justanotherdot •

Ryan James Spencer

What makes a good pull request?

on Fri 20 Jul 20:40:08 AEST 2018

Pull Requests (or PRs) are a tango between two parties; the code author and the code reviewer which I will simply refer to as the 'author' and 'reviewer' in the remainder of this article. In a pull request, the author has provided code to solve a particular problem and the reviewer is there to provide a feedback mechanism to the author.

Code review is not a gate keeping task

I once worked for an organisation whose code review process centered around a total lack of faith in its developers ability to deliver quality product; tech leads acted as the gate keepers to their respective stacks forcing the average developer to resort to underhanded tactics in order to get their changes merged, regardless of quality or prospective bugs. This, in turn, meant the gate keepers felt justified under the guise of 'keeping things safe'. Thus, code wasn't being checked properly and wrong or flimsy changes would trickle into master, making the code review process utterly broken.

As a software engineer, every line of code you ship is code you, or someone else, will need to maintain, and as such, you should be fighting to deliver the best quality you can offer, regardless of deadline. Code reviewers are there to help people bring their code into the light of day where asking questions is the chief tool a reviewer employs. This can include, but is not limited to, probing to see if:

  • Is the thought fully fleshed out?
  • Is this implementation correct for the problem it aims to solve?
  • Are the changes sound and principled?
  • Are there any performance or semantic concerns?

I'm purposefully leaving out stylistic choices here as the discussion often leads to the argument around adoption of some automated code-formatting tool.

Raise early and raise often

In "Debugging Team's", the authors kick off the start of the book with a simple analogy between two competing inventors. The one inventor does not want to share his ideas for fear of them being poached by others, while the rival inventor gleefully goes to local places where experts might hang out to get more information about how to build her inventions.

Software development is no different in that knowing things earlier is always better than knowing things later. Raising PRs even before they are 'complete' (and appropriately marking them as WIPs or 'works in progress') allows people to possibly, time permitting, look into your changes and see if there are any major red flags.

That said, if you are a reviewer and are asked (or not!) to look at a set of changes that is marked as a WIP, try to hold off on a more in-depth review until the author changes this status. And to PR raisers, don't keep things in WIP stage for too long, which brings me to my next point.

PRs are for small chunks of code to merge often

A PR should represent up to a day's worth of work. This is beneficial to both parties in that it facilitates a 'merge often' approach for devs (and devs get the little adrenaline kick from clicking that green merge button) and reviewers can much more easily review a smaller hunk of changes. A reviewer reviewing five PRs in the course of a week has to spend less time grokking those individual changes than to review five days worth of work in a single PR.

Massive projects poised around scaled tooling and reviews such as the Linux kernel would flat out reject a patch with, say, 3k additions and 1k removals. If a project of that size and calibre, and that many international hands involved, is marking code review of those dimensions as 'unmanageable', what hope does a startup have at making fast, rapid changes in the same light?

Small PRs are also focused on a clear intent. Asking the author to fix neighboring code 'just because' or refactoring/formatting several adjacent files that are not directly tied to the immediate effort of the PR wastes the both parties time. Opening a PR to refactor changes and another PR to add new functionality is a much better way to get appropriate attention from reviewers. As the joke goes:

10 lines = 10 possible bugs, 100 lines = lgtm

It's important to remember that a PR is not to encompass a single ticket or issue. Tickets can have several PRs attached to them and all it takes is lobbing [FOO-123] on top of one's PR title for Jira or marking #<issue number> in your description in GitHub. I like to call this act 'linking' and it's useful for stakeholders to track down all the changes that have fed into a particular ticket.

Context matters

Reviewers need to discuss with the author about the purpose of a set of changes and how close or far off they are from that goal, but if the reviewer is unclear about this goal, it's difficult for them to strike up a discussion with the author.

In the context of OSS, raising a patch directly to a project such as the Linux kernel is poor practice. If you want to make a change in any capacity it's best first to contact the people who own the code on public channels. This provides auditing and clear context for others. Your first instinct should be to raise a PR unless it's a feature. If you feel uncertain about whether or not your change is warranted, it's best to raise an issue first, instead.

That said, raising PRs should feel natural; PRs are cheap and can be closed and their branches pruned as need be, but regardless of the cost of raising a PR, it's critical to include appropriate information. Some important things to mention may be:

  • What does this set of changes solve?
  • Is there a specific task (issue/ticket) that this relates to?
  • Is this blocked or blocking any other PRs/issues/tickets?
  • Is there any additional information that will help the reviewer know about my manual testing of this ticket (screenshots, output from tooling, et. al.)?
  • Have you updated tests and documentation accordingly? Have you added tests that the reviewer can skip to first to immediately see how you're proposed changes are supposed to work and in what cases?
  • Is there current behaviour to contrast the new behaviour to?
  • Are there breaking changes present?

The traditional approach for this was to include commentary in your actual commit messages and headers. I don't think times have really changed in this regard and the more context you sprinkle about the better, so long as you are clear about your intent and you don't waste the reader's time.

Check out changes locally when it makes sense

A really healthy habit for reasonably sized changes is to always check out a PR and see if it works for you. Some projects have powerful processes for testing full 'e2e/raw hardware' scenarios such as Intel's zero-day testing bot which actually boots up machines to test out differing version of the Linux kernel. While continuous integration can catch a lot, it's important to sometimes get a human eye for regressions that may not, or cannot, be encoded in automated tests. We all make mistakes, and reviewers are there to add a layer of sanity checks to our changes.

Use common sense. If a change is pretty sensible (e.g. a single line change to update a variable name), you probably don't need to spend the time pulling the change down, compiling, running the tests, and so forth. As the developer's mantra goes, "Don't be the machine"!

Clean up your mess

Your changes have been merged and you can go on with your life, but before you reach for beverage of choice, you should prune your dead branches. I actually have a git bash script for this that I place in my PATH so I can call it as git wash. The script is here. When run without arguments, this will delete the current branch you are on locally and remotely so long as the branch specified to git wash is not master and the remote branch is not protected. If you need other git functionality like this, any script in your path with the name git-<thing> can be run as git <thing>.


Reviewing many small changes is much more manageable for a reviewer than reviewing large, tangled changes. Decomposition in programming gives us the ability to stitch together many small, verified solutions that lead up to an equally trustworthy result and the same is no different in the process of supplying code changes to a project; breaking up the changes you need to make into reasonable chunks that can fit in everyone's heads not only helps to provide code changes faster but it also paves foundations for robust and resilient code.