feat(remote_state): implement outputs fetching from gcs

This implements outputs fetching from the GCS backend, in a similar fashion to
what has been implemented for S3.

I've also added some misc fixes:
* correct typo: steateBody -> stateBody
* drop unused Path parameter on GCS config (it's not supported)

Tested on my architecture, everything seems to work!

Ref: https://developer.hashicorp.com/terraform/language/settings/backends/gcs

config/dependency.go

@@ -3,6 +3,7 @@ package config
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"encoding/json"
 	goErrors "errors"
 	"fmt"
@@ -16,6 +17,7 @@ import (
 
 	"github.com/gruntwork-io/terragrunt/internal/cache"
 
+	"cloud.google.com/go/storage"
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/hashicorp/go-getter"
@@ -911,6 +913,18 @@ func getTerragruntOutputJSONFromRemoteState(
 
 			ctx.TerragruntOptions.Logger.Debugf("Retrieved output from %s as json: %s using s3 bucket", targetTGOptions.TerragruntConfigPath, jsonBytes)
 
+			return jsonBytes, nil
+		case "gcs":
+			jsonBytes, err := getTerragruntOutputJSONFromRemoteStateGCS(
+				targetTGOptions,
+				remoteState,
+			)
+			if err != nil {
+				return nil, err
+			}
+
+			ctx.TerragruntOptions.Logger.Debugf("Retrieved output from %s as json: %s using GCS bucket", targetTGOptions.TerragruntConfigPath, jsonBytes)
+
 			return jsonBytes, nil
 		default:
 			ctx.TerragruntOptions.Logger.Errorf("FetchDependencyOutputFromState is not supported for backend %s, falling back to normal method", backend)
@@ -990,12 +1004,72 @@ func getTerragruntOutputJSONFromRemoteStateS3(terragruntOptions *options.Terragr
 		}
 	}(result.Body)
 
-	steateBody, err := io.ReadAll(result.Body)
+	stateBody, err := io.ReadAll(result.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	jsonState := string(stateBody)
+	jsonMap := make(map[string]interface{})
+
+	err = json.Unmarshal([]byte(jsonState), &jsonMap)
+	if err != nil {
+		return nil, err
+	}
+
+	jsonOutputs, err := json.Marshal(jsonMap["outputs"])
+	if err != nil {
+		return nil, err
+	}
+
+	return jsonOutputs, nil
+}
+
+// getTerragruntOutputJSONFromRemoteStateGCS pulls the output directly from a GCS bucket without calling Terraform
+func getTerragruntOutputJSONFromRemoteStateGCS(
+	terragruntOptions *options.TerragruntOptions,
+	remoteState *remote.RemoteState,
+) ([]byte, error) {
+	terragruntOptions.Logger.Debugf("Fetching outputs directly from gcs://%s/%s/default.tfstate", remoteState.Config["bucket"], remoteState.Config["prefix"])
+
+	gcsConfigExtended, err := remote.ParseExtendedGCSConfig(remoteState.Config)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := remote.ValidateGCSConfig(gcsConfigExtended); err != nil {
+		return nil, err
+	}
+
+	var gcsConfig = gcsConfigExtended.RemoteStateConfigGCS
+
+	gcsClient, err := remote.CreateGCSClient(gcsConfig)
+	if err != nil {
+		return nil, err
+	}
+
+	bucket := gcsClient.Bucket(gcsConfig.Bucket)
+	object := bucket.Object(gcsConfig.Prefix + "/default.tfstate")
+
+	reader, err := object.NewReader(context.Background())
+
+	if err != nil {
+		return nil, err
+	}
+
+	defer func(reader *storage.Reader) {
+		err := reader.Close()
+		if err != nil {
+			terragruntOptions.Logger.Warnf("Failed to close remote state response %v", err)
+		}
+	}(reader)
+
+	stateBody, err := io.ReadAll(reader)
 	if err != nil {
 		return nil, err
 	}
 
-	jsonState := string(steateBody)
+	jsonState := string(stateBody)
 	jsonMap := make(map[string]interface{})
 
 	err = json.Unmarshal([]byte(jsonState), &jsonMap)

remote/remote_state_gcs.go

@@ -31,7 +31,7 @@ import (
  * has to create them.
  */
 type ExtendedRemoteStateConfigGCS struct {
-	remoteStateConfigGCS RemoteStateConfigGCS
+	RemoteStateConfigGCS RemoteStateConfigGCS `mapstructure:"remote_state_config_gcs"`
 
