Pre-tested commits with Hudson and Git 31 Dec 2009
software development git hudson

A few months ago Kohsuke, author of the Hudson continuous integration server, introduced me to the concept of the “pre-tested commit”, a feature of the TeamCity build management and continuous integration system. The concept is simple, the build system stands as a roadblock between your commit entering trunk and only after the build system determines that your commit doesn’t break things does it allow the commit to be introduced into version control, where other developers will sync and integrate that change into their local working copies. The reasoning and workflow put forth by TeamCity for “pre-tested commits” is very dependent on a centralized version control system, it is solving an issue Git or Mercurial users don’t really run into. Those using Git can commit their hearts out all day long and it won’t affect their colleagues until they merge their commits with others.

In some cases, allowing buggy or broken code to be merged in from another developer’s Git repository can be worse than in a central version control system, since the recipient of the broken code might perform a knee-jerk git-revert(1) command on the merge! When you revert a merge commit in Git, what happens is you not only revert the merge, you revert the commits associated with that merge commit; in essence, you’re reverting everything you just merged in when you likely just wanted to get the broken code out of your local tree so you could continue working without interruption. To solve for this problem-case, I utilize a “pre-tested commit” or “pre-tested merge” workflow with Hudson.

My workflow with Hudson for pre-tested commits involves three separate Git repositories: my local repo (local), the canonical/central repo (origin) and my “world-readable” (inside the firewall) repo (public). For pre-tested commits, I utilize a constantly changing branch called “pu” (potential updates) on the world-readable repo. Inside of Hudson I created a job that polls the world-readable repo (public) for changes in the “pu” branch and will kick off builds when updates are pushed. Since the content of public/pu is constantly changing, the git-push(1) commands to it must be “forced-updates” since I am effectively rewriting history every time I push to public/pu.

To help forcefully pushing updates from my current local branch to public/pu I use the following git alias:

% git config alias.pup "\!f() { branch=\$(git symbolic-ref HEAD | sed 's/refs\\/heads\\///g');\
      git push -f \$1 +\${branch}:pu;}; f"

While a little obfuscated, thie pup alias forcefully pushes the contents of the current branch to the specified remote repository’s pu branch. I find this is easier than constantly typing out: git push -f public +topic:pu

In list form, my workflow for taking a change from inception to origin is:

  • hack, hack, hack
  • commit to local/topic
  • git pup public
  • Hudson polls public/pu
  • Hudson runs potential-updates job
  • Tests fail?
    • Yes: Rework commit, try again
    • No: Continue
  • Rebase onto local/master
  • Push to origin/master

Using this pre-tested commit workflow I can offload the majority of my testing requirements to the build system’s cluster of machines instead of running them locally, meaning I can spend the majority of my time writing code instead of waiting for tests to complete on my own machine in between coding iterations.