Feature Flags: key the wpcom override cache by blog

[louwie17]

Proposed changes

Alternative to #52555, for the same bug. Wpcom_Feature_Flags memoizes a site's flag overrides in a static property the first time any flag is resolved, and never re-reads them. On WordPress.com Simple, the public API process resolves flags before it switches to the requested site (premium-analytics-dashboard-composition is checked on init), so the empty map cached then answers for the requested site too. Overrides set on Tools → Feature Flags therefore apply in that site's wp-admin but not to its wpcom/v2 REST responses.

#52555 resets the cache on switch_blog. This PR keys it by blog instead:

• The cache is one slot plus the blog it was read for, checked against get_current_blog_id(). A resolution on a different blog re-reads that blog's option; a repeat on the same blog answers from the cache. Memory stays bounded — a CLI process that walks many blogs holds one map, not one per blog.
• No switch_blog hook, so the fix does not depend on that action firing, or on firing before another callback that resolves a flag. Re-reads match a reset-on-switch_blog whenever flags resolve inside the switched scope; the only saving is a switch_to_blog()/restore_current_blog() pair that resolves no flag, and after the first read that saving is an object-cache lookup, not a query.
• get_current_blog_id() is always 1 on single-site, so Atomic and self-hosted sites behave as before. get_overrides(), save_overrides() and reset_overrides_cache() keep their signatures.
• Tests: one visits blog 1, blog 2 and blog 1 again, each with its own override, never fires switch_blog, and asserts each blog's exact override map, so an empty map cannot pass (it fails against Feature Flags: re-read wpcom overrides after switch_to_blog() #52555's version). The WorDBless suite is single-site, so it moves get_current_blog_id() through $GLOBALS['blog_id'] and serves each blog's option through a pre_option_ filter instead of calling switch_to_blog(). A second test checks that repeat resolutions on one blog read the option once.

Behaviour change for reviewers: any process that resolves flags on more than one blog (the public API, cron, wp-cli walking sites) now gets each blog's own overrides, where it used to get the first blog's map for the whole process. That is the fix, but it reaches more than the REST path described here, while the changelog entry is patch/fixed.

Related product discussion/links

• Alternative to Feature Flags: re-read wpcom overrides after switch_to_blog() #52555; only one of the two should land.
• Follows Feature Flags: add an Automattician-only control screen on Simple and Atomic #51324, which added the screen.
• Unblocks Premium Analytics: let Automatticians see every section of a preview-scoped dashboard #52549 on WordPress.com Simple.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

Automated:

• jp test php packages/jetpack-mu-wpcom --filter Wpcom_Feature_Flags

On a WordPress.com Simple sandbox, with a site whose Premium Analytics preview is switched on through its own opt-in and no jetpack-premium-analytics override (Traffic and Insights only), and with #52549 applied:

1. Sandbox both the site's domain and public-api.wordpress.com.
2. As an Automattician, open Tools → Feature Flags (a8c), force premium-analytics-a11n-all-sections on, and save.
3. Open https://public-api.wordpress.com/wpcom/v2/sites/<blog-id>/dashboards/jetpack-premium-analytics_dashboard/sections.
4. On trunk, the response still lists only traffic and insights. With this PR, it lists every section the site qualifies for.
5. Set the flag back to Default. The response returns to the two preview tabs.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Ac4zq7xVjwnnmCNRV8rNeb

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (WordPress.com Site Helper), and enable the fix/wpcom-feature-flags-by-blog-cache branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/wpcom-feature-flags-by-blog-cache

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/wpcom-feature-flags/class-wpcom-feature-flags.php	217/228 (95.18%)	0.09%	0 💚

Full summary · PHP report · JS report

[dognose24]

Both threads are resolved in the follow-up commits: the docblock now argues from correctness rather than a saving that a single slot cannot deliver, and the blog-switching test pins each blog's exact map with blog 1 overriding to true, so an empty map fails every step. Merged onto trunk locally and ran the jetpack-mu-wpcom suite: 1225 tests pass.