 	Project                string            `mapstructure:"project"`
 	Location               string            `mapstructure:"location"`
@@ -59,7 +59,6 @@ type RemoteStateConfigGCS struct {
 	Credentials   string `mapstructure:"credentials"`
 	AccessToken   string `mapstructure:"access_token"`
 	Prefix        string `mapstructure:"prefix"`
-	Path          string `mapstructure:"path"`
 	EncryptionKey string `mapstructure:"encryption_key"`
 
 	ImpersonateServiceAccount          string   `mapstructure:"impersonate_service_account"`
@@ -178,16 +177,16 @@ func (initializer GCSInitializer) buildInitializerCacheKey(gcsConfig *RemoteStat
 // Initialize the remote state GCS bucket specified in the given config. This function will validate the config
 // parameters, create the GCS bucket if it doesn't already exist, and check that versioning is enabled.
 func (initializer GCSInitializer) Initialize(ctx context.Context, remoteState *RemoteState, terragruntOptions *options.TerragruntOptions) error {
-	gcsConfigExtended, err := parseExtendedGCSConfig(remoteState.Config)
+	gcsConfigExtended, err := ParseExtendedGCSConfig(remoteState.Config)
 	if err != nil {
 		return err
 	}
 
-	if err := validateGCSConfig(gcsConfigExtended); err != nil {
+	if err := ValidateGCSConfig(gcsConfigExtended); err != nil {
 		return err
 	}
 
-	var gcsConfig = gcsConfigExtended.remoteStateConfigGCS
+	var gcsConfig = gcsConfigExtended.RemoteStateConfigGCS
 
 	cacheKey := initializer.buildInitializerCacheKey(&gcsConfig)
 	if initialized, hit := initializedRemoteStateCache.Get(ctx, cacheKey); initialized && hit {
@@ -245,7 +244,7 @@ func (initializer GCSInitializer) GetTerraformInitArgs(config map[string]interfa
 	return filteredConfig
 }
 
-// Parse the given map into a GCS config
+// parseGCSConfig parses the given config map into a GCS config
 func parseGCSConfig(config map[string]interface{}) (*RemoteStateConfigGCS, error) {
 	var gcsConfig RemoteStateConfigGCS
 	if err := mapstructure.Decode(config, &gcsConfig); err != nil {
@@ -255,8 +254,8 @@ func parseGCSConfig(config map[string]interface{}) (*RemoteStateConfigGCS, error
 	return &gcsConfig, nil
 }
 
-// Parse the given map into a GCS config
-func parseExtendedGCSConfig(config map[string]interface{}) (*ExtendedRemoteStateConfigGCS, error) {
+// ParseExtendedGCSConfig parses the given config map into a GCS config
+func ParseExtendedGCSConfig(config map[string]interface{}) (*ExtendedRemoteStateConfigGCS, error) {
 	var (
 		gcsConfig      RemoteStateConfigGCS
 		extendedConfig ExtendedRemoteStateConfigGCS
@@ -270,14 +269,14 @@ func parseExtendedGCSConfig(config map[string]interface{}) (*ExtendedRemoteState
 		return nil, errors.WithStackTrace(err)
 	}
 
-	extendedConfig.remoteStateConfigGCS = gcsConfig
+	extendedConfig.RemoteStateConfigGCS = gcsConfig
 
 	return &extendedConfig, nil
 }
 
-// Validate all the parameters of the given GCS remote state configuration
-func validateGCSConfig(extendedConfig *ExtendedRemoteStateConfigGCS) error {
-	var config = extendedConfig.remoteStateConfigGCS
+// ValidateGCSConfig validates all the parameters of the given GCS remote state configuration
+func ValidateGCSConfig(extendedConfig *ExtendedRemoteStateConfigGCS) error {
+	var config = extendedConfig.RemoteStateConfigGCS
 
 	if config.Bucket == "" {
 		return errors.WithStackTrace(MissingRequiredGCSRemoteStateConfig("bucket"))
@@ -290,8 +289,8 @@ func validateGCSConfig(extendedConfig *ExtendedRemoteStateConfigGCS) error {
 // confirms, create the bucket and enable versioning for it.
 func createGCSBucketIfNecessary(ctx context.Context, gcsClient *storage.Client, config *ExtendedRemoteStateConfigGCS, terragruntOptions *options.TerragruntOptions) error {
 	// TODO: Remove lint suppression
-	if !DoesGCSBucketExist(gcsClient, &config.remoteStateConfigGCS) { //nolint:contextcheck
-		terragruntOptions.Logger.Debugf("Remote state GCS bucket %s does not exist. Attempting to create it", config.remoteStateConfigGCS.Bucket)
+	if !DoesGCSBucketExist(gcsClient, &config.RemoteStateConfigGCS) { //nolint:contextcheck
+		terragruntOptions.Logger.Debugf("Remote state GCS bucket %s does not exist. Attempting to create it", config.RemoteStateConfigGCS.Bucket)
 
