Core Data: Make persistent container thread-safe

[itsmeichigo]

Closes: #14533

Description

When checking an issue with screenshot testing [p1732623939631369-slack-C03L1NF1EA3], @hichamboushaba found that the way we instantiate persistentContainer is not thread-safe as we can create it when accessing both viewStorage and writerDerivedStorage, which are in different threads.

This PR updates CoreDataManager to be more thread-safe by making all lazy variables constants.

Steps to reproduce

It's not easy to reproduce the race condition of persistent container. The most straightforward way is to run the screenshot test and verify that it is consistently able to display the stats on the dashboard.

Additionally, please do some quick smoke tests on the critical flows: Dashboard, Orders, Products, and Payments to ensure everything works as expected. You can follow the testing plan in pe5sF9-3e2-p2 as a guidance.

Testing information

Tested on simulator iPhone 16 Pro iOS 18 and confirmed that:

• The screenshot test now works correctly and consistently.
• Smoked testing features following the testing plan pe5sF9-3e2-p2 and confirmed that everything works as before (payment features were skipped, would appreciate your help here @jaclync @staskus 🙏)

Screenshots

N/A

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr14534-4ba484c
	Version	21.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	4ba484c
	App Center Build	WooCommerce - Prototype Builds #11859

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[staskus]

Thank you for the improvement! I haven't found flaws in the code logic. As I understand, the only downside we could theoretically expect are longer load times for the app, since CoreDataManager will be loaded all at once together with the persistent container.

We can observe in the subsequent releases if there are any dramatic changes in average (50th percentile) and longest (90th percentile) times in Xcode Organizer.

• I reproduced the issue with WooScreenshots not loading the stats data, and I can confirm that these changes help avoid the issue
• I tried stress testing the solution with some concurrent local unit tests and they succeeded
• I also performed smoke testing for login, logout, dashboard, orders, products, payments.

I haven't reproduced any new issues.

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Do you remember why we had a lazy persistent container in the first place? Were there any scenarios that we tried to cover and now we need to make sure they continue working as expected?

[itsmeichigo]

Thanks @staskus for the thorough reviews!

Do you remember why we had a lazy persistent container in the first place? Were there any scenarios that we tried to cover and now we need to make sure they continue working as expected?

Honestly this code exists a few years before I joined, and I could not find any documentation about why the Core Data stack was built that way. My best guess is to optimize app start time as you pointed out.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ Error Domain=NSPOSIXErrorDomain Code=28 "No space left on device" `` View Issue

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