Replace post meta sync storage with dedicated sync_updates table

src/wp-admin/admin.php

@@ -113,6 +113,11 @@
 	wp_schedule_event( time(), 'daily', 'delete_expired_transients' );
 }
 
+// Schedule sync updates cleanup.
+if ( ! wp_next_scheduled( 'wp_delete_old_sync_updates' ) && ! wp_installing() ) {
+	wp_schedule_event( time(), 'daily', 'wp_delete_old_sync_updates' );
+}
+
 set_screen_options();
 
 $date_format = __( 'F j, Y' );

src/wp-admin/includes/schema.php

@@ -186,6 +186,14 @@ function wp_get_db_schema( $scope = 'all', $blog_id = null ) {
 	KEY post_parent (post_parent),
 	KEY post_author (post_author),
 	KEY type_status_author (post_type,post_status,post_author)
+) $charset_collate;
+CREATE TABLE $wpdb->sync_updates (
+	id bigint(20) unsigned NOT NULL auto_increment,
+	room_hash char(32) NOT NULL,
+	update_value longtext NOT NULL,
+	created_at datetime NOT NULL default '0000-00-00 00:00:00',
+	PRIMARY KEY  (id),
+	KEY room_hash (room_hash,id)
 ) $charset_collate;\n";
 
 	// Single site users table. The multisite flavor of the users table is handled below.

src/wp-admin/includes/upgrade.php

@@ -886,7 +886,7 @@ function upgrade_all() {
 		upgrade_682();
 	}
 
-	if ( $wp_current_db_version < 61644 ) {
+	if ( $wp_current_db_version < 61697 ) {
 		upgrade_700();
 	}
 

src/wp-includes/class-wpdb.php

@@ -299,6 +299,7 @@ class wpdb {
 		'term_relationships',
 		'termmeta',
 		'commentmeta',
+		'sync_updates',
 	);
 
 	/**
@@ -404,6 +405,15 @@ class wpdb {
 	 */
 	public $posts;
 
+	/**
+	 * WordPress Sync Updates table.
+	 *
+	 * @since 7.0.0
+	 *
+	 * @var string
+	 */
+	public $sync_updates;
+
 	/**
 	 * WordPress Terms table.
 	 *

src/wp-includes/collaboration.php

@@ -22,3 +22,34 @@ function wp_collaboration_inject_setting() {
 		);
 	}
 }
+
+/**
+ * Deletes sync updates older than 1 day from the wp_sync_updates table.
+ *
+ * Rows left behind by abandoned collaborative editing sessions are cleaned up
+ * to prevent unbounded table growth.
+ *
+ * @since 7.0.0
+ */
+function wp_delete_old_sync_updates() {
+	global $wpdb;
+
+	/**
+	 * Filters the lifetime, in seconds, of a sync update row.
+	 *
+	 * By default, the lifetime is 1 day. Once a row reaches that age, it will
+	 * automatically be deleted by a cron job.
+	 *
+	 * @since 7.0.0
+	 *
+	 * @param int $expiration The expiration age of a sync update row, in seconds.
+	 */
+	$expiration = apply_filters( 'wp_sync_updates_expiration', DAY_IN_SECONDS );
+
+	$wpdb->query(
+		$wpdb->prepare(
+			"DELETE FROM {$wpdb->sync_updates} WHERE created_at < %s",
+			gmdate( 'Y-m-d H:i:s', time() - $expiration )
+		)
+	);
+}