 		// A project must be specified in order for terragrunt to automatically create a storage bucket.
 		if config.Project == "" {
@@ -304,10 +303,10 @@ func createGCSBucketIfNecessary(ctx context.Context, gcsClient *storage.Client,
 		}
 
 		if terragruntOptions.FailIfBucketCreationRequired {
-			return BucketCreationNotAllowed(config.remoteStateConfigGCS.Bucket)
+			return BucketCreationNotAllowed(config.RemoteStateConfigGCS.Bucket)
 		}
 
-		prompt := fmt.Sprintf("Remote state GCS bucket %s does not exist or you don't have permissions to access it. Would you like Terragrunt to create it?", config.remoteStateConfigGCS.Bucket)
+		prompt := fmt.Sprintf("Remote state GCS bucket %s does not exist or you don't have permissions to access it. Would you like Terragrunt to create it?", config.RemoteStateConfigGCS.Bucket)
 
 		shouldCreateBucket, err := shell.PromptUserForYesNo(ctx, prompt, terragruntOptions)
 		if err != nil {
@@ -316,7 +315,7 @@ func createGCSBucketIfNecessary(ctx context.Context, gcsClient *storage.Client,
 
 		if shouldCreateBucket {
 			// To avoid any eventual consistency issues with creating a GCS bucket we use a retry loop.
-			description := "Create GCS bucket " + config.remoteStateConfigGCS.Bucket
+			description := "Create GCS bucket " + config.RemoteStateConfigGCS.Bucket
 
 			return util.DoWithRetry(ctx, description, gcpMaxRetries, gcpSleepBetweenRetries, terragruntOptions.Logger, log.DebugLevel, func(ctx context.Context) error {
 				// TODO: Remove lint suppression
@@ -354,7 +353,7 @@ func CreateGCSBucketWithVersioning(gcsClient *storage.Client, config *ExtendedRe
 		return err
 	}
 
-	if err := WaitUntilGCSBucketExists(gcsClient, &config.remoteStateConfigGCS, terragruntOptions); err != nil {
+	if err := WaitUntilGCSBucketExists(gcsClient, &config.RemoteStateConfigGCS, terragruntOptions); err != nil {
 		return err
 	}
 
@@ -367,14 +366,14 @@ func CreateGCSBucketWithVersioning(gcsClient *storage.Client, config *ExtendedRe
 
 func AddLabelsToGCSBucket(gcsClient *storage.Client, config *ExtendedRemoteStateConfigGCS, terragruntOptions *options.TerragruntOptions) error {
 	if len(config.GCSBucketLabels) == 0 {
-		terragruntOptions.Logger.Debugf("No labels specified for bucket %s.", config.remoteStateConfigGCS.Bucket)
+		terragruntOptions.Logger.Debugf("No labels specified for bucket %s.", config.RemoteStateConfigGCS.Bucket)
 		return nil
 	}
 
 	terragruntOptions.Logger.Debugf("Adding labels to GCS bucket with %s", config.GCSBucketLabels)
 
 	ctx := context.Background()
-	bucket := gcsClient.Bucket(config.remoteStateConfigGCS.Bucket)
+	bucket := gcsClient.Bucket(config.RemoteStateConfigGCS.Bucket)
 
 	bucketAttrs := storage.BucketAttrsToUpdate{}
 
@@ -393,15 +392,15 @@ func AddLabelsToGCSBucket(gcsClient *storage.Client, config *ExtendedRemoteState
 
 // CreateGCSBucket creates the GCS bucket specified in the given config.
 func CreateGCSBucket(gcsClient *storage.Client, config *ExtendedRemoteStateConfigGCS, terragruntOptions *options.TerragruntOptions) error {
-	terragruntOptions.Logger.Debugf("Creating GCS bucket %s in project %s", config.remoteStateConfigGCS.Bucket, config.Project)
+	terragruntOptions.Logger.Debugf("Creating GCS bucket %s in project %s", config.RemoteStateConfigGCS.Bucket, config.Project)
 
