private void calculateRoots() {
    final ModuleManager moduleManager = ModuleManager.getInstance(myProject);
    // assertion for read access inside
    final Module[] modules =
        ApplicationManager.getApplication()
            .runReadAction(
                new Computable<Module[]>() {
                  public Module[] compute() {
                    return moduleManager.getModules();
                  }
                });

    final TreeSet<VirtualFile> checkSet =
        new TreeSet<VirtualFile>(FilePathComparator.getInstance());
    myRoots = new HashSet<VirtualFile>();
    myRoots.addAll(myInitialRoots);
    checkSet.addAll(myInitialRoots);
    for (Module module : modules) {
      final VirtualFile[] files = ModuleRootManager.getInstance(module).getContentRoots();
      for (VirtualFile file : files) {
        final VirtualFile floor = checkSet.floor(file);
        if (floor != null) {
          myModulesSet.put(file, module.getName());
          myRoots.add(file);
        }
      }
    }
  }