[Issue] Don't assume a frontend cache section exists in the env.php file.

[m2-assistant]

This issue is automatically created based on existing pull request: #38119: Don't assume a frontend cache section exists in the env.php file.

---

Description (*)

Code exists in Magento that assumes your app/etc/env.php will always contain a frontend section if there is a cache section. Like this:

    'cache' => [
        'frontend' => [
            ...
        ]
     ]

But that's not always the case. All shops that were setup before Magento 2.3.1 didn't have a cache section in their env.php file in a default installation (if they didn't use Redis as caching).
If they now upgrade to Magento 2.4.4 or higher and use a graphql endpoint, Magento automatically writes cache > graphql > id_salt to the env.php file and so introduces a cache section without a frontend section.

This then leads to this crash on every execution within Magento (frontend, backoffice, bin/magento, ...):

Warning: Undefined array key "frontend" in lib/internal/Magento/Framework/App/Cache/Frontend/Pool.php on line 90

This PR fixes this.

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes [2.4.4] Missing Frontend CacheInfo breaks bin/magento #35812
2. Fixes graphql id_salt causing 500 server errors #35861

Manual testing scenarios (*)

1. Fully setup a working Magento
2. Edit the app/etc/env.php file and remove the entire frontend section in the cache section
3. Run bin/magento or visit the frontend
4. Expected is to not see an error/crash
5. Run added unit test with and without fix. Without the fix, the test will fail (vendor/bin/phpunit -c dev/tests/unit/phpunit.xml.dist lib/internal/Magento/Framework/App/Test/Unit/Cache/Frontend/PoolTest.php)

More elaborate testing scenario that can cause this situation:

1. Using PHP 7.2 and composer v1, setup Magento 2.3.0 and install it fully (don't configure it with Redis caching)
2. Double check that the app/etc/env.php file contains no cache section
3. Using PHP 8.2 and composer v2, upgrade this installation to Magento 2.4.6-p3 (or 2.4-develop)
4. Run bin/magento setup:upgrade
5. Your app/etc/env.php file will be updated to a new format, but will still not contain a cache section.
6. Confirm that Magento works correctly
7. On the frontend visit the /graphql url
8. Check your app/etc/env.php file and see that a cache section has appeared with a graphql section but no frontend section
9. Visit the homepage or run bin/magento
10. See errors/crashes

Questions or comments

Should we expect that when we run bin/magento setup:upgrade it will update your app/etc/env.php file with all missing sections? Would people expect this? Or will this cause issues in some scenario's? Running bin:magento setup:config:set without extra flags does exactly that. That seems to be some kind of hidden feature ...

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

[github-jira-sync-bot]

✅ Jira issue https://jira.corp.adobe.com/browse/AC-10865 is successfully created for this GitHub issue.

[m2-assistant]

✅ Confirmed by @engcom-Hotel. Thank you for verifying the issue.
Issue Available: @engcom-Hotel, You will be automatically unassigned. Contributors/Maintainers can claim this issue to continue. To reclaim and continue work, reassign the ticket to yourself.