git-Subversion bridge

It should be quite obvious to GitHub users why our team likes git - we branch, diff, merge and rebase heavily, work offline, stash, amend commits and do other git-specific things that make git so fun and useful.

However, our corporate standard is Subversion. It is simple and reliable, the history is immutable and the central repository lives in the secure datacenter. Project managers and salespeople can use TortoiseSVN to access the project repository as well, keeping project documents neatly organized alongside source repositories; administrators have easy tools for managing authorization and authentication etc. Everyone is generally happy with it.

To reap the best of both worlds, we have setup a git-Subversion bridge that synchronizes changes between our team git repository and the corporate Subversion repository. The obvious requirement is that our git usage has to be transparent to other Subversion users, not interfere with their work or damage the corporate repository.

The setup looks like this:

We have used this setup in production for more than a year (albeit in a somewhat simpler incarnation), so we have sorted out most of the problem points and are content with it. It may also work for you in a similar situation.

This document gives an overview of the setup.

If you have administrator access to the Subversion repository (we don't), be sure to check out SubGit. It may or may not make the setup simpler.

Overview and caveats

• Each update of master in the central git repository will trigger synchronization with Subversion. Additionally, there is a cron job that runs the synchronization every n minutes (so that the repository is updated even if there are no commits at git side). Concurrent synchronization is properly guarded with a lock in the bundled synchronization script.

• There is no exchange of branches between git and Subversion. The git side tracks the trunk or any other main Subversion branch. Branching happens at git side with short-lived task branches that need not to be shared with Subversion users. Separate git repositories are used for tracking other long-lived Subversion branches.

• As Subversion history is linear, git merge commits will be squashed into a single commit (see examples below). For us this is no problem as, in the spirit of continuous integration, we consider the task branches to be lightweight, ephemeral "units of work" that can go to mainline in a single chunk and as the branch history is retained in git.

• To properly set author information in commits between git and Subversion, Subversion user passwords need to be available to the synchronization script. A fairly secure program, git-svn-auth-manager, that keeps passwords in an encrypted SQLite database is bundled and the synchronization script uses that by default (see description below).

• git history is duplicated as commits first go up to and then come back down again with a git-svn-id from Subversion. Although this may sound confusing it has not been a big problem in practice (see examples below; note that --no-ff is used to record the merges). Subversion history remains clean.

• Rewriting history on master will probably mess up the git-Subversion synchronization so it is disabled with the update hook in the central git repository (we haven't tried though, this just seems a sane precaution).

• If the project is Windows-only then the git bridge repo must be configured to retain Windows line endings. (TODO: describe how.)

Subversion's view on history

Squashed branch merge commit to master from git (see dave.sh below):

$ svn log trunk@r9 -l 1
------------------------------------------------------------------------
r9 | dave | 2012-08-26 13:22:39 +0300 (Sun, 26 Aug 2012) | 10 lines

2012-08-26 13:22:36 +0300 | Merge branch 'payment-support'
 [Dave]
2012-08-26 13:22:36 +0300 | Add storage encryption for payments
 [Dave]
2012-08-26 13:22:36 +0300 | Implement credit card payments
 [Dave]
2012-08-26 13:22:36 +0300 | Implement PayPal payments
 [Dave]
2012-08-26 13:22:36 +0300 | Add payment processing interface
 [Dave]
------------------------------------------------------------------------

Single commit to master from git (see carol.sh below):

$ svn log trunk@r8 -l 1
------------------------------------------------------------------------
r8 | carol | 2012-08-26 13:22:36 +0300 (Sun, 26 Aug 2012) | 2 lines

2012-08-26 13:22:35 +0300 | Use template filters to represent amounts in localized format
 [Carol]
------------------------------------------------------------------------

Git's view on history

Single commit before synchronization:

$ git log a165c -1
commit a165c9857eebb168e44b22278950cd930259394c
Author: Carol <[email protected]>
Date:   Sun Aug 26 13:22:35 2012 +0300

    Use template filters to represent amounts in localized format

After synchronization, it will be duplicated with another commit that has come down from Subversion:

