Always Bring Your Pull Requests Up to Date

A Pull Request (PR) should always be up to date with the branch into which it is being merged. Another way of saying this is you should be able to merge your PR as a fast-forward merge (even if you decide not to do a fast-forward merge).

The Danger of Merging a PR that is Not Up To Date

If your PR is not up to date, you may get unexpected results after your merge.

Example of a Dangerous PR

I’ve created a GitHub repository, salcode/catastrophic-pr, which contains one PR.

If you check out the main branch, you’ll find everything looks good.

If you check out the PR branch (increase-heat-strain), you’ll find everything looks good.

If you merge the PR, you’ll find the code fails catastrophically.

How Did this Happen?

The increase-heat-strain branch was created from the main branch and a new commit was added “Increase heat strain to 5”.

Before increase-heat-strain was merged back into main a new commit (“Increase weight strain to 6”) was added to main.

The result is the main branch is still at a safe level (with “Increase weight strain to 6” added) and the increase-heat-branch is a safe level (because it does not have “Increase weigh strain to 6”).

Our PR to merge increase-heat-strain into main doesn’t have any merge conflicts because the values were incremented in two different areas.

When merged our PR, a merge commit (rather than a fast-forward commit) is created. The merge commit brings both branches together, so while each branch only had one of the incrementing commits – after our merge we now have both incrementing commits (“Increase heat strain to 5” and “Increase weight strain to 6”).

How it Should Have Been Handled

The increase-heat-strain branch should have been brought up to date with main before merging it into main.

This could have been handled by switching to the increase-heat-strain branch and either:

a. Merging main into the branch (with git merge main) or
b. Rebasing (with git rebase main)

Sal Ferrarello (@salcode)

Sal is a PHP developer with a focus on the WordPress platform. He is a conference speaker with a background including Piano Player, Radio DJ, Magician/Juggler, Beach Photographer, and High School Math Teacher. Sal can be found professionally at WebDevStudios, where he works as a senior backend engineer.