Address possible metadata duplication in 8.1 (backport)

[jorgeatorres]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

This PR applies a subset of the changes from #40129, which -among other things-, prevent duplication of metadata when using the save_meta_data() order method, which is common with 3rd party code (including popular plugins such as WC Subscriptions).

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Make sure both HPOS and sync are enabled in WooCommerce > Settings > Advanced > Features.

2. Run the following PHP snippet via WP-CLI (for example):

   $order = wc_create_order();
   $order->add_meta_data( 'this_gets_duplicated', 'for reals' );
   $order->save_meta_data();
   
   echo $order->get_id();

   Take note of the ID that gets printed to the screen.

3. Make sure that metadata this_gets_duplicated appears only once in the output of the following WP-CLI commands:

   1. wp db query "SELECT * FROM $(wp db prefix)wc_orders_meta WHERE order_id = ORDER_ID"
   2. wp post meta list ORDER_ID

   Replace ORDER_ID with the ID from step 2.

In 8.1 (without this fix), you'll see that steps 3 shows you the metadata duplication.

Changelog entry

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Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Comment

[github-actions]

Test Results Summary

Commit SHA: 57d08d7

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 52s
E2E Tests	195	0	0	15	0	210	19m 35s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[barryhughes]

I'm not sure it's essential, but should we do anything if this returns false? Atm I don't believe we do anything with $changes_applied.

[vedanshujain]

yeah, this should be included, adding a small commit for this.

[barryhughes]

I think we're good, so provisionally approving (did leave one question, though).

[vedanshujain]

Seems like PHP 7.4 tests with nightly are failing because of something in the build process. I'd recommend not fixing them here as this PR targets the release branch.

[peterfabian]

LGTM, cross-checked with #40129.