Version control system migrations are a fact of life for developers in any longer lived codebase. In fact, I’ve had a hand in quite a few migrations as newer, more workable version control systems became available. Also, like a lot of developers, I’ve got fragments of source code dating back quite some years floating around on various servers and development machines of mine. Not necessarily code that is still being used, but still code that I don’t want to just delete forever. Some of the oldest code I have uses RCS for source control and hasn’t been touched for a long, long time. As my machines generally don’t have anything as old as RCS installed for version control, I decided this might be a good time to migrate the code to my version control system of choice, Mercurial.

I’ve mentioned before that I prefer Mercurial to Git, at least for my own work. That said, git has a nice feature that allows you to cherry pick revisions to merge between branches. That’s extremely useful if you want to move a single change between branches and not do a full branch merge. Turns out mercurial has that ability, too, but it goes by a slightly different name.

There are actually two options in Mercurial - the older transplant extension and from Mercurial 2.0 onwards, the built-in graft command. I prefer to use the graft command, mainly because it is built into base mercurial and thus is available everywhere as long as one is running Mercurial 2.0 or up. Given that the current release is 4.0.1 as of the time of writing, you should really run something newer than 2.0. Also, graft uses mercurial’s merge abilities to cherry pick the change, you have a somewhat better chance of the change applying cleanly. Transplant uses the patch mechanism with works reasonably well, but in my opinion the merge system works better especially if you’re dealing with something that turns into a three way merge.

The default setup for the Mercurial DVCS on Windows with tortoisehg uses plink and Pageant to manage SSH keys when you are using ssh as the transport protocol for mercurial. That’s most likely the right choice for a normal Windows setup, but if you already have openssh installed and configured to talk to various servers, it’s easy to switch mercurial and tortoisehg to use openssh. It’s also very helpful if you’re forgetful like me and forget to start pageant, add new keys to it etc.