Random things I work on

From Mehdi Abaakouk (sileht)

Automate rebasing and merging of your PR with pastamaker

The story of pastamaker

Gnocchi project recently moved from the OpenStack infrastructure to GitHub.

The OpenStack infrastructure provides a very good set of tools for managing/reviewing/testing projects. To do so, they use Gerrit to review patches and Zuul to schedule testing and checking jobs. Zuul, the gating system, ensures that no commit failling the unit and functional tests are merged.

When we moved to GitHub, we obviously switched to the Pull Request system, instead of using Gerrit. We also switched ot Travis-CI to run tests. But the workflow around Pull Request and Travis-CI is not as good as Zuul was.

First, Travis-CI only tests the head of the Pull Request branch whatever the base branch commit is. The base branch commit of a Pull Request is never the last commit of your base branch. At this point, if you click on the GitHub Merge button you can’t known if the resulting merge or rebase will pass tests or not.

GitHub offers a branch merging protection call: “Require branches to be up to date before merging”. That means that each times a Pull Request is merged, all Pull Requests must be rebased… But nothing in GitHub allows to automate that. They just offer a “Update branch” button you have to click manually.

For the Gnocchi project, after some weeks of using GitHub, we realized that all reviewers had to talk about which PR should be merged next. Then one of them would have to take care of it, click on “Update branch”, then wait ~ 30~40 minutes for Travis-CI to post the new result and click on the Merge Button. So much time lost, especially when you come from OpenStack Infrastructure and Zuul is doing all of that for you.

That why we ended up writing a GitHub bot to manage this administrative work for us. It uses new GitHub Apps API. The code is under “Apache License v2” and it’s called pastamaker.

How it works

Pastamaker in composed of three small apps:

  • The engine triggered by GitHub events.
  • A dashboard of ongoing pull requests to merge for all registered projects.

The engine workflow

On each GitHub event, Pastamaker looks at the pull request information in the event to retrieve the GitHub project and branch. Then it builds the list of open Pull Requests for this branch. For each pull request, it retrieves the CI status, the reviews, and if the pull request is up to date with the base branch. With this information it computes a weight. This naturally creates a priority queue of pull requests to merge. For example, the rule for computing the weight are “do we have enough core reviewers approvals?”, “Does the CI pass?”, “Is the pull request conflicting?”, etc.

Some rules are customisable by project/branch with a configuration file, like the number of required core reviewers.

When the queue is ready, Pastamaker stores it in Redis so it’s accessible from API and from the dashboard. Then it processes the first element of the queue. If everything is OK, it merges the patch. If the patch needs to be rebased, it uses the GitHub “Update branch” button.

The engine could be smarter and just do special action depending on the event type and the pull request state. But for now, I decided to keep it simple to avoid any race condition. The queue ordering is stable whatever the order of events is, whatever are the human interaction on pull requests. Pastamaker is also designed stateless: everything is built from information given by GitHub Events and GitHub API.


The REST API passes the HTTP GitHub events to the engine through Redis, serves all the static files for the dashboard. It allows to retrieve the queues built by the engine, or to subscribe a stream of the queues update. You can also trigger the initial built of the queue for all registered projects.

The dashbord

It just displays the pull requests list of all registered projects with the weight of the Pull Requests. An example of the dashboard:

Pastamaker dasbhoard

Install your own

pastamaker offers helpers to be deployed on Heroku. For the Gnocchi project, we currently use only the free services offered by Travis-CI, Heroku and GitHub and it works well for the size of our project.

The setup steps are basically:

  • Create a GitHub App
  • Add your projects to this GitHub App
  • Configure branch protection on your project (this may be done automatically by pastamaker one day)
  • Install pastamaker somewhere

You’re ready, pastamaker will start to receive your projects events.

More information can be found here