 	// The project ID to which the bucket belongs. This is only used when creating a new bucket during initialization.
 	// Since buckets have globally unique names, the project ID is not required to access the bucket during normal
 	// operation.
 	projectID := config.Project
 
 	ctx := context.Background()
-	bucket := gcsClient.Bucket(config.remoteStateConfigGCS.Bucket)
+	bucket := gcsClient.Bucket(config.RemoteStateConfigGCS.Bucket)
 
 	bucketAttrs := &storage.BucketAttrs{}
 
@@ -411,22 +410,22 @@ func CreateGCSBucket(gcsClient *storage.Client, config *ExtendedRemoteStateConfi
 	}
 
 	if config.SkipBucketVersioning {
-		terragruntOptions.Logger.Debugf("Versioning is disabled for the remote state GCS bucket %s using 'skip_bucket_versioning' config.", config.remoteStateConfigGCS.Bucket)
+		terragruntOptions.Logger.Debugf("Versioning is disabled for the remote state GCS bucket %s using 'skip_bucket_versioning' config.", config.RemoteStateConfigGCS.Bucket)
 	} else {
-		terragruntOptions.Logger.Debugf("Enabling versioning on GCS bucket %s", config.remoteStateConfigGCS.Bucket)
+		terragruntOptions.Logger.Debugf("Enabling versioning on GCS bucket %s", config.RemoteStateConfigGCS.Bucket)
 
 		bucketAttrs.VersioningEnabled = true
 	}
 
 	if config.EnableBucketPolicyOnly {
-		terragruntOptions.Logger.Debugf("Enabling uniform bucket-level access on GCS bucket %s", config.remoteStateConfigGCS.Bucket)
+		terragruntOptions.Logger.Debugf("Enabling uniform bucket-level access on GCS bucket %s", config.RemoteStateConfigGCS.Bucket)
 
 		bucketAttrs.BucketPolicyOnly = storage.BucketPolicyOnly{Enabled: true}
 	}
 
 	err := bucket.Create(ctx, projectID, bucketAttrs)
 
-	return errors.WithStackTraceAndPrefix(err, "Error creating GCS bucket %s", config.remoteStateConfigGCS.Bucket)
+	return errors.WithStackTraceAndPrefix(err, "Error creating GCS bucket %s", config.RemoteStateConfigGCS.Bucket)
 }
 
 // WaitUntilGCSBucketExists waits for the GCS bucket specified in the given config to be created.

test/fixtures/gcs-output-from-remote-state/env1/app1/main.tf

@@ -0,0 +1,7 @@
+terraform {
+  backend "gcs" {}
+}
+
+output "app1_text" {
+  value = "app1 output"
+}

test/fixtures/gcs-output-from-remote-state/env1/app1/terragrunt.hcl

@@ -0,0 +1,7 @@
+include {
+  path = find_in_parent_folders()
+}
+
+dependencies {
+  paths = ["../app3"]
+}

test/fixtures/gcs-output-from-remote-state/env1/app2/main.tf

@@ -0,0 +1,15 @@
+terraform {
+  backend "gcs" {}
+}
+
+output "app1_text" {
+  value = var.app1_text
+}
+
+output "app2_text" {
+  value = "app2 output"
+}
+
+output "app3_text" {
+  value = var.app3_text
+}

test/fixtures/gcs-output-from-remote-state/env1/app2/terragrunt.hcl

@@ -0,0 +1,24 @@
+include {
+  path = find_in_parent_folders()
+}
+
+dependency "app1" {
+  config_path = "../app1"
+
+  mock_outputs = {
+    app1_text = "(known after apply-all)"
+  }
+}
+
+dependency "app3" {
+  config_path = "../app3"
+
+  mock_outputs = {
+    app3_text = "(known after apply-all)"
+  }
+}
+
+inputs = {
+  app1_text = dependency.app1.outputs.app1_text
+  app3_text = dependency.app3.outputs.app3_text
+}

test/fixtures/gcs-output-from-remote-state/env1/app2/variables.tf

@@ -0,0 +1,7 @@
+variable "app1_text" {
+  type = string
+}
+
+variable "app3_text" {
+  type = string
+}

test/fixtures/gcs-output-from-remote-state/env1/app3/main.tf

@@ -0,0 +1,7 @@
+terraform {
+  backend "gcs" {}
+}
+
+output "app3_text" {
+  value = "app3 output"
+}

test/fixtures/gcs-output-from-remote-state/env1/app3/terragrunt.hcl

@@ -0,0 +1,3 @@
+include {
+  path = find_in_parent_folders()
+}

test/fixtures/gcs-output-from-remote-state/terragrunt.hcl

@@ -0,0 +1,11 @@
+# Configure Terragrunt to automatically store tfstate files in a GCS bucket
+remote_state {
+  backend = "gcs"
+
+  config = {
+    project        = "__FILL_IN_PROJECT__"
+    location       = "__FILL_IN_LOCATION__"
+    bucket         = "__FILL_IN_BUCKET_NAME__"
+    prefix         = "${path_relative_to_include()}/terraform.tfstate"
+  }
+}