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Howto: fork a Subversion project with Git

This is a short excerpt on how you can use Git to fork a Subversion project.

I assume that:

• you want to maintain your own fork of a project, and intend to follow its evolutions
• you have no commit-access to the upstream Subversion server, or you do not intend to push you changes back.

We will take XStream as a running example.

Import the Subversion project into Git

git svn clone --stdlayout --prefix=svn/ http://svn.codehaus.org/xstream/ xstream-fork

This creates a complete clone of the XStream Subversion repository. Tags and branches will be imported as well (--stdlayout).

Git has local and remote branches. The local ones are those you make modifications too. They generally merge from remote branches, which you can think of as read-only copies of upstream branches.

Git-SVN turns Subversion branches as Git remote branches. The --prefix=svn/ makes it easier to differentiate them from regular Git branches.

What you get

You can check what was imported:

git branch -a
* master
  remotes/svn/pull-parser-merge
  remotes/svn/tags/XSTREAM_0_2
  remotes/svn/tags/XSTREAM_0_3
  remotes/svn/tags/XSTREAM_0_4
  remotes/svn/tags/XSTREAM_0_5
  remotes/svn/tags/XSTREAM_0_6
  remotes/svn/tags/XSTREAM_0_6_RC1
  remotes/svn/tags/XSTREAM_1_0_1
  remotes/svn/tags/XSTREAM_1_0_2
  remotes/svn/tags/XSTREAM_1_0_RC1
  remotes/svn/tags/XSTREAM_1_1
  remotes/svn/tags/XSTREAM_1_1_1
  remotes/svn/tags/XSTREAM_1_1_2
  remotes/svn/tags/XSTREAM_1_1_3
  remotes/svn/tags/XSTREAM_1_2
  remotes/svn/tags/XSTREAM_1_2_1
  remotes/svn/tags/XSTREAM_1_2_2
  remotes/svn/tags/XSTREAM_1_3
  remotes/svn/tags/XSTREAM_1_3_1
  remotes/svn/trunk
  remotes/svn/xmlencoder-spike

By default, your master branch tracks svn/trunk. You can easily make a local branch based on something else, such as the xmlencoder-spike branch:

git checkout -b xmlencoder-spike xmlencoder-spike

Another thing is that tags in Subversion are just folders, so Git-SVN imported them as branches. You may transform those into real Git tags:

git tag -m 'Tag for v0.2' -a v0.2 svn/tags/XSTREAM_0_2

Work!

The best practice in Git is to work with feature branches, so each time you need to work on something, you should really spin a branch!

Choose which local branch to fork, such as:

git checkout -b my-cool-feature master

Make your commits along the way, and when your work is ready to be merged:

git checkout master
git merge my-cool-feature

# Optional, if you don't need the feature branch anynore
git branch -d my-cool-feature

For quick one-offs, you may not need feature branches though.

Synchronize!

To update all Git-SVN remote branches:

git svn fetch

To merge onto local branches that track Git-SVN remote branches:

git svn fetch
git checkout master
git merge svn/trunk
git checkout 3.1
git merge svn/branch-3.1

You may also directly fetch and apply the changes if your branch was created from a Git-SVN branch:

git svn rebase

Make a real Git clone repository available

You can't push Subversion remote branches to another repository, so if you need to make your Git fork public somewhere (such as on GitHub), you will need to:

• create tags as mentioned above
• create a local branch for each Subversion branch that you want to republish such as stable branches
• push those tags and local branches to your public repository.

- (NSMutableString*) csvData {
    NSError *error = nil;
	if (![[self fetchedResultsController] performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
		abort();
	}
 
    NSMutableString* str = [[NSMutableString alloc] initWithString:@"timestamp,altitude,latitude,longitude,xAcceleration,yAcceleration,zAcceleration\n"];
    for (NSManagedObject* entry in self.fetchedResultsController.fetchedObjects) {
        [str appendFormat:@"%@,%@,%@,%@,%@,%@,%@\n",
         [entry valueForKey:@"timestamp"],
         [entry valueForKey:@"altitude"],
         [entry valueForKey:@"latitude"],
         [entry valueForKey:@"longitude"],
         [entry valueForKey:@"xAcceleration"],
         [entry valueForKey:@"yAcceleration"],
         [entry valueForKey:@"zAcceleration"]];
    }
 
    [str autorelease];
    return str;
}

- (void)sendMail {
    if (! [MFMailComposeViewController canSendMail]) {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Unsupported operation"
                                                            message:@"This device is not configured to send emails." 
                                                           delegate:nil cancelButtonTitle:@"Ok"
                                                  otherButtonTitles:nil];
        [alertView show];
        [alertView release];        
        return;
    }
 
    MFMailComposeViewController* mailController = [[MFMailComposeViewController alloc] init];
    [mailController setSubject:@"Entropia Data"];
    [mailController setMessageBody:@"This data was captured by Entropia.app" isHTML:NO];
    [mailController addAttachmentData:[[self csvData] dataUsingEncoding:NSUTF8StringEncoding]
                             mimeType:@"application/csv"
                             fileName:@"data.csv"];
 
    mailController.mailComposeDelegate = self;
    [self.navigationController presentModalViewController:mailController animated:YES];
}

- (void)mailComposeController:(MFMailComposeViewController *)controller 
          didFinishWithResult:(MFMailComposeResult)result 
                        error:(NSError *)error {    
    [self.navigationController dismissModalViewControllerAnimated:YES];
 
    if (result == MFMailComposeResultFailed) {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:[error localizedDescription]
                                                            message:[error localizedFailureReason]
                                                           delegate:nil cancelButtonTitle:@"Ok"
                                                  otherButtonTitles:nil];
        [alertView show];
        [alertView release];
    }
 
    [controller release];
}