$ git log
...
commit 10fb01c123851b02f2105c98cb7c9adc47a1bb39
Merge: fc656d9 a165c98
Author: Carol <[email protected]>
Date:   Sun Aug 26 13:22:36 2012 +0300

    2012-08-26 13:22:35 +0300 | Use template filters to represent amounts in localized format
     [Carol]
    
    git-svn-id: svn://localhost/trunk@8 49763079-ba47-4a7b-95a0-4af80b88d9d8
...
commit a165c9857eebb168e44b22278950cd930259394c
Author: Carol <[email protected]>
Date:   Sun Aug 26 13:22:35 2012 +0300

    Use template filters to represent amounts in localized format
...

For each branch merge, an additional squashed merge commit will come down from Subversion as shown in the previous section.

Setup

The following walkthrough is provided both for documentation and for hands-on testing. All of this can be run in one go with the test script. After you have prepared the envrionment, new bridge repositories for other Subversion branches can be set up with the branch setup script.

Start by creating the bridge user (use your actual email address instead of [email protected], it is used later during setup and testing):

$ sudo adduser git-svn-bridge
$ sudo su git-svn-bridge
$ set -u
$ [email protected]
$ git config --global user.name "Git-SVN Bridge (GIT SIDE)"
$ git config --global user.email "$YOUR_EMAIL"
$ cd
$ mkdir {bin,git,svn,test}

Subversion

Assure that Subversion caches passwords (only last git-svn-auth-manager reset user password will be cached; let me know if this does not meet your security requirements, there are ways around this):

$ echo 'store-plaintext-passwords = yes' >> ~/.subversion/servers

Create the Subversion repository (in real life you would simply use the existing central Subversion repository):

$ cd ~/svn
$ svnadmin create svn-repo
$ svn co file://`pwd`/svn-repo svn-checkout
Checked out revision 0.

Commit a test revision to Subversion:

$ cd svn-checkout
$ mkdir -p trunk/src
$ echo 'int main() { return 0; }' > trunk/src/main.cpp
$ svn add trunk
A         trunk
A         trunk/src
A         trunk/src/main.cpp
$ svn ci -m "First commit."
Adding         trunk
Adding         trunk/src
Adding         trunk/src/main.cpp
Transmitting file data .
Committed revision 1.

Setup svnserve to serve the repository:

$ cd ~/svn

$ SVNSERVE_PIDFILE="$HOME/svn/svnserve.pid"
$ SVNSERVE_LOGFILE="$HOME/svn/svnserve.log"
$ SVNSERVE_CONFFILE="$HOME/svn/svnserve.conf"
$ SVNSERVE_USERSFILE="$HOME/svn/svnserve.users"

$ >> $SVNSERVE_LOGFILE

$ cat > "$SVNSERVE_CONFFILE" << EOT
[general]
realm = git-SVN test
anon-access = none
password-db = $SVNSERVE_USERSFILE
EOT

$ cat > "$SVNSERVE_USERSFILE" << EOT
[users]
git-svn-bridge = git-svn-bridge
alice = alice
bob = bob
carol = carol
dave = dave
EOT

$ TAB="`printf '\t'`"

$ cat > ~/svn/Makefile << EOT
svnserve-start:
${TAB}svnserve -d \\
${TAB}${TAB}--pid-file "$SVNSERVE_PIDFILE" \\
${TAB}${TAB}--log-file "$SVNSERVE_LOGFILE" \\
${TAB}${TAB}--config-file "$SVNSERVE_CONFFILE" \\
${TAB}${TAB}-r ~/svn/svn-repo

svnserve-stop:
${TAB}kill \`cat "$SVNSERVE_PIDFILE"\`
EOT

Start svnserve:

$ make svnserve-start

Git

Setup the central repository that git users will use:

$ cd ~/git
$ git init --bare git-central-repo-trunk.git
Initialized empty Git repository in /home/git-svn-bridge/git/git-central-repo-trunk.git/
$ cd git-central-repo-trunk.git
$ git remote add svn-bridge ../git-svn-bridge-trunk

