Backport compaction processing fix that can cause user YDocs to diverge

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

@@ -498,9 +498,14 @@ private function process_sync_update( string $room, int $client_id, int $cursor,
 					return $this->add_update( $room, $client_id, $type, $data );
 				}
 
-				// Reaching this point means there's a newer compaction, so we can
-				// silently ignore this one.
-				return true;
+				/*
+				 * 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:

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

@@ -936,7 +936,7 @@ 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() {
+	public function test_sync_stale_compaction_is_stored_as_update_when_newer_compaction_exists() {
 		wp_set_current_user( self::$editor_id );
 
 		$room   = $this->get_post_room();
@@ -966,9 +966,14 @@ public function test_sync_stale_compaction_succeeds_when_newer_compaction_exists
 			)
 		);
 
-		// 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.
+		/*
+		 * Client 3 sends a stale compaction at cursor 0 (mirroring two offline
+		 * clients that reconnect from the same baseline cursor). The server
+		 * cannot run remove_updates_before_cursor because client 2 has already
+		 * advanced the frontier, but the bytes must still be stored as a
+		 * regular update so client 3's operations can propagate to other
+		 * clients via Yjs state-as-update merging.
+		 */
 		$stale_compaction = array(
 			'type' => 'compaction',
 			'data' => 'c3RhbGU=',
@@ -981,16 +986,31 @@ public function test_sync_stale_compaction_succeeds_when_newer_compaction_exists
 
 		$this->assertSame( 200, $response->get_status() );
 
-		// Verify the newer compaction is preserved and the stale one was not stored.
-		$response    = $this->dispatch_sync(
+		/*
+		 * Verify the newer compaction is preserved AND the stale compaction's
+		 * bytes were persisted (now as type=update so subsequent compactions
+		 * don't trip the has_newer_compaction check).
+		 */
+		$response = $this->dispatch_sync(
 			array(
 				$this->build_room( $room, 4, 0, array( 'user' => 'c4' ) ),
 			)
 		);
-		$update_data = wp_list_pluck( $response->get_data()['rooms'][0]['updates'], 'data' );
+		$updates  = $response->get_data()['rooms'][0]['updates'];
 
+		$update_data = wp_list_pluck( $updates, 'data' );
 		$this->assertContains( 'Y29tcGFjdGVk', $update_data, 'The newer compaction should be preserved.' );
-		$this->assertNotContains( 'c3RhbGU=', $update_data, 'The stale compaction should not be stored.' );
+		$this->assertContains( 'c3RhbGU=', $update_data, 'The stale compaction bytes should be stored so client 3\'s operations propagate.' );
+
+		$stale_entry = null;
+		foreach ( $updates as $entry ) {
+			if ( 'c3RhbGU=' === $entry['data'] ) {
+				$stale_entry = $entry;
+				break;
+			}
+		}
+		$this->assertNotNull( $stale_entry, 'The stale compaction entry should be present in the room.' );
+		$this->assertSame( 'update', $stale_entry['type'], 'The stale compaction should be stored as type=update, not type=compaction.' );
 	}
 
 	/*