allow terraform init to fail in additional codepaths

allow `terraform init` to fail -- as expected -- in instances where a
non-legacy Terraform is used against a legacy TF state for the
purposes of `state replace-provider`-ing _also_ when invoking `terraform
init` from within `OverrideBackendToLocal`.

This addresses the _second_ `init` error seen in the logs:

```
2023/08/28 14:52:35 [INFO] [runner] load migration file: test.hcl
2023/08/28 14:52:35 [INFO] [migrator] start state migrator plan
2023/08/28 14:52:35 [INFO] [migrator@.] terraform version: 1.5.5
2023/08/28 14:52:35 [INFO] [migrator@.] initialize work dir
2023/08/28 14:52:35 [INFO] [migrator@.] ignoring error 'Error: Invalid legacy provider address' initilizing work dir; the error is expected when using Terraform >= 0.13 with a legacy Terraform state
2023/08/28 14:52:35 [INFO] [migrator@.] get the current remote state
2023/08/28 14:52:35 [INFO] [migrator@.] override backend to local
2023/08/28 14:52:35 [INFO] [executor@.] create an override file
2023/08/28 14:52:35 [INFO] [migrator@.] creating local workspace folder in: terraform.tfstate.d/default
2023/08/28 14:52:35 [INFO] [executor@.] switch backend to local
2023/08/28 14:52:35 [INFO] [migrator@.] compute a new state
2023/08/28 14:52:36 [INFO] [migrator@.] check diffs
2023/08/28 14:52:36 [INFO] [executor@.] remove the override file
2023/08/28 14:52:36 [INFO] [executor@.] remove the workspace state folder
2023/08/28 14:52:36 [INFO] [executor@.] switch back to remote
2023/08/28 14:52:36 [ERROR] [executor@.] failed to switch back to remote: failed to run command (exited 1): terraform init -input=false -no-color -reconfigure
stdout:

Initializing the backend...

Successfully configured the backend "s3"! Terraform will automatically
use this backend unless the backend configuration changes.

stderr:

Error: Invalid legacy provider address

This configuration or its associated state refers to the unqualified provider
"null".

You must complete the Terraform 0.13 upgrade process before upgrading to
later versions.

2023/08/28 14:52:36 [ERROR] [executor@.] please re-run terraform init -reconfigure
2023/08/28 14:52:36 [INFO] [migrator] state migrator plan success!
```

This seeks to address feedback from @minamijoyo:
#145 (comment)

Signed-off-by: Mike Ball <mikedball@gmail.com>

tfexec/terraform.go

@@ -136,7 +136,7 @@ type TerraformCLI interface {
 	// so we need to switch the backend to local for temporary state operations.
 	// The filename argument must meet constraints for override file.
 	// (e.g.) _tfexec_override.tf
-	OverrideBackendToLocal(ctx context.Context, filename string, workspace string, isBackendTerraformCloud bool, backendConfig []string) (func(), error)
+	OverrideBackendToLocal(ctx context.Context, filename string, workspace string, isBackendTerraformCloud bool, backendConfig []string, supportsStateReplaceProvider bool) (func(), error)
 
 	// PlanHasChange is a helper method which runs plan and return true if the plan has change.
 	PlanHasChange(ctx context.Context, state *State, opts ...string) (bool, error)
@@ -213,7 +213,7 @@ func (c *terraformCLI) SetExecPath(execPath string) {
 // The filename argument must meet constraints in order to override the file.
 // (e.g.) _tfexec_override.tf
 func (c *terraformCLI) OverrideBackendToLocal(ctx context.Context, filename string,
-	workspace string, isBackendTerraformCloud bool, backendConfig []string) (func(), error) {
+	workspace string, isBackendTerraformCloud bool, backendConfig []string, supportsStateReplaceProvider bool) (func(), error) {
 	// create local backend override file.
 	path := filepath.Join(c.Dir(), filename)
 	contents := `
@@ -277,12 +277,16 @@ terraform {
 		if !isBackendTerraformCloud {
 			args = append(args, "-reconfigure")
 		}
-		err = c.Init(ctx, args...)
 
+		err = c.Init(ctx, args...)
 		if err != nil {
-			// we cannot return error here.
-			log.Printf("[ERROR] [executor@%s] failed to switch back to remote: %s\n", c.Dir(), err)
-			log.Printf("[ERROR] [executor@%s] please re-run terraform init -reconfigure\n", c.Dir())
+			if supportsStateReplaceProvider && strings.Contains(err.Error(), AcceptableLegacyStateInitError) {
+				log.Printf("[INFO] [migrator@%s] ignoring error '%s'; the error is expected when using Terraform with a legacy Terraform state\n", c.Dir(), AcceptableLegacyStateInitError)
+			} else {
+				// we cannot return error here.
+				log.Printf("[ERROR] [executor@%s] failed to switch back to remote: %s\n", c.Dir(), err)
+				log.Printf("[ERROR] [executor@%s] please re-run terraform init -reconfigure\n", c.Dir())
+			}
 		}
 	}
 

