Bluish Coder

Programming Languages, Martials Arts and Computers. The Weblog of Chris Double.


2011-10-28

Performing Compatible Updates of Mozilla Central Git Forks

Earlier this year I posted about how I created my git mirror of the mozilla-central mercurial repository. I've been keeping a fork on github updated regularly since then that a number of people have started using.

Users of my mirror have wanted to be able to keep up to date with the mercurial mozilla-central repository themselves, or add updates from other branches of the Mozilla source. It takes a large amount of time to start a conversion from scratch but it's possible to start from the existing git mirror, perform the incremental updates from mercurial yourself, and still stay compatible with the incremental updates I push to my github mirror.

hg-git uses a git-mapfile in the .hg directory of the mercurial clone to keep track of the mapping between mercurial and git commits. By using the same git-mapfile that I use for my mirror you can start your own incremental update from the latest data in the git-mapfile instead of from the beginning. I keep an up-to-date git-mapfile in git-mapfile.bz2. It's compressed with bzip2 as the uncompressed version is quite large.

The steps to start your own update process are:

  1. Clone the mercurial mozilla-central repository
  2. Clone my git mirror as a bare repository in .hg/git
  3. Place the git-mapfile in .hg
  4. Do an hg bookmark -f -r default master to mark the commit to convert to
  5. Perform hg gexport to update .hg/git with recent mercurial commits

The .hg/git directory should now be up-to-date with respect to the mercurial repository. And the additional git commits will have the same SHA id as any commits I push into my mirror when I perform my own update.

The steps to do an incremental update are the normal:

  1. Pull from the mercurial mozilla-central repository
  2. Run hg bookmark -f -r default master
  3. Run hg gexport
  4. Push or pull to/from .hg/git as needed

As a working example, the following shell commands on a Linux system should set up your own repository ready for incremental updating:

$ hg clone https://hg.mozilla.org/mozilla-central
$ cd mozilla-central/.hg
$ git clone --bare git://github.com/doublec/mozilla-central.git git
$ wget http://www.bluishcoder.co.nz/git-mapfile.bz2
$ bunzip2 git-mapfile.bz2
$ cd ..
$ hg bookmark -f -r default master
$ hg gexport
$ cd .hg/git
$ git push --all ~/some/other/repo.git

Incremental updates just repeat the last few commands above:

$ hg pull -u
$ hg bookmark -f -r default master
$ hg gexport
$ cd .hg/git
$ git push --all ~/some/other/repo.git

Note that I use hg gexport and push from the .hg/git repository using the git command instead of doing an hg push and relying on hg-git to do the conversion and push. In my original article I did the latter. The hg-git push method uses slower python based routines to do the push which can take a long time and large amounts of memory on big repositories like mozilla-central. By splitting this up into gexport and using the native git command I save a lot of time and memory.

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