Terragrunt OpenTofu: reset terraform_version when changing tool

spacelift/internal/testhelpers/helpers.go

@@ -179,6 +179,17 @@ func Equals(expected string) ValueCheck {
 	}
 }
 
+// NotEquals checks for inequality against the unexpected value.
+func NotEquals(unexpected string) ValueCheck {
+	return func(actual string) error {
+		if actual != unexpected {
+			return nil
+		}
+
+		return errors.Errorf("unexpectedly got %q", actual)
+	}
+}
+
 // IsEmpty checks that the expected value is empty.
 func IsEmpty() ValueCheck {
 	return func(actual string) error {

spacelift/resource_stack.go

@@ -973,6 +973,10 @@ func getVendorConfig(d *schema.ResourceData) *structs.VendorConfigInput {
 			terragruntConfig.Tool = toOptionalString(tool)
 		}
 
+		if shouldWeReComputeTerraformVersionForTerragrunt(d) {
+			terragruntConfig.TerraformVersion = nil
+		}
+
 		return &structs.VendorConfigInput{
 			TerragruntInput: &terragruntConfig,
 		}
@@ -986,6 +990,9 @@ func getVendorConfig(d *schema.ResourceData) *structs.VendorConfigInput {
 
 	if terraformWorkflowTool, ok := d.GetOk("terraform_workflow_tool"); ok {
 		terraformConfig.WorkflowTool = toOptionalString(terraformWorkflowTool)
+		if shouldWeReComputeTerraformVersionForTerraformWorkflowTool(d) {
+			terraformConfig.Version = nil
+		}
 	}
 
 	if terraformWorkspace, ok := d.GetOk("terraform_workspace"); ok {
@@ -1007,6 +1014,38 @@ func getVendorConfig(d *schema.ResourceData) *structs.VendorConfigInput {
 	return &structs.VendorConfigInput{Terraform: terraformConfig}
 }
 
+func shouldWeReComputeTerraformVersionForTerragrunt(d *schema.ResourceData) bool {
+	// When tool is changed, we need to recompute terraform version
+	oldTool, newTool := d.GetChange("terragrunt.0.tool")
+	if oldTool.(string) != newTool.(string) {
+		// but only if version isn't provided manually in the config
+		inConf := d.GetRawConfig().AsValueMap()["terragrunt"].AsValueSlice()[0].AsValueMap()
+		if value, ok := inConf["terraform_version"]; ok {
+			if value.IsNull() || value.AsString() == "" {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
+func shouldWeReComputeTerraformVersionForTerraformWorkflowTool(d *schema.ResourceData) bool {
+	// When tool is changed, we need to recompute terraform version
+	oldTool, newTool := d.GetChange("terraform_workflow_tool")
+	if oldTool.(string) != newTool.(string) {
+		// but only if version isn't provided manually in the config
+		inConfig := d.GetRawConfig().AsValueMap()
+		if value, ok := inConfig["terraform_version"]; ok {
+			if value.IsNull() || value.AsString() == "" {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
 func getStrings(d *schema.ResourceData, fieldName string) []graphql.String {
 	values := []graphql.String{}
 	if commands, ok := d.GetOk(fieldName); ok {

spacelift/resource_stack_test.go

@@ -663,6 +663,114 @@ func TestStackResource(t *testing.T) {
 		})
 	})
 
