docs: protected final QueryEngine queryEngine() {
          if (queryEngine == null) {
              try {
                  engineInitLock.lock();
                  if (queryEngine == null) {
                      QueryEngineBuilder builder = null;
                      if (!repoConfig.getIndexProviders().isEmpty()) {
                          // There is at least one index provider ...
                          builder = IndexQueryEngine.builder();
                          logger.debug("Queries with indexes are enabled for the '{0}' repository. Executing queries may require scanning the repository contents when the query cannot use the defined indexes.",
                                       repoConfig.getName());
                      } else {
                          // There are no indexes ...
                          builder = ScanningQueryEngine.builder();
                          logger.debug("Queries with no indexes are enabled for the '{0}' repository. Executing queries will always scan the repository contents.",
                                       repoConfig.getName());
                      }
                      queryEngine = builder.using(repoConfig, indexManager, runningState.context()).build();
                  }
              } finally {
                  engineInitLock.unlock();
              }
          }
          return queryEngine;
      }