[Backport] Fix layout xml and page layout caching issue on redis cluster under high load

[andrey-legayev]

Original Pull Request

#22766

Description (*)

Bugs which were fixed:

• $this->pageLayout was not checked after reading from cache, but was used as is
• two cache items were used in once place instead of one (performance impact)

Changes:

• replace 2 cache items by 1 - it should improve performance
• add "_MERGED" to cache key suffix to have compatibility with old cache keys during deployment of new version

Fixed Issues (if relevant)

1. Layout corruption with php-fpm (a Magento 1 problem returns) #6942 Layout corruption (Title is wrong, but issue is real)
2. Corrupt layout cache causes white page on customer account login #22976 Corrupt layout cache causes white page on customer account login

Manual testing scenarios (*)

N/A

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)
• All automated tests passed successfully (all builds on Travis CI are green)

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[magento-cicd2]

All committers have signed the CLA.

[orlangur]

LGTM, previous attempt to fix this issue was performed in #16428 (comment), this one looks better from perfomance perspective.

[andrey-legayev]

I've found two issues in previous fix for the issue:

• It tries to load two cache items even if 1st is not available. It does small impact on performance.
• Comparison if ($result && $pageLayoutResult) { ... } doesn't catch the case when $pageLayoutResult === "" (which is ok for Magento layouts) and it generates layouts again and again without using cached data. It had to be if ($result && $pageLayoutResult !== false) { ... }

[andrey-legayev]

Hi there!
I've submitted changes 2 days ago, but unit tests are still be pending in Travis.
Is it ok?

[magento-engcom-team]

@andrey-legayev thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[orlangur]

@andrey-legayev nope, it is not good but EngCom guys are aware of it and it should be fixed soon.

Sorry, just noticed, we need to apply changes against 2.3-develop first. Could you please prepare such a PR? This one will be on hold for now.

[andrey-legayev]

@orlangur I've created pull request for 2.3-develop: #22766

[andrey-legayev]

This fix has been already included into v2.2.9 by cherry-picking commit.
I'm closing pull request.

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