Add OpenTofu support

[brikis98]

Describe the solution you'd like
As recently announced, the OpenTofu fork should drop soon. We should make sure that Terragrunt supports it out of the box.

Here's an idea for an initial implementation: anywhere Terragrunt calls out to the terraform binary, we should update it to check for the terraform binary first; if that's not found, then try the tofu binary; and if that's not found, show an error (as we do today). Whatever we pick, we should mention in the debug log.

Describe alternatives you've considered
In theory, you can already use OpenTofu by using the terraform_binary config or --terragrunt-tfpath CLI flag or TERRAGRUNT_TFPATH env var, but those all require extra steps, whereas I'd like OpenTofu to "just work" with Terragrunt.

Additional context
OpenTofu should work as a "drop in" replacement for Terraform, so in theory, no other changes should be necessary. However, we should run our tests to make sure once the tofu binary is available for testing. One place where OpenTofu will differ is stdout and stderr output will say OpenTofu instead of Terraform; if Terragrunt is parsing that log output anywhere, we'll need to update the code accordingly.

This is just the initial implementation in the sense that it should be backward compatible. In the future, we can reverse the lookup order, and perhaps even drop support for terraform, but those would be significant breaking changes (especially the latter), so we'd want to see how things go with OpenTofu and what the community wants first.

Note on registry usage
HashiCorp recently did an update (one that was totally unannounced, BTW) to its Registry terms to say:

You may download providers, modules, policy libraries and/or other Services or Content from this website solely for use with, or in support of, HashiCorp Terraform.

Terragrunt uses go-getter to download modules specified in the source URL, and it's possible that URL points to the Terraform Registry. For now, Terragrunt only works with Terraform, so that is still inline with the terms. If we add support for OpenTofu, then in theory, anyone using Terragrunt with OpenTofu might be violating the registry terms.

Possible solutions:

1. Cheapest/fastest: If we detect a registry source URL and tofu as the binary, give the user a warning.
2. Better solution: If we detect a registry source URL and tofu as the binary, automatically use OpenTofu's registry (coming soon) instead of HashiCorp's.

[lorengordon]

How about modifying the tfpath argument to support a list (presuming it doesn't support that already), and just adding opentf to the list?

---

Fwiw, personally and for my own teams, I'm likely to use an alias, or the terraform_binary config argument, or both!

[skyzyx]

From a technical perspective, I expect that it will be a no-op for the time being as OpenTF anticipates maintaining compat/interop for the foreseeable future. (However, OpenTF has stated that some functionality may be added to OpenTF that is a superset of what's in base Terraform. [source])

From a process perspective:

1. If there is a bug in OpenTF that does not exist in Terraform (or vice-versa), will bug reports (as they pertain to this project) be treated equally on both platforms?

2. Will testing be performed with code targeting both platforms?

3. For OpenTF's potential future "superset" functionality, will that be treated as a first-class citizen?

[skyzyx]

Came across this comment last night.

https://www.linkedin.com/feed/update/urn:li:ugcPost:7102220371797389313?commentUrn=urn%3Ali%3Acomment%3A%28ugcPost%3A7102220371797389313%2C7102263805740503040%29&replyUrn=urn%3Ali%3Acomment%3A%28ugcPost%3A7102220371797389313%2C7102264759957213184%29&dashCommentUrn=urn%3Ali%3Afsd_comment%3A%287102263805740503040%2Curn%3Ali%3AugcPost%3A7102220371797389313%29&dashReplyUrn=urn%3Ali%3Afsd_comment%3A%287102264759957213184%2Curn%3Ali%3AugcPost%3A7102220371797389313%29

[brikis98]

I just updated the issue description with:

Note on registry usage
HashiCorp recently did an update (one that was totally unannounced, BTW) to its Registry terms to say:

You may download providers, modules, policy libraries and/or other Services or Content from this website solely for use with, or in support of, HashiCorp Terraform.

Terragrunt uses go-getter to download modules specified in the source URL, and it's possible that URL points to the Terraform Registry. For now, Terragrunt only works with Terraform, so that is still inline with the terms. If we add support for OpenTF, then in theory, anyone using Terragrunt with OpenTF might be violating the registry terms.

Possible solutions:

1. Cheapest/fastest: If we detect a registry source URL and opentf as the binary, give the user a warning.
2. Better solution: If we detect a registry source URL and opentf as the binary, automatically use OpenTF's registry (coming soon) instead of HashiCorp's.

[brikis98]

From a technical perspective, I expect that it will be a no-op for the time being as OpenTF anticipates maintaining compat/interop for the foreseeable future. (However, OpenTF has stated that some functionality may be added to OpenTF that is a superset of what's in base Terraform. [source])

From a process perspective:

1. If there is a bug in OpenTF that does not exist in Terraform (or vice-versa), will bug reports (as they pertain to this project) be treated equally on both platforms?

Yup. Until otherwise announced, we should support both platforms equally.

2. Will testing be performed with code targeting both platforms?

Yes, we should test both platforms.

3. For OpenTF's potential future "superset" functionality, will that be treated as a first-class citizen?

Probably, but hard to say until we know what that functionality is. I think we'll have to decide on a case by case basis, with input from the community.

[skyzyx]

Your response is greatly appreciated.

This whole situation has created uncertainty at a time when we could really do without it. My company leverages several open-source and SaaS offerings in the Terraform ecosystem, and I've been reaching out across the ecosystem to understand where everyone is on this topic, and identifying what we need to watch out for.

Hoping for the very best from this whole situation, but planning ahead for the absolute worst.

[RobertKosten]

I've been using the unofficial dev snap OpenTF v1.6.0-dev, but even with --terragrunt-tfpath set to /snap/bin/opentf I get an error: Unable to parse Terraform version output: OpenTF v1.6.0-dev on linux_amd64. I'd say parsing that was bound to be brittle :-P

[denis256]

Released in https://github.com/gruntwork-io/terragrunt/releases/tag/v0.52.0