Setup the git-Subversion bridge repository:

$ cd ~/git
$ SVN_REPO_URL="svn://localhost/trunk"
$ git svn init --prefix=svn/ $SVN_REPO_URL git-svn-bridge-trunk
Initialized empty Git repository in /home/git-svn-bridge/git/git-svn-bridge-trunk/.git/

Fetch changes from Subversion:

$ cd git-svn-bridge-trunk
$ AUTHORS='/tmp/git-svn-bridge-authors'
$ echo "git-svn-bridge = Git SVN Bridge <${YOUR_EMAIL}>" > $AUTHORS
$ git svn fetch --authors-file="$AUTHORS" --log-window-size 10000
Authentication realm: <svn://localhost:3690> git-SVN test
Password for 'git-svn-bridge': git-svn-bridge
   A   src/main.cpp
r1 = 061725282bdccf7f4a8efa66ee34b195ca7070fc (refs/remotes/svn/git-svn)
Checked out HEAD:
  file:///home/git-svn-bridge/svn/svn-repo/trunk r1

Verify that the result is OK:

$ git branch -a -v
* master              0617252 First commit.
  remotes/svn/git-svn 0617252 First commit.

Add the central repository as a remote to the bridge repository and push changes from Subversion to the central repository:

$ git remote add git-central-repo ../git-central-repo-trunk.git
$ git push --all git-central-repo
Counting objects: 4, done.
Writing objects: 100% (4/4), 332 bytes, done.
Total 4 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
To ../git-central-repo-trunk.git
 * [new branch]      master -> master

Clone the central repository and verify that the Subversion test commit is there:

$ cd ~/git
$ git clone git-central-repo-trunk.git git-central-repo-clone
Cloning into 'git-central-repo-clone'...
done.

$ cd git-central-repo-clone
$ git log
commit 061725282bdccf7f4a8efa66ee34b195ca7070fc
Author: git-svn-bridge <git-svn-bridge>
Date:   Wed Aug 15 11:38:57 2012 +0000

   First commit.

   git-svn-id: file:///home/git-svn-bridge/svn/svn-repo/trunk@1 b4f7b086-5416-...

Create the git hook that blocks non-fast-forward commits in the central repository:

$ cd ~/git/git-central-repo-trunk.git
$ cat > hooks/update << 'EOT'
#!/bin/bash
set -u
refname=$1
shaold=$2
shanew=$3

# we are only interested in commits to master
[[ "$refname" = "refs/heads/master" ]] || exit 0

# don't allow non-fast-forward commits
if [[ $(git merge-base "$shanew" "$shaold") != "$shaold" ]]; then
    echo "Non-fast-forward commits to master are not allowed"
    exit 1
fi
EOT

$ chmod 755 hooks/update

Create the git hook that triggers synchronization:

$ cat > hooks/post-update << 'EOT'
#!/bin/bash

# trigger synchronization only on commit to master
for arg in "$@"; do
    if [[ "$arg" = "refs/heads/master" ]]; then
        /home/git-svn-bridge/bin/synchronize-git-svn.sh GIT_HOOK
        exit $?
    fi
done
EOT

$ chmod 755 hooks/post-update

$ cat > ~/bin/synchronize-git-svn.sh << 'EOT'
# test script to verify that the git hook works properly
echo "Commit from $1 to master" > /tmp/test-synchronize-git-svn
exit 1 # test that error exit does not abort the update
EOT

$ chmod 755 ~/bin/synchronize-git-svn.sh

Test that the hook works:

$ cd ~/git/git-central-repo-clone
$ echo "void do_nothing() { }" >> src/main.cpp

$ git commit -am "Update main.cpp"
[master 2c833e2] Update main.cpp
 1 file changed, 1 insertion(+)

$ git push
Counting objects: 7, done.
Writing objects: 100% (4/4), 341 bytes, done.
Total 4 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
To /home/git-svn-bridge/git/git-central-repo-trunk.git
   5b73892..2c833e2  master -> master

