No problem at all, and that's the proper solution I believe. I'm actually glad that you say it before I propose it, because I came to the same conclusion independently :-)
For the story, the problem it was causing was that during some time, the file size is not in phase with the content. In other terms, the file is sparse and this was not supported at the time. Now it is. So what we'll have in case of races between a reader and a writer is that the reader may get 0s for the parts he reads too quickly. I believe this is absolutely fine.