Support OpenTofu auto download

[nitrocode]

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---

• I'd be willing to implement this feature (contributing guide)

Describe the user story

Please support downloading of the opentf terraform fork instead of hashicorp terraform

https://github.com/opentffoundation/opentf

This may already be supported using --tf-download-url but it would be nice to have this officially documented so people can start using opentf instead of terraform for future releases.

https://www.runatlantis.io/docs/server-configuration.html#tf-download-url

Describe the solution you'd like

See above

Describe the drawbacks of your solution

N/A

Describe alternatives you've considered

N/A

---

• epic Support OpenTofu (epic) #3741
• Previous ticket What is the stance of atlantis on OpenTofu ?  #3727
• feat: Download terraform versions using >= and ~> specifiers #1776
• https://github.com/warrensbox/terraform-switcher
• OpenTofu Support warrensbox/terraform-switcher#315

---

atlantis/cmd/server.go

Line 173 in a793620

DefaultTFDownloadURL = "https://releases.hashicorp.com"

atlantis/server/server.go

Line 405 in a793620

terraformClient, err := terraform.NewClient(

atlantis/server/core/terraform/terraform_client.go

Lines 239 to 253 in a793620

	func NewClient(
	log logging.SimpleLogging,
	binDir string,
	cacheDir string,
	tfeToken string,
	tfeHostname string,
	defaultVersionStr string,
	defaultVersionFlagName string,
	tfDownloadURL string,
	tfDownloader Downloader,
	tfDownloadAllowed bool,
	usePluginCache bool,
	projectCmdOutputHandler jobs.ProjectCommandOutputHandler,
	) (*DefaultClient, error) {
	return NewClientWithDefaultVersion(

atlantis/server/core/terraform/terraform_client.go

Line 379 in 3ea2914

_, err = ensureVersion(log, c.downloader, c.versions, v, c.binDir, c.downloadBaseURL, c.downloadAllowed)

atlantis/server/core/terraform/terraform_client.go

Lines 555 to 562 in 3ea2914

	log.Info("Could not find terraform version %s in PATH or %s, downloading from %s", v.String(), binDir, downloadURL)
	urlPrefix := fmt.Sprintf("%s/terraform/%s/terraform_%s", downloadURL, v.String(), v.String())
	binURL := fmt.Sprintf("%s_%s_%s.zip", urlPrefix, runtime.GOOS, runtime.GOARCH)
	checksumURL := fmt.Sprintf("%s_SHA256SUMS", urlPrefix)
	fullSrcURL := fmt.Sprintf("%s?checksum=file:%s", binURL, checksumURL)
	if err := dl.GetFile(dest, fullSrcURL); err != nil {
	return "", errors.Wrapf(err, "downloading terraform version %s at %q", v.String(), fullSrcURL)
	}

atlantis/server/core/terraform/terraform_client.go

Line 538 in 3ea2914

binFile := "terraform" + v.String()

Listing versions

atlantis/server/core/terraform/terraform_client.go

Line 304 in 3ea2914

versions, err := lib.GetTFList(url, true)

---

The terraform url is constructed

• url = https://releases.hashicorp.com
• format string = %s/terraform/%s/terraform_%s, prefix, version, version
• %s_%s_%s.zip, prefix, os, arch
• full url https://releases.hashicorp.com/terraform/1.5.7/terraform_1.5.7_darwin_amd64.zip

Opentofu

• url = https://github.com/opentofu/opentofu/releases
• format string = %s/download/%s/tofu_%s, prefix, version, version
• %s_%s_%s.zip, prefix, os, arch
• full url https://github.com/opentofu/opentofu/releases/download/v1.6.0-alpha4/tofu_1.6.0-alpha4_darwin_amd64.zip

Seems like the following changes are needed

• overriding the url string

  • this can be done already

• overriding the format string with a new flag

• simplifying default format string to use %[2]s to remove redundant argument

• a way to search opentf versions to know which version to download

  • https://releases-page.opentofu-get.pages.dev/tofu/
  • Introduce some sort of API to list all versions of opentofu un-paginated opentofu/opentofu#928
  • Fixes opentofu/opentofu#1131: Provide releases page opentofu/get.opentofu.org#16
  • Support OpenTofu auto download #4339
  • Statically generating opentofu releases page: Fixes opentofu/opentofu#1131 opentofu/get.opentofu.org#22

• opentofu-enabled flag

• change the name of the binary downloaded

• allow a required_version block at or below a certain version such as 1.5.x which would download terraform binaries instead of opentofu if needed

  • if we don't implement this then users with older and newer versions would get blocked for all older root dirs

[janosdebugs]

@nitrocode this may be helpful: opentofu/get.opentofu.org#25