$ cat /tmp/test-synchronize-git-svn
Commit from GIT_HOOK to master

Verify that non-fast-forward commits to master are not allowed:

$ echo "void do_nothing() { }" >> src/main.cpp
$ git add src/
$ git commit --amend
[master d2f9a16] Update main.cpp
 1 file changed, 2 insertions(+)

$ git push --force
Counting objects: 7, done.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (4/4), 345 bytes, done.
Total 4 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
remote: Non-fast-forward commits to master are not allowed
remote: error: hook declined to update refs/heads/master
To /home/git-svn-bridge/git/git-central-repo-trunk.git
 ! [remote rejected] master -> master (hook declined)
error: failed to push some refs to '/home/git-svn-bridge/git/git-central-repo-trunk.git'

$ git reset --hard origin/master

So far, so good. Let's wire in the real synchronization utilities now.

Synchronization utilities

Real synchronization relies on

• the synchronization script that controls the actual synchronization

• git-svn-auth-manager, a utility that manages Subversion authentication and commit author mapping between git and Subversion (note that this is the sweet spot of the solution); it is described in more detail in a separate README.

Start by cloning this repository:

$ cd ~/git
$ git clone --recursive git://github.com/mrts/git-svn-bridge.git github-git-svn-bridge-utils

Warning to Ubuntu 16.04 users
The versions of Mono and Git provided in Ubuntu 16.04 cause problems as described below, please use latest Mono and Git if you run into problems.

git-svn-auth-manager

Install required libraries and tools:

$ sudo apt-get install build-essential mono-devel libssl-dev tcl

Change the encryption key:

$ cd github-git-svn-bridge-utils/git-svn-auth-manager
$ ENCRYPTION_KEY=`tr -dc '[:alnum:]' < /dev/urandom | head -c 16`
$ sed -i "s/CHANGETHIS/$ENCRYPTION_KEY/" src/EncryptedUserRepository.cs
$ git diff src
...
-               private const string ENCRYPTION_KEY = "CHANGETHIS";
+               private const string ENCRYPTION_KEY = "TNwwmT2Wc3xVTole";
...

This is generally not necessary, but if you have an old database lying around from previous runs, it should be removed now as the encryption key has changed (careful with your actual user information):

$ make mrproper
...
rm -f ~/.config/git-svn-auth-manager/userinfo.db
...

Build and install git-svn-auth-manager:

$ make install
...
install -m 711 -D bin/git-svn-auth-manager ~/bin/git-svn-auth-manager

Verify that it works:

$ ~/bin/git-svn-auth-manager
git-svn-auth-manager: too few, too many or invalid arguments

Helper utility for running a git-SVN bridge.
Manages SVN authentication for git and user mapping between git and SVN.

Usage:
    either with a single non-option argument to output user
    name and email suitable for `git --authors-prog`:

        git-svn-auth-manager SVN_USERNAME

    or with a single option to add users or change passwords:

        git-svn-auth-manager OPTION=SVN_USERNAME

    or with a single option and single non-option argument to reset
    SVN authentication cache:

        git-svn-auth-manager --reset-auth-for=EMAIL SVN_URL

Options:
      --help, -h             Show help
      --add-user, -a=VALUE   Add user information to the database
      --change-passwd-for, -p=VALUE
                             Change user's password in the database
      --reset-auth-for, -r=VALUE
                             Reset SVN auth cache with user's credentials;
                               option argument is user's email; SVN URL
                               required as non-option argument

Note
If ~/bin/git-svn-auth-manager crashes, then this is caused by Mono problems, please update Mono as described above

Secure the key - as encryption key is embedded in git-svn-auth-manager, it needs to be owned by root and be made execute-only (make install took care of the execute-only part already, but let's be extra safe and explicit here):

$ sudo chown root: ~/bin/git-svn-auth-manager
$ sudo chmod 711 ~/bin/git-svn-auth-manager
$ ls -l ~/bin/git-svn-auth-manager
-rwx--x--x 1 root root 697208 Aug 23 17:38 git-svn-auth-manager

