When I started on the Firebird team at Bazaarvoice, I was happy to learn that they host their code on GitHub and review and land changes via pull requests. I was less happy to learn that they merged pull requests with the big green button. I was able to convince the team to try out a new, rebase-oriented, workflow that keeps the mainline branch linear and clean. While the new workflow was a hit with the team, it was much more complicated than just clicking a button, so I automated the workflow with a simple git extension, git land, which we have released as an open source tool.
What’s Wrong With the Big Green Button?
The big green button is the “Merge pull request” button that GitHub provides to merge pull requests. Clicking it prompts the user to enter a commit message (or accept a default provided by GitHub) and then confirm the merge. When the user confirms the merge, the pull request branch is merged using the –no-ff option, which always creates a merge commit. Finally, GitHub closes the pull request.
For example, given a master branch like this:
…and a feature branch that diverges from the second commit:
…this is the result of doing a –no-ff merge:
Merging with the big green button is frowned upon by many; for detailed discussions of why this is, see Isaac Z. Schlueter and Benjamin Sandofsky. In addition to the problems with merge commits that Isaac and Benjamin point out, the big green button has another downside: it merges the pull request without an opportunity to squash commits or otherwise clean up the branch.
This causes a couple of problems. First, because only the pull request author can clean up the PR branch, merging often became a tedious and drawn out process as reviewers cajoled the author to update their branch to a state that would keep `master`’s history relatively clean. Worse, sometimes messy pull requests were hastily or mistakenly merged.
As a result, the team was encouraged to keep their pull requests squashed into one or two clean commits at all times. This solved one problem, but introduced another: when an author responds to comments by pushing up a new version of the pull request, the latest changes are squashed together into one or two commits. As a result, reviewers had to hunt through the entire diff to ensure that their comments were fully addressed.
An Alternate Workflow
After some lively discussion, the team adopted a new workflow centered on fast-forward merging squashed and rebased pull request branches. Developers create topic branches and pull requests as before, but when updating their pull request, they never squash commits. This preserves detailed history of the changes the author makes in response to review feedback.
When the PR is ready to be merged, the merger interactively rebases it on the latest master, squashes it down to one or two commits, and does a fast-forward merge. The result is a clean, linear, and atomic history for `master`.
One hiccup is that GitHub can’t easily tell that the rebased and squashed commit contains the changes in the pull request, so it doesn’t close the PR automatically. Fortunately, GitHub will close pull requests that contain special keywords. So, the merger has a final task: adding “[closes #<PR number>]” to one of the squashed commit’s message.
The biggest downside to the new workflow is that it transformed merging a PR from a simple operation (pushing a button) to a somewhat tricky multi-step process:
- update local master to latest from upstream
- check out pull request branch
- do an interactive rebase on top of master, squashing down to one or two commits
- add “[closes #<PR number>]” to the last commit message for the most recent squashed commit
- do a fast-forward merge of the pull request branch into master
- push local master to upstream
This process was too lengthy and error-prone to be reliable unless automated. To address this problem, I created a simple git extension: git-land. The Firebird team has been using this tool for a little over a year with very few problems. In fact, it has spread to other teams at Bazaarvoice. We are excited to release it as an open source tool for the public to use.