Real-time collaboration: Prevent fall-through in sync polling server

src/wp-includes/collaboration/class-wp-http-polling-sync-server.php

@@ -402,7 +402,10 @@ private function process_sync_update( string $room, int $client_id, int $cursor,
 
 					return $this->add_update( $room, $client_id, $type, $data );
 				}
-				break;
+
+				// Reaching this point means there's a newer compaction, so we can
+				// silently ignore this one.
+				return true;
 
 			case self::UPDATE_TYPE_SYNC_STEP1:
 			case self::UPDATE_TYPE_SYNC_STEP2:

tests/phpunit/tests/rest-api/rest-sync-server.php

@@ -623,6 +623,48 @@ public function test_sync_should_compact_is_false_for_non_compactor() {
 		$this->assertFalse( $data['rooms'][0]['should_compact'] );
 	}
 
+	public function test_sync_stale_compaction_succeeds_when_newer_compaction_exists() {
+		wp_set_current_user( self::$editor_id );
+
+		$room   = $this->get_post_room();
+		$update = array(
+			'type' => 'update',
+			'data' => 'dGVzdA==',
+		);
+
+		// Client 1 sends an update to seed the room.
+		$response = $this->dispatch_sync(
+			array(
+				$this->build_room( $room, 1, 0, array( 'user' => 'c1' ), array( $update ) ),
+			)
+		);
+
+		$end_cursor = $response->get_data()['rooms'][0]['end_cursor'];
+
+		// Client 2 sends a compaction at the current cursor.
+		$compaction = array(
+			'type' => 'compaction',
+			'data' => 'Y29tcGFjdGVk',
+		);
+
+		$this->dispatch_sync(
+			array(
+				$this->build_room( $room, 2, $end_cursor, array( 'user' => 'c2' ), array( $compaction ) ),
+			)
+		);
+
+		// Client 3 sends a stale compaction at cursor 0. The server should find
+		// client 2's compaction in the updates after cursor 0 and silently discard
+		// this one.
+		$response = $this->dispatch_sync(
+			array(
+				$this->build_room( $room, 3, 0, array( 'user' => 'c3' ), array( $compaction ) ),
+			)
+		);
+
+		$this->assertSame( 200, $response->get_status() );
+	}
+
 	/*
 	 * Awareness tests.
 	 */