WP_HTTP_Polling_Sync_Server::process_sync_update( string $room, int $client_id, int $cursor,  $update ): true|WP_Error

Source

private function process_sync_update( string $room, int $client_id, int $cursor, array $update ) {
	$data = $update['data'];
	$type = $update['type'];

	switch ( $type ) {
		case self::UPDATE_TYPE_COMPACTION:
			/*
			 * Compaction replaces updates the client has already seen. Only remove
			 * updates with markers before the client's cursor to preserve updates
			 * that arrived since the client's last sync.
			 *
			 * Check for a newer compaction update first. If one exists, skip this
			 * compaction to avoid overwriting it.
			 */
			$updates_after_cursor = $this->storage->get_updates_after_cursor( $room, $cursor );
			$has_newer_compaction = false;

			foreach ( $updates_after_cursor as $existing ) {
				if ( self::UPDATE_TYPE_COMPACTION === $existing['type'] ) {
					$has_newer_compaction = true;
					break;
				}
			}

			if ( ! $has_newer_compaction ) {
				if ( ! $this->storage->remove_updates_before_cursor( $room, $cursor ) ) {
					return new WP_Error(
						'rest_sync_storage_error',
						__( 'Failed to remove updates during compaction.' ),
						array( 'status' => 500 )
					);
				}

				return $this->add_update( $room, $client_id, $type, $data );
			}

			/*
			 * A newer compaction already advanced the cursor, but we
			 * can not safely drop an update. The incoming bytes still encode
			 * operations other clients may not have seen, so store them as a
			 * regular update. Y.applyUpdateV2 merges state-as-update blobs
			 * idempotently, so overlap with the existing compaction is safe.
			 */
			return $this->add_update( $room, $client_id, self::UPDATE_TYPE_UPDATE, $data );

		case self::UPDATE_TYPE_SYNC_STEP1:
		case self::UPDATE_TYPE_SYNC_STEP2:
		case self::UPDATE_TYPE_UPDATE:
			/*
			 * Sync step 1 announces a client's state vector. Other clients need
			 * to see it so they can respond with sync_step2 containing missing
			 * updates. The cursor-based filtering prevents re-delivery.
			 *
			 * Sync step 2 contains updates for a specific client.
			 *
			 * All updates are stored persistently.
			 */
			return $this->add_update( $room, $client_id, $type, $data );
	}

	return new WP_Error(
		'rest_invalid_update_type',
		__( 'Invalid sync update type.' ),
		array( 'status' => 400 )
	);
}

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