docs: public void doProgressText(StaplerRequest req, StaplerResponse rsp) throws IOException {
          rsp.setContentType("text/plain");
          rsp.setStatus(HttpServletResponse.SC_OK);
  
          if(!source.exists()) {
              // file doesn't exist yet
              rsp.addHeader("X-Text-Size","0");
              rsp.addHeader("X-More-Data","true");
              return;
          }
  
          long start = 0;
          String s = req.getParameter("start");
          if(s!=null)
              start = Long.parseLong(s);
  
          if(source.length() < start )
              start = 0;  // text rolled over
  
          CharSpool spool = new CharSpool();
          long r = writeLogTo(start,spool);
  
          rsp.addHeader("X-Text-Size",String.valueOf(r));
          if(!completed)
              rsp.addHeader("X-More-Data","true");
  
          // when sending big text, try compression. don't bother if it's small
          Writer w;
          if(r-start>4096)
              w = rsp.getCompressedWriter(req);
          else
              w = rsp.getWriter();
          spool.writeTo(new LineEndNormalizingWriter(w));
          w.close();
  
      }