+	t.Run("with GitHub and Terragrunt changing configuration scenarios", func(t *testing.T) {
+		name := "terragrunt-switch-testing-" + acctest.RandStringFromCharSet(5, acctest.CharSetAlphaNum)
+		config := func(body string) string {
+			return `
+				resource "spacelift_stack" "test" {
+					administrative = true
+					branch = "master"
+					name = "` + name + `"
+					project_root = "root"
+					repository = "demo"
+					runner_image = "custom_image:runner"
+					autodeploy = true
+					terragrunt {
+						` + body + `
+					}
+				}
+			`
+		}
+		testSteps(t, []resource.TestStep{
+			{
+				Config: config(`
+					tool = "TERRAFORM_FOSS"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("TERRAFORM_FOSS")),
+					Attribute("terragrunt.0.terraform_version", IsNotEmpty()),
+				),
+			},
+			{ // Change to OPEN_TOFU
+				Config: config(`
+					tool = "OPEN_TOFU"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("OPEN_TOFU")),
+					Attribute("terragrunt.0.terraform_version", IsNotEmpty()),
+				),
+			},
+			{ // Change to TERRAFORM with specific version
+				Config: config(`
+					tool = "TERRAFORM_FOSS"
+					terraform_version = "1.5.6"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("TERRAFORM_FOSS")),
+					Attribute("terragrunt.0.terraform_version", Equals("1.5.6")),
+				),
+			},
+			{ // Change to OPEN_TOFU with specific version
+				Config: config(`
+					tool = "OPEN_TOFU"
+					terraform_version = "1.6.2"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("OPEN_TOFU")),
+					Attribute("terragrunt.0.terraform_version", Equals("1.6.2")),
+				),
+			},
+			{ // Change to TERRAFORM without version specified
+				Config: config(`
+					tool = "TERRAFORM_FOSS"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("TERRAFORM_FOSS")),
+					Attribute("terragrunt.0.terraform_version", NotEquals("1.6.2")),
+				),
+			},
+			{ // Change to OPEN_TODU with invalid version
+				Config: config(`
+					tool = "OPEN_TOFU"
+					terraform_version = "1.5.6"
+				`),
+				ExpectError: regexp.MustCompile(`could not update stack: stack has 1 error: terragrunt: no supported OpenTofu version ([^ ]* - [^ ]*) satisfies constraints "1.5.6"`),
+			},
+			{ // Change to TERRAFORM with invalid version
+				Config: config(`
+					tool = "TERRAFORM_FOSS"
+					terraform_version = "1.6.2"
+				`),
+				ExpectError: regexp.MustCompile(`could not update stack: stack has 1 error: terragrunt: no supported Terraform version ([^ ]* - [^ ]*) satisfies constraints "1.6.2"`),
+			},
+			{ // Change to MANUALLY PROVISIONED
+				Config: config(`
+					tool = "MANUALLY_PROVISIONED"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("MANUALLY_PROVISIONED")),
+					Attribute("terragrunt.0.terraform_version", IsEmpty()),
+				),
+			},
+			{ // Back to OPEN_TOFU
+				Config: config(`
+					tool = "OPEN_TOFU"
+				`),
+				Check: Resource(
+					"spacelift_stack.test",
+					Attribute("terragrunt.0.tool", Equals("OPEN_TOFU")),
+					Attribute("terragrunt.0.terraform_version", IsNotEmpty()),
+				),
+			},
+		})
+	})
+
 	t.Run("with GitHub and no vendor-specific configuration", func(t *testing.T) {
 		testSteps(t, []resource.TestStep{
 			{
@@ -1380,6 +1488,43 @@ func TestStackResourceSpace(t *testing.T) {
 					Attribute("terraform_workflow_tool", Equals("CUSTOM")),
 				),
 			},
+			// Check to change from TERRAFORM_FOSS to CUSTOM with a specific version
+			// Actually, we don't need to specify the version, but when we don't do it, it will be
+			// evaluated on the first run. So, it's simpler just to specify a version for that test case.
+			{
+				Config: fmt.Sprintf(`
+					resource "spacelift_stack" "terraform_workflow_tool_custom_with_run" {
+						branch                  = "master"
+						name                    = "Provider test stack workflow_tool with run %s"
+						project_root            = "root"
+						repository              = "demo"
+						terraform_workflow_tool = "TERRAFORM_FOSS"
+						terraform_version       = "1.5.7"
+						autodeploy              = true
+					}
+				`, randomID),
+				Check: Resource(
+					"spacelift_stack.terraform_workflow_tool_custom_with_run",
+					Attribute("terraform_workflow_tool", Equals("TERRAFORM_FOSS")),
+				),
+			},
+			{
+				Config: fmt.Sprintf(`
+					resource "spacelift_stack" "terraform_workflow_tool_custom_with_run" {
+						branch                  = "master"
+						name                    = "Provider test stack workflow_tool with run %s"
+						project_root            = "root"
+						repository              = "demo"
+						terraform_workflow_tool = "CUSTOM"
+						terraform_version       = ""
+						autodeploy              = true
+					}
+				`, randomID),
+				Check: Resource(
+					"spacelift_stack.terraform_workflow_tool_custom_with_run",
+					Attribute("terraform_workflow_tool", Equals("CUSTOM")),
+				),
+			},
 		})
 	})
 }