Testing go.mod Tidiness in CI

Updated August 18 per discussion on /r/golang. Thanks to user Bake_Jailey for noting that running go mod tidy can do more than just remove unneeded module requirements.

Now that Go modules are a thing, I’m starting to use them for my Go projects. So far it’s been a nice improvement from dep and before that, godep.

With Go modules you end up with two files in your repo, go.mod and go.sum. The former file stores a list of your dependencies. The go binary will automatically keep these files up to date when you run things like go build. Or at least mostly up to date. When you remove a dependency it does not get automatically removed from the go.mod file. Instead, you need to run go mod tidy to remove unneeded deps from this file. And of course if you haven’t run go build yet, then a new dependency won’t be added at commit time.

Unfortunately, go mod tidy always exits with a 0, even if it added or removed a dependency. So if you want to test that this file is up to date in CI, you need to do it yourself. Here’s my little script for doing so:

set -e
go mod tidy
STATUS=$( git status --porcelain go.mod go.sum )
if [ ! -z "$STATUS" ]; then
    echo "Running go mod tidy modified go.mod and/or go.sum"
    exit 1
exit 0

This runs go mod tidy and checks to see if it modified either of the files we care about.

Of course, the right way to do this is to add an option to go mod tidy to do these sorts of checks. See https://github.com/golang/go/issues/27005 for a discussion of exactly that.

You can add this as a script step in your .travis file:

  - go test -v ./…
  - check-go-mod.sh