Add the git-svn-bridge user for testing (as before, use your actual email address instead of [email protected] and 'git-svn-bridge' as password):

$ ~/bin/git-svn-auth-manager -a git-svn-bridge
Adding/overwriting SVN user git-svn-bridge
SVN password: git-svn-bridge
SVN password (confirm): git-svn-bridge
Email: [email protected]
Name: Git-SVN Bridge

Verify that the database is really encrypted:

$ echo .dump | sqlite3 ~/.config/git-svn-auth-manager/userinfo.db
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
/**** ERROR: (26) file is encrypted or is not a database *****/
ROLLBACK; -- due to errors

Create configuration files and enable email notifications to users for Subversion authentication failures (substitute YOURGMAILPASSWORD with real GMail password, the credentials will be used to authenticate GMail SMTP connections):

$ make install_config
...
install -m 600 -D config-just-enough ~/.config/git-svn-auth-manager/config
$ GITSVNAUTHMGRCONF="$HOME/.config/git-svn-auth-manager/config"
$ sed -i "s/[email protected]/${YOUR_EMAIL}/" "$GITSVNAUTHMGRCONF"
$ sed -i 's/userpassword/YOURGMAILPASSWORD/' "$GITSVNAUTHMGRCONF"

$ cat "$GITSVNAUTHMGRCONF"
<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <appSettings>
    <add key="mail_sending_enabled" value="true" />
    <add key="smtp_username" value="[email protected]" />
    <add key="smtp_password" value="YOURGMAILPASSWORD" />
  </appSettings>
</configuration>

Test that email sending works (the invalid SVN repository URL triggers an error that will cause the email to be sent):

$ ~/bin/git-svn-auth-manager -r ${YOUR_EMAIL} non-existing-path
git-svn-auth-manager: error email sent
git-svn-auth-manager: System.ApplicationException: Error executing `svn info --username "git-svn-bridge" --password "*****" "non-existing-path"`:
svn: 'non-existing-path' is not a working copy

Verify that the error email arrives to your mailbox. It should look like this sample.

synchronize-git-svn.sh

Start by copying the script and sample configuration to ~/bin:

$ cd ~/bin
$ cp ../git/github-git-svn-bridge-utils/scripts/synchronize-git-svn.sh .
$ cp ../git/github-git-svn-bridge-utils/scripts/synchronize-git-svn.sh.config .

And test it all:

$ ./synchronize-git-svn.sh

$ cd ~/git/git-central-repo-clone
$ git pull --rebase
$ echo "void more_do_nothing() { }" >> src/main.cpp
$ git commit -am "Add more_do_nothing() to main.cpp"
[master 0c6e72a] Add more_do_nothing() to main.cpp
 1 file changed, 1 insertion(+)
$ git push
Counting objects: 7, done.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (4/4), 374 bytes, done.
Total 4 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
To /home/git-svn-bridge/git/git-central-repo-trunk.git
   001c5c9..0c6e72a  master -> master

Note
If you see Not a git repository errors during push, then this is caused by problems with some versions of Git, please update Git as described above

We are done with the setup now and will proceed with semi-realistic virtual developer testing in the next section.

Test synchronization

The scenario:

• Alice commits to trunk in Subversion

• Bob commits to trunk in Subversion

• Carol commits a number of changes directly to master and pushes in git (triggers synchronization with the update hook)

• Dave works on the task branch payment-support, merges it to master and pushes in git (triggers synchronization with the update hook)

• Finally, cron triggers synchronization explicitly.

