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Friday, August 7, 2009

The Journey to Git, Part IX--Communicating from Git to Subversion

In this second part of the Git/Subversion interaction guide, we'll explore the commands that let you do the equivalent of "svn update" and "svn commit". You need to already have a Git repository that's linked to a Subversion repository. The previous post in this series will help you with that if you need it.


Articles in this series:

Getting a Git copy of a Subversion repository and making local commits/branches/whatever in it is great. How do you get further updates of commits that others have made to Subversion? What do you do when you're ready to send your changes back to Subversion? First, decide which branch it is you want to send or receive commits for, and make sure it's checked out. Both the commands I'm going to discuss work within your current branch.

Before committing anything to Subversion, it's never a bad idea to update first and see if there have been any changes, so let's look at that command first.

Updating from Subversion with Git

The Rebase

It may not seem apparent at first, but when we pull changes from SVN to Git, what we really want is a rebase. Why? To maintain a perfectly linear history for the sake of Subversion. We've seen the git rebase command before. Recall that it's the one that lets you freely squash, edit, delete, and move commits around in a branch. To pull the latest changes from Subversion into your current branch--every Git branch can be traced back to a Subversion branch from which it originated, and that's the one it pulls commits from--you first need a clean index and working tree, and then run:

git svn rebase

Note: See the note at the beginning of my previous post to learn how "git svn rebase" is currently broken in Cygwin and to find a workaround.

Your index and working tree have to be clean because of what the rebase does, which I'll get to in a minute. First, let's examine why we have to use a rebase more in depth. When you're working out of a Subversion repository with Git, you'll always be building on top of Subversion commits. Say you're on a branch where there are two Subversion commits: A and B. Then you make commit C in Git locally. Meanwhile, someone else has made a new commit to Subversion--call it X. When you go to pull that change from Subversion down to your local Git repo, where should it go? Your first inclination might be that your local history should become A -> B -> C -> X, but this is 100% wrong. Subversion already has A -> B -> X, and you're not easily going to convince it to put C in front of X. Git, on the other hand, has no problem at all sticking the X before the C. That's exactly what "git rebase" is for. Therefore, when you pull commit X from Subversion, you want to end up with A -> B -> X -> C. That is, you want Subversion commits to all be on top of each other and ahead of any of your local commits.

Now, a "git svn rebase" behaves much like a normal "git rebase". First, it moves all of your local commits--the ones that Subversion doesn't know about yet--out of the way, effectively taking them out of the branch and making HEAD point at the latest SVN commit that's in your Git repo. Then it pulls down all of the SVN commits that aren't represented locally and applies them one at a time on the HEAD of the current branch. This part should all go smoothly because it's basically just copying history from Subversion into Git. After that's done, the rebase puts your commits back on to the HEAD of the current branch, again one at a time.

When Conflict Occurs

When your commits are being reapplied, it's quite possible that you'll experience conflicts with the new commits you got from Subversion, just as you would with "svn update", and this is the main reason that your working tree and index must be clean before you start any kind of rebase. In the event of a conflict, you use your working tree and index to resolve it.

It's very important that you pay attention to Git's messages during any kind of rebase because if there is a conflict, it becomes an interactive process. The rebase will stop, and you'll be looking at a dirty working tree with unmerged files and possibly some staged changes in the index. All the changes you see are the content of the commit Git was trying to apply when the conflict happened. It's waiting for you to resolve the conflict somehow and then tell it to continue the rebase with:

git rebase --continue

Conflict resolution works exactly as I described in my post on merging. Note that we continue the rebase with the "git rebase" command and not "git svn rebase". Once the rebase is kicked off, it acts just like any other rebase you'd perform in Git, and as with any other rebase, resolving the conflicts and continuing is only one of your three options. You can also skip the current commit with:

git rebase --skip

You'd use this if, for instance, the current commit is no longer applicable because of an upstream commit that you've received. The commit is effectively deleted, and it won't be in the branch when the rebase is complete. The third option is to abort the rebase entirely with:

git rebase --abort

You can always abort up until the rebase is completely finished. An abort takes you back to the state you had before you started the rebase. If you really get yourself in a bind, or if you decide you just don't have the knowledge to resolve the conflicts effectively, you can always abort and come back to it later.

The really important thing about rebasing is that when it stops in the middle, you must see it through to the end one way or another. Don't go off to work on something else until the rebase is complete, or you're really going to confuse yourself.

Now that we've got all the incoming changes, let's see how to send changes back to Subversion.

Commiting to Subversion with Git

Compared to what we've seen so far about Git-SVN interaction, sending your local commits back to SVN is a breeze. Just make sure you have a clean working tree and index, then run:

git svn dcommit

There's not much that can go wrong with this command. The working tree and index have to be clean because a dcommit ends with a rebase or a reset, and we've already seen why you have to clean those up before a rebase. I'm not certain exactly what the rebase/reset does, but I think it has to do with putting the "SVN version" of the commit in your branch in place of your local commit.

When the dcommit is complete, there will be a commit in Subversion matching each of the local commits you had in Git, and the commits in Git will all reflect a git-svn-id, indicating that they're recorded in Subversion.

That's it. Two commands are all you need to swap commits with a Subversion repository. Next up, I'll talk about interacting with remote Git repositories. You'll find that the basics are quite similar to the Subversion interaction, but it's a lot more powerful.

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