A mergebot for pull-request review
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@TravisCI @AelitaBot @Gitter

The Not Rocket Science Rule Of Software Engineering

Automatically maintain a repository of code that always passes all the tests.

-- Graydon Hoare (summarizing Ben Elliston)

Aelita is an implementation of this rule on top of pull requests and a traditional CI system such as Travis or Jenkins. Contributors make a pull request (obviously) and reviewers leave a magic comment like @aelita-mergebot r+.

This program is very experimental at this point, but it's already in production-esque use as @cerebrum-mergebot.

But don't GitHub's Protected Branches already do this?

Travis and Jenkins both run the test suite on every branch after it's pushed to and every pull request when it's opened, and GitHub can block the pull requests if the tests fail on them. To understand why this is insufficient to get an evergreen master, imagine this:

  • Pull Request #1: Rename bifurcate() to bifurcateCrab()

    Change the name of this function, as well as every call site that currently exists in master. I've thought of making it a method on Crab instead of on Sword, but then it would be bifurcateWithSword(), which hardly seems like an improvement.

  • Pull Request #2: bifurcate() after landing, in addition to before

    Adds another call to bifurcate(), to make sure it gets done even if we skip the pre-landing procedure.

When both of these pull requests are sitting open in the backlog, they will both be tested against master. Assuming they both pass, GitHub will happily present the Big Green Merge Button. Once they both get merged master will go red (Method bifurcate() not found).

In addition to the testing requirements, GitHub can also be set to block pull requests that are not "up to date" with master, meaning that problems like this can show up. This fixes the problem, by requiring that master only contain a snapshot of the code that has passed the tests, but it requires maintainers to manually:

  1. "Update the pull requests," merging them onto master without changing master itself
  2. Wait for the test suite to finish
  3. Merge the pull request when it's done, which is a trivial operation that can't break the test suite thanks to step 1

And it has to be done for every pull request one at a time.

This is essentially the process that aelita automates. Instead of merging, you add reviewed pull requests to a "merge queue" of pull requests that are tested against master by copying master to a staging branch and merging into that. When the status of staging is determined (either pass or fail), aelita reports the result back as a comment and merges staging into master if it was a pass. Then it goes on to the next one.

Note that aelita is not a replacement for Jenkins or Travis. It just implements this workflow.


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