googlecloudstorage: fall back to default application credentials

Fall back to default application credentials when all other credentials sources fail

This change allows users with default application credentials
configured (notably when running on google compute instances) to
dispense with explicitly configuring google cloud storage credentials
in rclone's own configuration.

backend/googlecloudstorage/googlecloudstorage.go

@@ -16,6 +16,7 @@ FIXME Patch/Delete/Get isn't working with files with spaces in - giving 404 erro
 */
 
 import (
+	"context"
 	"encoding/base64"
 	"encoding/hex"
 	"fmt"
@@ -45,6 +46,8 @@ import (
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"google.golang.org/api/googleapi"
+
+	// NOTE: This API is deprecated
 	storage "google.golang.org/api/storage/v1"
 )
 
@@ -381,7 +384,11 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 	} else {
 		oAuthClient, _, err = oauthutil.NewClient(name, m, storageConfig)
 		if err != nil {
-			return nil, errors.Wrap(err, "failed to configure Google Cloud Storage")
+			ctx := context.Background()
+			oAuthClient, err = google.DefaultClient(ctx, storage.DevstorageFullControlScope)
+			if err != nil {
+				return nil, errors.Wrap(err, "failed to configure Google Cloud Storage")
+			}
 		}
 	}
 

docs/content/googlecloudstorage.md

@@ -217,6 +217,20 @@ the rclone config file, you can set `service_account_credentials` with
 the actual contents of the file instead, or set the equivalent
 environment variable.
 
+### Application Default Credentials ###
+
+If no other source of credentials is provided, rclone will fall back
+to
+[Application Default Credentials](https://cloud.google.com/video-intelligence/docs/common/auth#authenticating_with_application_default_credentials)
+this is useful both when you already have configured authentication
+for your developer account, or in production when running on a google
+compute host. Note that if running in docker, you may need to run
+additional commands on your google compute machine -
+[see this page](https://cloud.google.com/container-registry/docs/advanced-authentication#gcloud_as_a_docker_credential_helper).
+
+Note that in the case application default credentials are used, there
+is no need to explicitly configure a project number.
+
 ### --fast-list ###
 
 This remote supports `--fast-list` which allows you to use fewer