Google Drive SDK does not bail on bad configuration (#733)

internal/storage/googledrive/googledrive.go

@@ -12,13 +12,14 @@ import (
 	"time"
 
 	"crypto/tls"
+	"net/http"
+
 	"github.com/offen/docker-volume-backup/internal/errwrap"
 	"github.com/offen/docker-volume-backup/internal/storage"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"google.golang.org/api/drive/v3"
 	"google.golang.org/api/option"
-	"net/http"
 )
 
 type googleDriveStorage struct {
@@ -47,6 +48,8 @@ func NewStorageBackend(opts Config, logFunc storage.Log) (storage.Backend, error
 	}
 	if opts.ImpersonateSubject != "" {
 		config.Subject = opts.ImpersonateSubject
+	} else {
+		return nil, errwrap.Wrap(nil, "GOOGLE_DRIVE_IMPERSONATE_SUBJECT cannot be left blank")
 	}
 	if opts.TokenURL != "" {
 		config.TokenURL = opts.TokenURL

test/googledrive/docker-compose.yml

@@ -36,6 +36,7 @@ services:
       GOOGLE_DRIVE_ENDPOINT: http://openapi_mock:8080
       GOOGLE_DRIVE_TOKEN_URL: http://oauth2_mock:8090/issuer1/token
       GOOGLE_DRIVE_CREDENTIALS_JSON_FILE: /etc/gdrive/credentials.json
+      GOOGLE_DRIVE_IMPERSONATE_SUBJECT: me@example.com
       GOOGLE_DRIVE_FOLDER_ID: "root"
     volumes:
       - app_data:/backup/app_data:ro