Real-time collaboration: Compaction race condition in the default polling provider

[czarate]

There is a race condition in the post meta storage engine (WP_Sync_Post_Meta_Storage) for the default RTC polling provider.

When handling compaction updates, all updates for a room are deleted before being partially restored along with the compaction update. The race condition is between these two lines:

• ​https://github.com/WordPress/wordpress-develop/blob/8a82e67cf65766fcb8a11e3fe5c6e2f48083fcdb/src/wp-includes/collaboration/class-wp-sync-post-meta-storage.php#L305
• ​https://github.com/WordPress/wordpress-develop/blob/8a82e67cf65766fcb8a11e3fe5c6e2f48083fcdb/src/wp-includes/collaboration/class-wp-sync-post-meta-storage.php#L308

In other words, while Client A is performing compaction, Client B could contribute an update that gets lost because it is not in Client A's in-memory copy of the update set.

Instead of using WordPress functions, we can use post meta meta_ids as cursors and conduct direct DELETE queries.

Incidentally, moving to meta_id cursors also fixes a bug with 32-bit PHP. The current timestamp-based cursors exceed 32-bit integer space.

Props to @mindctrl and @JoeFusco for spotting the race condition and exploring fixes.

Fix race condition in RTC default polling provider. This solution is pulled from @mindctrl's PR ​https://github.com/WordPress/wordpress-develop/pull/11067

This PR is targeted as narrowly as possible at the solving the race condition and does not attempt to solve the cache invalidation issues. If ​https://github.com/WordPress/wordpress-develop/pull/11290 is merged, then we can consider it solved separately.

This race condition is also solved by @josephfusco's PR ​https://github.com/WordPress/wordpress-develop/pull/11256 to replace post meta storage with a dedicated table. If that PR is merged, this one can be closed.

[ellatrix]

In 62064:

Real-time collaboration: fix race condition in default polling provider.

See also: ​https://github.com/WordPress/wordpress-develop/pull/11067.
Developed in: ​https://github.com/WordPress/wordpress-develop/pull/11292.

Fixes #64887.
Props czarate, westonruter, mindctrl, peterwilsoncc, joefusco.

Closing since this change has been committed.