Helper scripts to set up git to svn mirroring
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This is a bunch of scripts attempting to set up Git to SVN mirroring, without the branching/merging/pulling constraints set by the standard git-svn tool.

Git-svn caveats

Git-svn is a well known option to use Git as an SVN client, but it comes with some caveats and constraints. To quote the git-svn manual page:

For the sake of simplicity and interoperating with a less-capable system (SVN), it is recommended that all git svn users clone, fetch and dcommit directly from the SVN server, and avoid all git clone/pull/merge/push operations between git repositories and branches. [...] Running git merge or git pull is NOT recommended on a branch you plan to dcommit from.

A related problem is that git-svn dcommit rewrites git commits (e.g. it adds SVN metadata to the commit message). This means that Git can get easily confused when it tries to merge the branch with the rewritten commits with a branch with the original commits (for example when the original branch was forked or pushed before the git-svn dcommit). I've been there, it's not pretty.

Eventhough the git-svn documentation warns not do git branching, merging and pulling when you use git-svn, it makes the usefulness of git-svn pretty limited in real projects and workflows.

Mirroring a read/write Git repo to a read-only SVN repo mirror

This was the initial target use case for these sync scripts. The aim is to allow all standard git operations like branching, merging, pushing, pulling and still be able to mirror it to a read-only SVN repo. "Read-only SVN repo" means here that only the Git commits should be pushed ("svn commit") to it through the sync tools, but all other "users" of the SVN repo should only read ("svn update") from it.

The trick to allow full git branching functionality combined with git-svn syncing is to keep a dedicated git-svn sync branch completely separate from the bunch of normal git branches ("master", "develop", topic branches, integration branches, ...). All normal, daily work and commits should happen on the normal branches, the git-svn sync branch is only for internal housekeeping and should not be committed on directly. To avoid merging issues due to commit rewriting, there should also be no merging between the dedicated git-svn sync branch and the normal branches. Instead, to keep things in sync, commits are ported/transferred by the sync script with a "git rebase --onto" trick. This has the added benefit that git rebase will try to create a linear commit-history on the git-svn sync branch from a possibly non-linear history in the bunch of normal branches.

The script

This script should be executed from within the central Git repo. It assumes a certain working context, based on two Git branches that are used for interal housekeeping:

  • svn-sync/svn-side: this is the Git branch that will be synced to the SVN repo with the git-svn tool. This implies that the original/initial git-svn remote tracking branch should be an ancestor of the svn-sync/svn-side branch.
  • svn-sync/git-side, which tracks the commits on the git master branch that are already synced.

It is vital that these the content pointed to by these two branches is the same, meaning that the diff between the two should be empty. However, this does not mean that these two branches point at the same Git commit. On the contrary, they will point to the same Git commit, only at initialisation or, in some cases, they won't share any history at all. Due to the syncing approach, the branches will follow their own seprate history with separate git commits. As already noted, they should never be merged, so from the standpoint of Git, they diverge in terms of commits. But in terms of raw content, they will be "in sync".

A typical initialization is starting from an existing SVN repo and convert it to Git, like so:

# Convert SVN repo to Git repo
git svn clone --prefix=svn/ $svn-repo-url git-repo
cd git-repo
# Initialize the svn-sync housekeeping branches
git branch svn-sync/svn-side master
git branch svn-sync/git-side master

From here we can work on the master branch using standard Git workflow tools like branching, merging, rebasing etc.

At some point, we want to sync our work to SVN and just call:

This can be on each commit/merge on master, or less frequently (e.g. once a day). The script does not make an assumption here.

TODO ...