setUp(): void
+    {
+        $this->objectManager = Bootstrap::getObjectManager();
+        $this->identifierForSave = $this->objectManager->get(IdentifierForSave::class);
+        $this->fixtures = $this->objectManager->get(DataFixtureStorageManager::class)->getStorage();
+        $this->context = $this->objectManager->get(Context::class);
+        $this->cookieManager = $this->objectManager->get(CookieManagerInterface::class);
+        $this->cookieMetadataFactory = $this->objectManager->get(CookieMetadataFactory::class);
+    }
+
+    /**
+     * Test that cache identifier properly handles logged-in customers
+     */
+    #[
+        ConfigFixture('system/full_page_cache/caching_application', '1', 'store'),
+        ConfigFixture('system/full_page_cache/enabled', '1', 'store'),
+        DataFixture(CustomerFixture::class, as: 'customer')
+    ]
+    public function testAfterGetValueWithLoggedInCustomer()
+    {
+        // Get customer and login
+        $customer = $this->fixtures->get('customer');
+        $customerSession = $this->objectManager->get(Session::class);
+        $customerSession->loginById($customer->getId());
+
+        // Get cache identifiers
+        $result = $this->identifierForSave->getValue();
+
+        // Verify that both cache keys are not empty and contain customer context
+        $this->assertNotEmpty($result, 'Cache identifier for save should not be empty for logged-in user');
+
+        // Test scenario: Simulate context vary string being empty but cookie vary string present
+        // Get the current vary string from context
+        $originalVaryString = $this->context->getVaryString();
+        $this->assertNotEmpty($originalVaryString, 'Context vary string should not be empty for logged-in user');
+
+        // Set the vary cookie to simulate a previous request
+        $cookieMetadata = $this->cookieMetadataFactory->createSensitiveCookieMetadata()->setPath('/');
+        $this->cookieManager->setSensitiveCookie(
+            self::COOKIE_VARY_STRING,
+            $originalVaryString,
+            $cookieMetadata
+        );
+
+        // Clear the context vary string to simulate depersonalization
+        $this->context->_resetState();
+
+        // Verify context vary string is now empty
+        $this->assertEmpty($this->context->getVaryString(), 'Context vary string should be empty after reset');
+
+        // Get cache identifiers again - should still work due to cookie fallback
+        $resultWithEmptyContext = $this->identifierForSave->getValue();
+
+        // Both should still generate valid cache keys due to cookie fallback
+        $this->assertNotEmpty(
+            $resultWithEmptyContext,
+            'Cache identifier for save should work with empty context due to cookie fallback'
+        );
+
+        // Both cache key should be same even after context vary string is empty because it use cookie vary string
+        $this->assertEquals($result, $resultWithEmptyContext);
+
+        // Clean up
+        $this->cookieManager->deleteCookie(self::COOKIE_VARY_STRING, $cookieMetadata);
+    }
+}