@Override
  public boolean isOpen() {
    return channelOpen.get();
  }
Copies bytes from {@code src} into the composite buf.
@param src the source {@link ByteBuf} to copy bytes from.
@return the number of bytes copied.
@throws ClosedChannelException if the channel is closed when this function was called.
ambry-utils/src/main/java/com/github/ambry/utils/ByteBufChannel.java