With the Subversion VCS, one way to import external modules or libraries into a code tree is by defining the svn:externals property of your repository. Subversion will then check out the specified revision (or the latest revision) of the other repository into your source tree when checking out your code.

Submodules are basically the same thing in the "git" world.

And since git can talk to subversion repositories with git svn, we should be able to specify a third-party SVN repository as a submodule in our own git repository, right? Sadly the answer is currently: No.

Here is a workaround that I have been using to at least achieve a similar effect, while keeping both SVN and git happy. This assumes that you have a local git repository that is also available somewhere else "up-stream" (such as github), and you want to import an external SVN repository into your source tree.

1: Add a tracking branch for SVN

Add a section referring to your desired SVN repository to your .git/config file:

(...)
[svn-remote "product-details"]
    url = http://svn.mozilla.org/libs
    fetch = product-details:refs/remotes/product-details-svn

Note that in the fetch line, the part before the colon refers to the branch you want to check out of SVN (for example: trunk), and the part after that will be our local remote branch location, i.e. product-details-svn will be our remote branch name.

Now, fetch the remote data from SVN, specifying a revision range unless you want to check out the entire history of that repository:

git svn fetch product-details -r59506:HEAD

git will check out the remote branch.

2: clone the tracking branch locally

Now we have a checked-out SVN tracking branch, but to use it as a submodule, we must make a real git repository from it -- a branch of our current repository will keep everything in one place and work as well. So let's check out the tracking branch into a new branch:

git checkout -b product-details-git product-details-svn

As git status can confirm, you'll now have (at least) two branches: master and product-details-git.

3: Import the branch as a submodule

Now let's make the new branch available upstream:

git push --all

After that's been pushed, we can import the new branch as a submodule where we want it in the tree:

git checkout master
git submodule add -b product-details-git ../reponame.git my/submodules/dir/product-details

Note that ../reponame.git refers to the up-stream repository's name, and -b ... defines the name of the branch we've created earlier. Git will check out your remote repository and point to the right branch automatically.

Don't forget to git commit and you're done!

Updating from SVN

Updating the "external" from SVN is unfortunately a tedious three-step process :(. First, fetch changes from SVN:

git svn fetch product-details

Second, merge these changes into your local git branch and push the changes up-stream:

git checkout product-details-git
git merge product-details-svn
git push origin HEAD

And finally, update the submodule and "pin it" at the newer revision:

git checkout master
cd my/submodules/dir/product-details
git pull origin product-details-git
cd ..
git add product-details
git commit -m 'updating product details'

Improvements?

This post is as much a set of instructions as it is a call for improvements. If you have an easier way to do this, or if you know how to speed up or simplify any of this, a comment to this post would be very much appreciated!

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Here at Mozilla, a bunch of webdevs use git svn instead of plain Subversion to interact with our svn repositories -- mostly because of in-place branching, better merging, and all these things that make a dev's life happier.

As you probably know when reading this article, you push all uncommitted changes to the remote svn repository by typing git svn dcommit. This will take your last, say, five commits, push them to SVN, and mark them locally as committed. But what if you only want to dcommit some of your changes?

(If you don't need explanations, jump straight to the summary).

Step 1 (optional): Reorder commits

I am not going to go into a lot of detail on interactive rebasing (git rebase -i), but to start off, make sure your commits are ordered, so that the ones you do want to commit to SVN are before the ones you do not want to push up-stream for now. Example: If in the following history, you want to commit all changes but a023fea, you want to rebase your commits so a023fea is last:

In git rebase -i HEAD~4, change...

pick 07c26c5 some de L10n
pick a023fea adding free-text messages to localizer dashboards
pick 8597f47 adding featured collections as l10n category
pick 19f3df3 making existing localizer pages work with amo2009 layout

... to...

pick 07c26c5 some de L10n
pick 8597f47 adding featured collections as l10n category
pick 19f3df3 making existing localizer pages work with amo2009 layout
pick a023fea adding free-text messages to localizer dashboards

Make sure to resolve any merging problems that might occur due to the reordering.

Step 2: Step in between commits

To only push the desired commits to svn, execute another git rebase -i and mark the last desired commit for editing (by changing pick to edit):

pick 07c26c5 some de L10n
pick 8597f47 adding featured collections as l10n category
edit 19f3df3 making existing localizer pages work with amo2009 layout
pick a023fea adding free-text messages to localizer dashboards

When exiting the editor, git will drop you off after the marked commit, but before the one you don't want, as a quick look at git log can tell you.

Step 3: dcommit desired changes

After making sure this is really what you want, just execute git svn dcommit as usual and watch git push all desired changes to SVN, while omitting the rest.

Step 4: Fast-forward to HEAD

When the dcommit is done, remember we are still in the middle of a rebase, so just run git rebase --continue to fast-forward to the HEAD of your branch. Again, a quick look at git log can reassure you that only the changes you wanted to have been pushed to SVN.

Success!

Summary: Quick cheat sheet

Here's a quick cheat sheet for you (and me) to come back to in case you forget:

• Reorder commits (git rebase -i HEAD~4) so that the undesired ones are after the ones you want to push
• In your commit history, jump right after the last wanted commit by marking it for editing in git rebase -i
• git svn dcommit
• git rebase --continue

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Whenever I apply somebody else's big patch to review it, I obviously go ahead and revert the changes to my local Subversion working copy afterwards. Here's the line I use to do it, maybe somebody will find it useful (though I mainly blog it not to forget it myself):

svn st -q | awk '{print $2;}' | xargs svn revert

(To be honest, I also expect about 17 comments now that tell me how I can do this more efficiently, or what I didn't consider when writing this in about 5 seconds -- so, don't disappoint me, fire away! :) )

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