Migrating patches from darcs to git

Suppose ghc/darcs is a darcs GHC tree containing patches that need to be migrated to git, and ghc/git is a git GHC tree.

This assumes that all your changes are in the ghc repo itself. Otherwise, if you have made changes to other repos (e.g. libraries/base) you'll need to do a similar procedure for those repos too.

We'll assume the following directory setup, where $root is some directory in your file system

• $root/my-darcs: the Darcs repo that contains the patches you'd like to move over into Git. We will make no changes to this repo.
• $root/baseline-git (initially non-existent): the Git repo at the moment of switchover
• $root/migrate (initially non-existent): during the transfer process this is going to be both a Darcs repo and a Git repo

Step 1: create migrate:

cd $root
darcs get http://darcs.haskell.org/ghc
mv ghc migrate

This gets a Darcs copy of the repo, precisely at the switchover point. (We don't need a tag, because the Darcs repos were frozen precisely at switchover, so the HEAD will do.)

Step 2: create baseline-git:

cd $root
git clone http://darcs.haskell.org/ghc.git/ baseline-git
cd baseline-git
git checkout -b "some-descriptive-name" ghc-darcs-git-switchover

This gets a Git copy of the repo, precisely at the point of switchover (that's what the ghc-darcs-git-switchover tag does). At this moment all the source files in migrate and baseline-git should be bit-for-bit identical. Check this.

Step 3: make migrate/ into a Git repo too! All we need do is to copy over Git's meta-data.

cd $root/migrate
mv ../baseline-git/.git .

Step 4: transfer patches. For each patch (or group thereof) you want to transfer, do this:

• Pull the patch into tmp-git:

  cd $root/migrate
  darcs pull ../my-darcs
   ...interactively pull the patches you want...

• Record a Git patch for this patch group

  git commit -a

Step 5: merge the patches into the git master:

git checkout master
git merge "some-descriptive-name"

Step 6: validate and push to Git.