The case is that sometimes you want to add some cool feature to an open source project. But you don't have commit rights to that project. So you checked out a copy of the code and do some local hacking. The problem is that you'll have to track the updates of the original code and merge the changes to your local copy constantly.

One example is the multi-tty support of GNU Emacs (now officially available in Emacs). While the Emacs code base is rapidly evolving, the author of multi-tty branch should track the changes and merge them to his local copy. Another example is YARV when it is still not part of the official Ruby implementation.

Things will be simple if the project is under a distributed version control system (like git), in fact, GitHub is encouraging you to fork the project you want to hack (with simply one click). But the situation is: the official Ruby repository is under subversion while Emacs is still under CVS. :(

But dealing with svn repository is not too hard. The tool to use is git-svn (I still remember that 华华 mentioned the great tool to me). The work flow is simple:

1. Check out the repository with git-svn.

   git-svn clone http://svn.ruby-lang.org/repos/ruby/trunk ruby19
   

   git-svn try to fetch every history commits of the svn repository. This is a time-consuming process when the repository of that project exists a long time. Generally we don't need the history information, so we can tell git-svn to just retrieve the latest version:

   git-svn -r HEAD clone http://svn.ruby-lang.org/repos/ruby/trunk ruby19
   

2. Now you have a git repository locally. Create your own branch for your happy hackings:

   git checkout -b hacking
   

3. Now do your hacking on the hacking branch and commit locally.
4. You should still update and merge changes from the official repository constantly, the process is fairly simple:
   1. Commit your local copy to your hacking branch:

      git commit -a
      

   2. Switch to the master branch:

      git checkout master
      

   3. Fetch the changes from remote svn repository:

      git-svn rebase
      

   4. Switch back to your hacking branch:

      git checkout hacking
      

   5. Merge changes to your hacking branch:

      git merge master
      

      You are lucky if everything goes fine. But if conflicts occurred, just resolve them and commit.
5. When you are ready with your hacking and want to release the patch against the latest official repository, just run git diff:

   git diff master > hacking.patch
   

   Then put the hacking.patch somewhere publicly available. Other people with a copy of the official repository of a right revision number can apply your patch by simple:

   patch < hacking.patch
   

   Cool!

:D Now go and fork your favourite project!

git-svn -r HEAD clone http://svn.apache.org/repos/asf/mina/trunk/ trunk

$ git-svn clone http://svn.apache.org/repos/asf/mina/trunk/ trunk      Initialized empty Git repository in .git/

W: Ignoring error from SVN, path probably does not exist: (175007): HTTP Path Not Found: REPORT request failed on '/repos/asf/!svn/bc/100/mina/trunk': '/repos/asf/!svn/bc/100/mina/trunk' path not found

W: Do not be alarmed at the above message git-svn is just searching aggressively for old history.

This may take a while on large repositories