How to debug dependencies in Go projects

June 19, 2024

Debugging dependencies

If you want to debug how a dependency is used in a Go project run go mod vendor to download all your projects dependencies to a vendor folder in your project. You can then add breakpoints to the vendored code just like your project code.

Replacing dependencies with your forks

If you’re working on a contribution to a dependency (for example fixing a bug you’ve discovered) then you might want to test that out locally with your project. To do this you can use the replace keyword in your go.mod file.

Today I wanted to test a change to how sweepers are filtered in the hashicorp/terraform-plugin-sdk module, as part of my work on the Google Terraform provider. The snippet below shows how I replaced use of hashicorp/terraform-plugin-sdk with my fork of that module containing a change. In the go,mod file I specify a commit containing an idea for a contribution/bug fix (d188bcc). That commit SHA is used below:

module github.com/hashicorp/terraform-provider-google-beta

go 1.21

require (
  ...
	github.com/hashicorp/terraform-plugin-sdk/v2 v2.33.0
  ...
)

// Use my fork of the hashicorp/terraform-plugin-sdk repository instead
replace github.com/hashicorp/terraform-plugin-sdk/v2 => github.com/SarahFrench/terraform-plugin-sdk/v2 d188bcc3c75e66bd775b98fc512a5706ccf930e8

After a quick go mod tidy this is changed to a valid reference, which in my case seems to take advantage of exissting tags in the repository; your milage may vary:

- replace github.com/hashicorp/terraform-plugin-sdk/v2 => github.com/SarahFrench/terraform-plugin-sdk/v2 d188bcc3c75e66bd775b98fc512a5706ccf930e8
+ replace github.com/hashicorp/terraform-plugin-sdk/v2 => github.com/SarahFrench/terraform-plugin-sdk/v2 v2.34.1-0.20240619163930-d188bcc3c75e

Debugging the replaced dependencies

After adding replace to your go.mod file you can run go mod vendor again. In the vendor folder the directory structure will use the replaced dependency’s name (vendor/github.com/hashicorp/terraform-plugin-sdk/… in my case) but the actual code inside will be the contents of your fork!

Using these steps I was able to find my contribution’s regex needed to be updated to account for a suffix present on sweeper names, which is used internally by the SDK code.