docs: @Override
      public R put(R r) {
          // Defense against JENKINS-23152 and its ilk.
          File rootDir = r.getRootDir();
          if (rootDir.isDirectory()) {
              throw new IllegalStateException("JENKINS-23152: " + rootDir + " already existed; will not overwrite with " + r);
          }
          if (!r.getClass().getName().equals("hudson.matrix.MatrixRun")) { // JENKINS-26739: grandfathered in
              proposeNewNumber(r.getNumber());
          }
          rootDir.mkdirs();
          return super._put(r);
      }