Let's setup the stage:

$ cd ~/test
$ mkdir {alice,bob,carol,dave}

$ for name in alice bob; do
    echo "Use '$name' as password and '[email protected]' as email"
    ~/bin/git-svn-auth-manager -a $name
    pushd $name
    svn --username $name --password $name co $SVN_REPO_URL
    popd
done

$ for name in carol dave; do
    echo "Use '$name' as password and '[email protected]' as email"
    ~/bin/git-svn-auth-manager -a $name
    pushd $name
    git clone ~/git/git-central-repo-trunk.git git-trunk
    cd git-trunk
    git config user.name `~/bin/git-svn-auth-manager $name | sed 's/ <.*//'`
    git config user.email `~/bin/git-svn-auth-manager $name | sed 's/.*<\(.*\)>/\1/'`
    popd
done

$ cat > alice.sh << 'EOT'
#!/bin/bash
pushd alice/trunk
echo 'void alice() { }' >> src/alice.cpp
svn --username alice --password alice up
svn add src/alice.cpp
svn --username alice --password alice ci -m "Protect the global cache with a mutex"
popd
EOT

$ cat > bob.sh << 'EOT'
#!/bin/bash
pushd bob/trunk
echo 'void bob() { }' >> src/bob.cpp
svn --username bob --password bob up
svn add src/bob.cpp
svn --username bob --password bob ci -m "Cache rendered templates"
echo 'void bob2() { }' >> src/bob.cpp
svn --username bob --password bob up
svn --username bob --password bob ci -m "Add tags to articles"
popd
EOT

$ cat > carol.sh << 'EOT'
#!/bin/bash
pushd carol/git-trunk
echo 'void carol1() { }' >> src/carol.cpp
git add src/carol.cpp
git commit -m "Add template tag library"
echo 'void carol2() { }' >> src/carol.cpp
git commit -am "Use template tag library for localized date format"
git pull --rebase
git push
echo 'void carol3() { }' >> src/carol.cpp
git commit -am "Use template filters to represent amounts in localized format"
git pull --rebase
git push
popd
EOT

$ cat > dave.sh << 'EOT'
#!/bin/bash
# dave is working on a task branch
pushd dave/git-trunk
git checkout -b payment-support
echo 'void dave1() { }' >> src/dave.cpp
git add src/dave.cpp
git commit -m "Add payment processing interface"
echo 'void dave2() { }' >> src/dave.cpp
git commit -am "Implement PayPal payments"
echo 'void dave3() { }' >> src/dave.cpp
git commit -am "Implement credit card payments"
git fetch
git rebase origin/master
echo 'void dave4() { }' >> src/dave.cpp
git commit -am "Add storage encryption for payments"
git checkout master
git pull --rebase
git merge --no-ff payment-support
git push
popd
EOT

$ cat > cron.sh << 'EOT'
#!/bin/bash
~/bin/synchronize-git-svn.sh CRON
EOT

$ chmod 755 *.sh

$ cat > Makefile << 'EOT'
all: alice bob carol dave cron
.PHONY: all alice bob carol dave cron

EOT

$ for name in alice bob carol dave cron; do
    echo -e "${name}:\n\t./${name}.sh\n" >> Makefile
done

And now we let our imaginary developers loose to the source control land:

make

Or, to test that mutual exclusion works, run the scripts in parallel:

make -j 5

Finally, shut down svnserve:

make -f ~/svn/Makefile svnserve-stop

Verify that all went well (you should see clean history according to the examples in the Overview section):

$ cd ~/svn/svn-checkout
$ svn up
$ svn log

$ cd ~/git/git-central-repo-clone
$ git pull --rebase
$ git log

Test setting up repos for other branches

$ make -f ~/svn/Makefile svnserve-start

$ cd ~/svn/svn-checkout
$ svn mkdir branches
$ svn cp trunk branches/1.x
$ svn ci -m "Branch trunk to 1.x"

$ cd ~/git
$ ./github-git-svn-bridge-utils/scripts/setup-svn-branch-git-bridge.sh
$ git clone central-repo-1.x.git central-repo-1.x-clone
$ cd central-repo-1.x-clone
$ git log
commit 095e7a01f102f79224df4283a67c4624986679a1
Author: [email protected] <[email protected]>
Date:   Sun Aug 26 18:38:34 2012 +0000

    Branch trunk to 1.x

    git-svn-id: svn://localhost/branches/1.x@10 1db59d55-421c-46dd...

$ make -f ~/svn/Makefile svnserve-stop