In my opinion, there's just not enough Subversion servers on the internet.
That's an opinion you'll probably never hear me express - I much rather everyone move over to a DVCS (*ahem* mercurial) - but Subversion has, some would say quite rightly, earned its place as the dominant VCS for nearly every part of the IT community (apart from big, lumbering, financial companies).
Recently, I've been slowly switching away from subversion for my personal projects into Mercurial. This, in itself, a painless process, but with my public projects - particularly South - this has the unintended effect of slightly alienating some of my users. Most are alright with adapting - after all, it's quickly becoming the norm to use a DVCS - but there's still a few people left out.
One of the particular issues I have is with svn:externals. A lot of apps - including some we have at work - rely on svn:externals to pull in external dependencies into a libs folder along with the project itself. Externals is one of the few features of subversion that I thought was pretty much perfect, and it was sad to see my move to Mercurial break it.
This week, at DjangoCon, Chris Wanstrath did a nice talk on DVCSen. One of the questions led him onto hg-git - the awesome git backend plugin for Mercurial, that the GitHub guys wrote - and how they first investigated the idea of an svn gateway to expose their repositories transparently to subversion users. From what I gathered, subversion's wire format proved too tricky to deal with, and so they turned elsewhere.
However, the idea of a subversion gateway intrigued me greatly. What better way to transparently serve South so people can still pull it using svn:externals while still developing in Mercurial? With that in mind, I started looking into Subversion's wire protocols and their delta format (svndiff) sometime yesterday afternoon.
After some digging, reading an obscure academic paper and liberal application of Wireshark, I grew confident enough that I could at least implement something. A day later, and I'd like to present the very first version of what I'm calling Heechee (if you get the pun, ten nerd points).
Heechee is a transparent mercurial-as-subversion gateway. It serves a Mercurial repository as a Subversion WebDAV-based repository. It's still in the early stages, but at the moment it will serve its own mercurial repository to subversion in such a way that you can check out the repository, and update to various revisions within it.
You can check it out at BitBucket. It's pretty alpha code, and make sure you have the dependencies mentioned in the README, but it works, which greatly surprises me. I plan to much improve the code to support more 'advanced' features, like being able to do more than checkout and update, as well as exposing tags and branches correctly. There's even the chance I'll stick Git support in, when I've had a play with Dulwich.
Still, hopefully I'll soon have it so it can serve South's code via svn, and thus restore harmony to my land of VCS users.
So, please, take the code and have a play with it. It's not formally licensed and released yet, but it will probably be Apache licensed. Feedback and suggestions for features, as well as how to linearise trees into svn histories, much welcomed!