Add __unstable_wp_sync_storage filter for pluggable storage backends

[josephfusco]

What

Adds __unstable_wp_sync_storage filter to allow custom storage backends for collaborative editing data.

Why

Current RTC stores sync data in post meta, causing site-wide cache invalidation on every edit (Trac #64696). Plugins can now provide alternative storage without forking Gutenberg.

Use case: proposed realtime-collaboration:

• Presence API for awareness (cursors, user metadata)
• Dedicated wp_collaboration table for CRDT updates
• Eliminates cache invalidation while maintaining Gutenberg's HTTP polling provider

How

/**
 * Filters the sync storage implementation for collaborative editing.
 *
 * Allows plugins to replace the default post meta storage with alternative
 * backends. The primary use case is the realtime-collaboration plugin,
 * which uses Presence API for awareness and a dedicated wp_collaboration
 * table for CRDT updates, eliminating cache side effects.
 *
 * @since Gutenberg 21.x
 *
 * @param WP_Sync_Storage $sync_storage Storage implementation. Must implement
 *                                      the WP_Sync_Storage interface.
 */
$sync_storage = apply_filters( '__unstable_wp_sync_storage', new WP_Sync_Post_Meta_Storage() );

[github-actions]

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Co-authored-by: josephfusco <joefusco@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: chriszarate <czarate@git.wordpress.org>

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[github-actions]

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[josephfusco]

The failing Playwright - 3 test is unrelated to this PR's one-line filter change:

Failing test: specs/editor/blocks/tabs.spec.js:133:3 › Tabs › Editor functionality › adds and activates a new tab when pressing Enter at the end of a tab label

This test has a known flaky history (see issue #80106, closed Aug 6). The PR only adds apply_filters( 'gutenberg_sync_storage', new WP_Sync_Post_Meta_Storage() ) and doesn't touch any UI code or tabs block functionality.

Could a maintainer re-run the Playwright - 3 job?

[Mamaduka]

Tabs failure will be resolved via #81696. If that's only failing check it's not a blocker for merge.

[josephfusco]

@Mamaduka @chriszarate Thanks for taking a look at this. This unblocks Gutenberg presence integration for realtime-collaboration feature plugin development.

Happy to answer any questions or concerns.

[chriszarate]

My only concern here is that RTC is headed back to experiment status while we explore some fundamental changes to how syncing works. The current interface is flexible but it assumes a pure naïve relay, and this could change. This filter should probably communicate its unstable nature in name (__unstable_wp_sync_storage?) and description.

[josephfusco]

Good call, added the unstable prefix.

This filter is the integration point for the sync storage exploration. Once a solution is validated, it could replace the current post-meta default.

[chriszarate]

This is ok with me, but adding a filter (even an experimental one) probably deserves another set of eyes / approval.

[Mamaduka]

As long as it doesn't end up in Core by accident as __unstable_, I think it's good to merge. The APIs in lib/experiments don't need to adhere to the backward compatibility policy.