code: @CheckReturnValue
      private boolean loadMasterKillSwitchFile(@Nonnull File f) {
          try {
              if (!f.exists())    return true;
              return Boolean.parseBoolean(FileUtils.readFileToString(f).trim());
          } catch (IOException e) {
              LOGGER.log(WARNING, "Failed to read "+f, e);
              return false;
          }
      }
docs: Reads the master kill switch from a file.
  
  Instead of {@link FileBoolean}, we use a text file so that the admin can prevent Jenkins from
  writing this to file.
  @param f File to load
  @return {@code true} if the file was loaded, {@code false} otherwise