RTC: Backport race condition fix

backport-changelog/7.0/11292.md

@@ -0,0 +1,3 @@
+https://github.com/WordPress/wordpress-develop/pull/11292
+
+* https://github.com/WordPress/gutenberg/pull/76649

lib/compat/wordpress-7.0/class-wp-sync-post-meta-storage.php

@@ -81,48 +81,9 @@ public function add_update( string $room, $update ): bool {
 				return false;
 			}
 
-			// Create an envelope and stamp each update to enable cursor-based filtering.
-			$envelope = array(
-				'timestamp' => $this->get_time_marker(),
-				'value'     => $update,
-			);
+			$meta_id = add_post_meta( $post_id, self::SYNC_UPDATE_META_KEY, $update, false );
 
-			return (bool) add_post_meta( $post_id, self::SYNC_UPDATE_META_KEY, $envelope, false );
-		}
-
-		/**
-		 * Retrieves all sync updates for a given room.
-		 *
-		 * @since 7.0.0
-		 *
-		 * @param string $room Room identifier.
-		 * @return array<int, array{ timestamp: int, value: mixed }> Sync updates.
-		 */
-		private function get_all_updates( string $room ): array {
-			$this->room_cursors[ $room ] = $this->get_time_marker() - 100; // Small buffer to ensure consistency.
-
-			$post_id = $this->get_storage_post_id( $room );
-			if ( null === $post_id ) {
-				return array();
-			}
-
-			$updates = get_post_meta( $post_id, self::SYNC_UPDATE_META_KEY, false );
-
-			if ( ! is_array( $updates ) ) {
-				$updates = array();
-			}
-
-			// Filter out any updates that don't have the expected structure.
-			$updates = array_filter(
-				$updates,
-				static function ( $update ): bool {
-					return is_array( $update ) && isset( $update['timestamp'], $update['value'] ) && is_int( $update['timestamp'] );
-				}
-			);
-
-			$this->room_update_counts[ $room ] = count( $updates );
-
-			return $updates;
+			return (bool) $meta_id;
 		}
 
 		/**
@@ -172,8 +133,7 @@ public function set_awareness_state( string $room, array $awareness ): bool {
 		 * Gets the current cursor for a given room.
 		 *
 		 * The cursor is set during get_updates_after_cursor() and represents the
-		 * point in time just before the updates were retrieved, with a small buffer
-		 * to ensure consistency.
+		 * highest meta_id seen for the room's sync updates.
 		 *
 		 * @since 7.0.0
 		 *
@@ -243,17 +203,6 @@ private function get_storage_post_id( string $room ): ?int {
 			return null;
 		}
 
-		/**
-		 * Gets the current time in milliseconds as a comparable time marker.
-		 *
-		 * @since 7.0.0
-		 *
-		 * @return int Current time in milliseconds.
-		 */
-		private function get_time_marker(): int {
-			return (int) floor( microtime( true ) * 1000 );
-		}
-
 		/**
 		 * Gets the number of updates stored for a given room.
 		 *
@@ -267,32 +216,63 @@ public function get_update_count( string $room ): int {
 		}
 
 		/**
-		 * Retrieves sync updates from a room for a given client and cursor. Updates
-		 * from the specified client should be excluded.
+		 * Retrieves sync updates from a room after the given cursor.
 		 *
 		 * @since 7.0.0
 		 *
 		 * @param string $room   Room identifier.
-		 * @param int    $cursor Return updates after this cursor.
+		 * @param int    $cursor Return updates after this cursor (meta_id).
 		 * @return array<int, mixed> Sync updates.
 		 */
 		public function get_updates_after_cursor( string $room, int $cursor ): array {
-			$all_updates = $this->get_all_updates( $room );
-			$updates     = array();
+			global $wpdb;
+
+			$post_id = $this->get_storage_post_id( $room );
+			if ( null === $post_id ) {
+				$this->room_cursors[ $room ]       = 0;
+				$this->room_update_counts[ $room ] = 0;
+				return array();
+			}
 
-			foreach ( $all_updates as $update ) {
-				if ( $update['timestamp'] > $cursor ) {
-					$updates[] = $update;
-				}
+			// Capture the current room state first so the returned cursor is race-safe.
+			$stats = $wpdb->get_row(
+				$wpdb->prepare(
+					"SELECT COUNT(*) AS total_updates, COALESCE( MAX(meta_id), 0 ) AS max_meta_id FROM {$wpdb->postmeta} WHERE post_id = %d AND meta_key = %s",
+					$post_id,
+					self::SYNC_UPDATE_META_KEY
+				)
+			);
+
+			$total_updates = $stats ? (int) $stats->total_updates : 0;
+			$max_meta_id   = $stats ? (int) $stats->max_meta_id : 0;
+
+			$this->room_update_counts[ $room ] = $total_updates;
+			$this->room_cursors[ $room ]       = $max_meta_id;
+
+			if ( $max_meta_id <= $cursor ) {
+				return array();
 			}
 
-			// Sort by timestamp to ensure order.
-			usort(
-				$updates,
-				fn ( $a, $b ) => $a['timestamp'] <=> $b['timestamp']
+			$rows = $wpdb->get_results(
+				$wpdb->prepare(
+					"SELECT meta_value FROM {$wpdb->postmeta} WHERE post_id = %d AND meta_key = %s AND meta_id > %d AND meta_id <= %d ORDER BY meta_id ASC",
+					$post_id,
+					self::SYNC_UPDATE_META_KEY,
+					$cursor,
+					$max_meta_id
+				)
 			);
 
-			return wp_list_pluck( $updates, 'value' );
+			if ( ! $rows ) {
+				return array();
+			}
+
+			$updates = array();
+			foreach ( $rows as $row ) {
+				$updates[] = maybe_unserialize( $row->meta_value );
+			}
+
+			return $updates;
 		}
 
 		/**
@@ -301,31 +281,31 @@ public function get_updates_after_cursor( string $room, int $cursor ): array {
 		 * @since 7.0.0
 		 *
 		 * @param string $room   Room identifier.
-		 * @param int    $cursor Remove updates with markers < this cursor.
+		 * @param int    $cursor Remove updates with meta_id < this cursor.
 		 * @return bool True on success, false on failure.
 		 */
 		public function remove_updates_before_cursor( string $room, int $cursor ): bool {
+			global $wpdb;
+
 			$post_id = $this->get_storage_post_id( $room );
 			if ( null === $post_id ) {
 				return false;
 			}
 
-			$all_updates = $this->get_all_updates( $room );
+			$deleted_rows = $wpdb->query(
+				$wpdb->prepare(
+					"DELETE FROM {$wpdb->postmeta} WHERE post_id = %d AND meta_key = %s AND meta_id < %d",
+					$post_id,
+					self::SYNC_UPDATE_META_KEY,
+					$cursor
+				)
+			);
 
-			// Remove all updates for the room and re-store only those that are newer than the cursor.
-			if ( ! delete_post_meta( $post_id, self::SYNC_UPDATE_META_KEY ) ) {
+			if ( false === $deleted_rows ) {
 				return false;
 			}
 
-			// Re-store envelopes directly to avoid double-wrapping by add_update().
-			$add_result = true;
-			foreach ( $all_updates as $envelope ) {
-				if ( $add_result && $envelope['timestamp'] >= $cursor ) {
-					$add_result = (bool) add_post_meta( $post_id, self::SYNC_UPDATE_META_KEY, $envelope, false );
-				}
-			}
-
-			return $add_result;
+			return true;
 		}
 	}
 }