Strict Merge

The strict merge workflow is a workflow that prevents merging broken pull requests. That situation can arise when outdated pull requests are being merged in their base branch.

Understanding the Problem

To understand what the problem is, imagining the following situation: the base branch (e.g., master) has its continuous integration testing passing correctly. A passing pull request is created then. The state of the repository can be represented like this:

Pull request is open

While the pull request stays open, another commit is pushed to master. That can be a local commit or a merge commit from another pull request; who knows. The tests are run against master and they pass. The state of the continuous integration system is now reflected as this:

Base branch adds a new commit

The pull request is still marked as valid by the continuous integration system since it did not change. Therefore, the pull request is considered as _mergeable_: the merge button is green.

If you click that merge button, this is what might happens:

Base branch is broken

As a new merge commit is created to merge the pull request, it is possible that the continuous integration testing will fail. Indeed: a new test might have been added in master while the pull request was open, and that pull request may not have the correct code to pass this new test.

By pressing the merge button, you just broke your continuous integration system, and probably your software. Good work.

The Strict Workflow Solution

The strict workflow solves that issue by updating any pull request that is not up-to-date with its base branch before being merged. That forces the continuous integration system to test again the pull request with the new code.

In the previous example, the continuous integration system would have marked the pull request as failing the test, removing it from the mergeable candidate.

Rebase make CI fails

When the strict workflow is enabled, Mergify takes care of merging the target branch in any pull request that is not up-to-date with its target branch. If multiple pull requests are mergeable, they are scheduled to be merged sequentially, and they will be updated on top of each other.

The pull request branch update is only done when the pull request is ready to be merged by the engine, e.g., when the status checks are OK and the number of postive reviews are enough.

Enabling the Strict Workflow

To enable the strict workflow, you need to set the value of strict to true in the merge action.

To enable it on a pull request, you can write a mergify.yml like this:

  - name: automatic merge with strict
      - "#approved-reviews-by>=2"
      - status-success=continuous-integration/travis-ci/pr
        method: merge
        strict: true