Code Review Guidelines

Getting your merge request reviewed, approved, and merged

There are a few rules to get your merge request accepted:

Your merge request should only be merged by a maintainer.
If your merge request includes only backend changes [^1], it must be approved by a backend maintainer.
If your merge request includes only frontend changes [^1], it must be approved by a frontend maintainer.
If your merge request includes frontend and backend changes [^1], it must be approved by a frontend and a backend maintainer.
If your merge request includes a new dependency or a filesystem change, it must be approved by a Build team member. See how to work with the Build team for more details.
To lower the amount of merge requests maintainers need to review, you can ask or assign any reviewers for a first review.
If you need some guidance (e.g. it's your first merge request), feel free to ask one of the Merge request coaches.
The reviewer will assign the merge request to a maintainer once the reviewer is satisfied with the state of the merge request.

For more guidance, see CONTRIBUTING.md.

Best practices

This guide contains advice and best practices for performing code review, and having your code reviewed.

All merge requests for GitLab CE and EE, whether written by a GitLab team member or a volunteer contributor, must go through a code review process to ensure the code is effective, understandable, and maintainable.

Any developer can, and is encouraged to, perform code review on merge requests of colleagues and contributors. However, the final decision to accept a merge request is up to one the project's maintainers, denoted on the engineering projects.

Everyone

Accept that many programming decisions are opinions. Discuss tradeoffs, which you prefer, and reach a resolution quickly.
Ask questions; don't make demands. ("What do you think about naming this :user_id?")
Ask for clarification. ("I didn't understand. Can you clarify?")
Avoid selective ownership of code. ("mine", "not mine", "yours")
Avoid using terms that could be seen as referring to personal traits. ("dumb", "stupid"). Assume everyone is attractive, intelligent, and well-meaning.
Be explicit. Remember people don't always understand your intentions online.
Be humble. ("I'm not sure - let's look it up.")
Don't use hyperbole. ("always", "never", "endlessly", "nothing")
Be careful about the use of sarcasm. Everything we do is public; what seems like good-natured ribbing to you and a long-time colleague might come off as mean and unwelcoming to a person new to the project.
Consider one-on-one chats or video calls if there are too many "I didn't understand" or "Alternative solution:" comments. Post a follow-up comment summarizing one-on-one discussion.
If you ask a question to a specific person, always start the comment by mentioning them; this will ensure they see it if their notification level is set to "mentioned" and other people will understand they don't have to respond.

Having your code reviewed

Please keep in mind that code review is a process that can take multiple iterations, and reviewers may spot things later that they may not have seen the first time.

The first reviewer of your code is you. Before you perform that first push of your shiny new branch, read through the entire diff. Does it make sense? Did you include something unrelated to the overall purpose of the changes? Did you forget to remove any debugging code?
Be grateful for the reviewer's suggestions. ("Good call. I'll make that change.")
Don't take it personally. The review is of the code, not of you.
Explain why the code exists. ("It's like that because of these reasons. Would it be more clear if I rename this class/file/method/variable?")
Extract unrelated changes and refactorings into future merge requests/issues.
Seek to understand the reviewer's perspective.
Try to respond to every comment.
Let the reviewer select the "Resolve discussion" buttons.
Push commits based on earlier rounds of feedback as isolated commits to the branch. Do not squash until the branch is ready to merge. Reviewers should be able to read individual updates based on their earlier feedback.

Reviewing code

Understand why the change is necessary (fixes a bug, improves the user experience, refactors the existing code). Then:

Try to be thorough in your reviews to reduce the number of iterations.
Communicate which ideas you feel strongly about and those you don't.
Identify ways to simplify the code while still solving the problem.
Offer alternative implementations, but assume the author already considered them. ("What do you think about using a custom validator here?")
Seek to understand the author's perspective.
If you don't understand a piece of code, say so. There's a good chance someone else would be confused by it as well.
After a round of line notes, it can be helpful to post a summary note such as "LGTM 👍", or "Just a couple things to address."
Assign the merge request to the author if changes are required following your review.
Set the milestone before merging a merge request.
Avoid accepting a merge request before the job succeeds. Of course, "Merge When Pipeline Succeeds" (MWPS) is fine.
If you set the MR to "Merge When Pipeline Succeeds", you should take over subsequent revisions for anything that would be spotted after that.
Consider using the Squash and merge feature when the merge request has a lot of commits.

The right balance

One of the most difficult things during code review is finding the right balance in how deep the reviewer can interfere with the code created by a reviewee.

Learning how to find the right balance takes time; that is why we have reviewers that become maintainers after some time spent on reviewing merge requests.
Finding bugs and improving code style is important, but thinking about good design is important as well. Building abstractions and good design is what makes it possible to hide complexity and makes future changes easier.
Asking the reviewee to change the design sometimes means the complete rewrite of the contributed code. It's usually a good idea to ask another maintainer or reviewer before doing it, but have the courage to do it when you believe it is important.
There is a difference in doing things right and doing things right now. Ideally, we should do the former, but in the real world we need the latter as well. A good example is a security fix which should be released as soon as possible. Asking the reviewee to do the major refactoring in the merge request that is an urgent fix should be avoided.
Doing things well today is usually better than doing something perfectly tomorrow. Shipping a kludge today is usually worse than doing something well tomorrow. When you are not able to find the right balance, ask other people about their opinion.

GitLab-specific concerns

GitLab is used in a lot of places. Many users use our Omnibus packages, but some use the Docker images, some are installed from source, and there are other installation methods available. GitLab.com itself is a large Enterprise Edition instance. This has some implications:

Query changes should be tested to ensure that they don't result in worse performance at the scale of GitLab.com:
Generating large quantities of data locally can help.
Asking for query plans from GitLab.com is the most reliable way to validate these.
Database migrations must be:
Reversible.
Performant at the scale of GitLab.com - ask a maintainer to test the migration on the staging environment if you aren't sure.
Categorised correctly:
- Regular migrations run before the new code is running on the instance.
- Post-deployment migrations run after the new code is deployed, when the instance is configured to do that.
- Background migrations run in Sidekiq, and should only be done for migrations that would take an extreme amount of time at GitLab.com scale.
Sidekiq workers cannot change in a backwards-incompatible way:
Sidekiq queues are not drained before a deploy happens, so there will be workers in the queue from the previous version of GitLab.
If you need to change a method signature, try to do so across two releases, and accept both the old and new arguments in the first of those.
Similarly, if you need to remove a worker, stop it from being scheduled in one release, then remove it in the next. This will allow existing jobs to execute.
Don't forget, not every instance will upgrade to every intermediate version (some people may go from X.1.0 to X.10.0, or even try bigger upgrades!), so try to be liberal in accepting the old format if it is cheap to do so.
Cached values may persist across releases. If you are changing the type a cached value returns (say, from a string or nil to an array), change the cache key at the same time.
Settings should be added as a last resort. If you're adding a new setting in gitlab.yml:
Try to avoid that, and add to ApplicationSetting instead.
Ensure that it is also added to Omnibus.
Filesystem access can be slow, so try to avoid shared files when an alternative solution is available.

Credits

Largely based on the thoughtbot code review guide.

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