tfexec/terraform_state_replace_provider.go

@@ -12,6 +12,13 @@ const (
 	// MinimumTerraformVersionForStateReplaceProvider specifies the minimum
 	// supported Terraform version for StateReplaceProvider.
 	MinimumTerraformVersionForStateReplaceProvider = "0.13"
+
+	// AcceptableLegacyStateInitError is the error message returned by `terraform
+	// init` when a non-legacy Terraform CLI is used against a legacy Terraform state.
+	// When invoking `state replace-provider`, it's necessary to first
+	// invoke `terraform init`. However, when using a non-legacy Terraform CLI
+	// against a legacy Terraform state, this error is expected.
+	AcceptableLegacyStateInitError = "Error: Invalid legacy provider address"
 )
 
 // SupportsStateReplaceProvider returns true if the terraform version is greater

tfexec/terraform_test.go

@@ -126,7 +126,12 @@ resource "null_resource" "bar" {}
 		t.Fatalf("an override file already exists: %s", err)
 	}
 
-	switchBackToRemotekFunc, err := terraformCLI.OverrideBackendToLocal(context.Background(), filename, workspace, false, nil)
+	supportsStateReplaceProvider, _, err := terraformCLI.SupportsStateReplaceProvider(context.Background())
+	if err != nil {
+		t.Fatalf("failed to determine if Terraform version supports state replace-provider: %s", err)
+	}
+
+	switchBackToRemotekFunc, err := terraformCLI.OverrideBackendToLocal(context.Background(), filename, workspace, false, nil, supportsStateReplaceProvider)
 	if err != nil {
 		t.Fatalf("failed to run OverrideBackendToLocal: %s", err)
 	}
@@ -254,7 +259,12 @@ resource "null_resource" "bar" {}
 		t.Fatalf("an override file already exists: %s", err)
 	}
 
-	switchBackToRemotekFunc, err := terraformCLI.OverrideBackendToLocal(context.Background(), filename, workspace, false, backendConfig)
+	supportsStateReplaceProvider, _, err := terraformCLI.SupportsStateReplaceProvider(context.Background())
+	if err != nil {
+		t.Fatalf("failed to determine if Terraform version supports state replace-provider: %s", err)
+	}
+
+	switchBackToRemotekFunc, err := terraformCLI.OverrideBackendToLocal(context.Background(), filename, workspace, false, backendConfig, supportsStateReplaceProvider)
 	if err != nil {
 		t.Fatalf("failed to run OverrideBackendToLocal: %s", err)
 	}

tfexec/test_helper.go

@@ -306,7 +306,7 @@ func SetupTestAccForStateReplaceProvider(t *testing.T, workspace string, source
 
 	err := tf.Init(ctx, "-input=false", "-no-color")
 
-	if err != nil && !strings.Contains(err.Error(), "Error: Invalid legacy provider address") {
+	if err != nil && !strings.Contains(err.Error(), AcceptableLegacyStateInitError) {
 		t.Fatalf("failed to run terraform init: %s", err)
 	}
 

tfmigrate/migrator.go

@@ -40,9 +40,8 @@ func setupWorkDir(ctx context.Context, tf tfexec.TerraformCLI, workspace string,
 	log.Printf("[INFO] [migrator@%s] initialize work dir\n", tf.Dir())
 	err = tf.Init(ctx, "-input=false", "-no-color")
 	if err != nil {
-		acceptable := "Error: Invalid legacy provider address"
-		if supportsStateReplaceProvider && strings.Contains(err.Error(), acceptable) {
-			log.Printf("[INFO] [migrator@%s] ignoring error '%s' initilizing work dir; the error is expected when using Terraform %s with a legacy Terraform state\n", tf.Dir(), acceptable, constraints)
+		if supportsStateReplaceProvider && strings.Contains(err.Error(), tfexec.AcceptableLegacyStateInitError) {
+			log.Printf("[INFO] [migrator@%s] ignoring error '%s' initilizing work dir; the error is expected when using Terraform %s with a legacy Terraform state\n", tf.Dir(), tfexec.AcceptableLegacyStateInitError, constraints)
 		} else {
 			return nil, nil, err
 		}
@@ -71,7 +70,7 @@ func setupWorkDir(ctx context.Context, tf tfexec.TerraformCLI, workspace string,
 	}
 	// override backend to local
 	log.Printf("[INFO] [migrator@%s] override backend to local\n", tf.Dir())
-	switchBackToRemoteFunc, err := tf.OverrideBackendToLocal(ctx, "_tfmigrate_override.tf", workspace, isBackendTerraformCloud, backendConfig)
+	switchBackToRemoteFunc, err := tf.OverrideBackendToLocal(ctx, "_tfmigrate_override.tf", workspace, isBackendTerraformCloud, backendConfig, supportsStateReplaceProvider)
 	if err != nil {
 		return nil, nil, err
 	}