[GraphQL] Wrong tags are set for CategoryList query

[magento-engcom-team]

We have an integration test that verifies pretty strange behavior: "category tag is present in X-Magento-Tags for current Query if it was present in the previous query"

Steps to reproduce

1. Run only 1 test-case "testRequestCacheTagsForCategoryListOnMultipleIds":
   Magento\GraphQlCache\Controller\Catalog\CategoryListCacheTest::testRequestCacheTagsForCategoryListOnMultipleIds

Magento\GraphQlCache\Controller\Catalog\CategoryListCacheTest::testRequestCacheTagsForCategoryListOnMultipleIds
Failed asserting that two arrays are equal.
--- Expected
<ins></ins><ins> Actual
@@ @@
 Array (
     0 => ‘cat_c’
-    1 => ‘cat*c*333’
-    2 => ‘cat*c*400’
-    3 => ‘cat*c*401’
-    4 => ‘FPC’
</ins>    1 => ‘cat*c*400’
<ins>    2 => ‘cat*c*401’
</ins>    3 => ‘FPC’
 )
/magento/dev/tests/integration/testsuite/Magento/GraphQlCache/Controller/Catalog/CategoryListCacheTest.php:108
FAILURES!
Tests: 1, Assertions: 2, Failures: 1.
{code}

2. Run all tests-cases Magento\GraphQlCache\Controller\Catalog\CategoryListCacheTest:
{code}
testsuite/Magento/GraphQlCache/Controller/Catalog/CategoryListCacheTest.php
PHPUnit 9.1.5 by Sebastian Bergmann and contributors.
Warning:       Using a custom test suite loader is deprecated
....                                                                                                                           4 / 4 (100%)
Time: 01:02.900, Memory: 98.00 MB
OK (4 tests, 12 assertions)

Expected behaviour: tag "333" form First request should not be "remembered" from one request to another and present in response for Second request

Note:

• possible this is issue in integration framework, so need to check this behavior without tests
• possible this issue is present in Luma as well (not only for GQL) and it should be escalated with their priority

[magento-engcom-team]

The issue was exported from the internal JIRA. The link to the original JIRA issue: https://jira.corp.magento.com/browse/MC-35798

[m2-assistant]

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[m2-assistant]

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[pmarjan]

From Description
The way that this issue is described in the first place

• run only one test (testRequestCacheTagsForCategoryListOnMultipleIds) from the test class
• run the full class

and the $actualCacheTags result are different for single test case run and the full class, suggests that the tests are NOT independent of each other.

Production behavior
I checked the behavior without tests, and on Luma too, and as far as I can tell there is no production code issue.
The problem is strictly in the tests.

Independence Problem explanation
The class that accumulates cache tags is Magento\GraphQlCache\Model\CacheableQuery.
This is instantiated in Magento\GraphQlCache\Model\CacheableQueryHandler, which on its own is instantiated in Magento\GraphQlCache\Model\Plugin\Query\Resolver.

These three objects are singletons, as the should be, as it is expected that tags are accumulated for different entities.
New Resolver instance, and for that matter CacheableQueryHandler and CacheableQuery instanes, are created only and only for new payloads.
Therefore if two test functions, have the same payload, for the first, yes new Resolver will be constructed, but for the second no, the same Resolver instance is used. And how CacheableQuery::addCacheTags() method is defined, it will combine tags from all previous test functions.

CategoryListCacheTest uses the same payload 4 times in a row - only once the Resolver is instantiated for the first test function testRequestCacheTagsForCategoryList.
For comparison, ProductsCacheTest uses the same payload in methods testRequestCacheTagsForProducts and testConsecutiveRequestsAreServedFromThePageCache which results in missed instantiation of the Resolver for testConsecutiveRequestsAreServedFromThePageCache.

Take away
Any test class in the GraphQlCache domain, that extends AbstractGraphqlCacheTest will have to be careful NOT to invoke same payload in different test functions as this will break test Independence principle. One phpunit call is one application request and the business implementation of singletons simply leads to this undesired consequence.