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Learn to Use replace in go.mod

📺 https://youtu.be/k_nx_zPf84A

The replace keyword in go.mod allows you to work on several packages, with different repos, at the same time, or, to test changes locally before pushing them.

Unfortunately, it is rather easy to leave the replace in your go.mod file and forget and commit it. Until rather recently, that meant other projects depending on your module would fail to compile trying to locate your local path on their build machines. These days go won’t even compile or install the module if there is a replace in the go.mod file.

:warning Leaving replace in a go.mod file will force a failure of go install as of Go 1.17. The Kubernetes kompose project is current failing because of this

$ go install github.com/kubernetes/kompose@latest
go install: github.com/kubernetes/kompose@latest (in github.com/kubernetes/kompose@v1.26.1):
	The go.mod file for the module providing named packages contains one
  or more replace directives. It must not contain directives that 
  would cause it to be interpreted differently than if it were the 
  main module.

Even so, committing to a production repo with replace could be embarrassing so might want to add the following .git/hooks/pre-commit hook (don’t forget to chmod +x) to prevent any commit of go.mod that has the word replace in it, which you could easily modify to check that it isn’t commented out (// replace should be allowed). This script checks for jq and if not found uses grep instead:

#!/bin/bash

jq=$(command -v jq)
if [[ -n "$jq" ]] ; then
  rv="$(go mod edit -json | $jq .Replace)"
  [[ $rv = 'null' ]] && exit 0
  echo "replace found in go.mod"
  exit 1
fi

git diff --cached go.mod | grep replace && exit 1

Tags:

#golang #tips #replace

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