Add OAuth2 token support for GCS backend

backend/remote-state/gcs/backend.go

@@ -14,6 +14,7 @@ import (
 	"github.com/hashicorp/terraform/helper/pathorcontents"
 	"github.com/hashicorp/terraform/helper/schema"
 	"github.com/hashicorp/terraform/httpclient"
+	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/jwt"
 	"google.golang.org/api/option"
 )
@@ -65,6 +66,15 @@ func New() backend.Backend {
 				Default:     "",
 			},
 
+			"access_token": {
+				Type:     schema.TypeString,
+				Optional: true,
+				DefaultFunc: schema.MultiEnvDefaultFunc([]string{
+					"GOOGLE_OAUTH_ACCESS_TOKEN",
+				}, nil),
+				Description: "An OAuth2 token used for GCP authentication",
+			},
+
 			"encryption_key": {
 				Type:        schema.TypeString,
 				Optional:    true,
@@ -116,12 +126,23 @@ func (b *Backend) configure(ctx context.Context) error {
 
 	var opts []option.ClientOption
 
-	creds := data.Get("credentials").(string)
-	if creds == "" {
+	// Add credential source
+	var creds string
+	var tokenSource oauth2.TokenSource
+
+	if v, ok := data.GetOk("access_token"); ok {
+		tokenSource = oauth2.StaticTokenSource(&oauth2.Token{
+			AccessToken: v.(string),
+		})
+	} else if v, ok := data.GetOk("credentials"); ok {
+		creds = v.(string)
+	} else {
 		creds = os.Getenv("GOOGLE_CREDENTIALS")
 	}
 
-	if creds != "" {
+	if tokenSource != nil {
+		opts = append(opts, option.WithTokenSource(tokenSource))
+	} else if creds != "" {
 		var account accountFile
 
 		// to mirror how the provider works, we accept the file path or the contents

website/docs/backends/types/gcs.html.md

@@ -58,6 +58,9 @@ The following configuration options are supported:
  *  `credentials` / `GOOGLE_CREDENTIALS` - (Optional) Local path to Google Cloud Platform account credentials in JSON format.
     If unset, [Google Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials) are used.
     The provided credentials need to have the `devstorage.read_write` scope and `WRITER` permissions on the bucket.
+ * `access_token` - (Optional) A temporary [OAuth 2.0 access token] obtained from
+    the Google Authorization server, i.e. the `Authorization: Bearer` token used to
+    authenticate HTTP requests to GCP APIs. This is an alternative to `credentials`. If both are specified, `access_token` will be used over the `credentials` field.
  *  `prefix` - (Optional) GCS prefix inside the bucket. Named states for workspaces are stored in an object called `<prefix>/<name>.tfstate`.
  *  `path` - (Deprecated) GCS path to the state file of the default state. For backwards compatibility only, use `prefix` instead.
  *  `encryption_key` / `GOOGLE_ENCRYPTION_KEY` - (Optional) A 32 byte base64 encoded 'customer supplied encryption key' used to encrypt all state. For more information see [Customer Supplied Encryption Keys](https://cloud.google.com/storage/docs/encryption#customer-supplied).