code: function maybeReadMore(stream, state) {
    if (!state.readingMore) {
      state.readingMore = true;
      timers.setImmediate(function() {
        maybeReadMore_(stream, state);
      });
    }
  }
docs: at this point, the user has presumably seen the 'readable' event, and called read() to consume some data.  that may have triggered in turn another _read(n) call, in which case reading = true if it's in progress. However, if we're not ended, or reading, and the length < hwm, then go ahead and try to read some more preemptively.