hpack/src/main/java/com/twitter/hpack/Encoder.java
private void ensureCapacity(int headerSize) throws IOException {
    while (size + headerSize > capacity) {
      int index = length();
      if (index == 0) {
        break;
      }
      remove();
    }
  }
private void ensureCapacity(int headerSize) throws IOException {
    while (size + headerSize > capacity) {
      int index = length();
      if (index == 0) {
        break;
      }
      remove();
    }
  }
Ensure that the dynamic table has enough room to hold 'headerSize' more bytes.
Removes the oldest entry from the dynamic table until sufficient space is available.