Enable the ability to connect as a service account

main.go

@@ -12,18 +12,43 @@ import (
 	"google.golang.org/api/drive/v3"
 	"google.golang.org/api/googleapi"
 	"google.golang.org/api/option"
+	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
 )
 
 type TokenCache struct {
+	CredentialPath   string
+	CredentialScopes []string
 	ConfigPath       string
 	TokenPath        string
 	configLoaded     bool
+	CredentialData   *google.Credentials
 	ConfigData       oauth2.Config
 }
 
+func (c *TokenCache) Credentials() (*google.Credentials, error) {
+	if c.CredentialData != nil {
+		f, err := os.Open(c.CredentialPath)
+		if err != nil {
+			return nil, fmt.Errorf("could not open credentials: %v", err)
+		}
+		defer func() { _ = f.Close() }()
+		data, err := ioutil.ReadAll(f)
+		if err != nil {
+			return nil, fmt.Errorf("could not read credentials: %v", err)
+		}
+		c.CredentialData, err = google.CredentialsFromJSON(context.Background(), data, c.CredentialScopes...)
+		err = json.NewDecoder(f).Decode(&c.ConfigData)
+		if err != nil {
+			return nil, fmt.Errorf("could not decode config: %v", err)
+		}
+		c.configLoaded = true
+	}
+	return c.CredentialData, nil
+}
+
 func (c *TokenCache) Config() (*oauth2.Config, error) {
 	if !c.configLoaded {
 		f, err := os.Open(c.ConfigPath)
@@ -40,11 +65,29 @@ func (c *TokenCache) Config() (*oauth2.Config, error) {
 	return &c.ConfigData, nil
 }
 
-// Retrieves a token and saves the token.
+func (c *TokenCache) CredentialToken() (*oauth2.Token, error) {
+	log.Printf("Getting token from credentials file %s", c.CredentialPath)
+	creds, err := c.Credentials()
+	if err != nil {
+		return nil, err
+	}
+	return creds.TokenSource.Token()
+}
+
 func (c *TokenCache) Token() (*oauth2.Token, error) {
+	if c.CredentialPath != "" {
+		return c.CredentialToken()
+	} else {
+		return c.StandardToken()
+	}
+}
+
+// Retrieves a token and saves the token.
+func (c *TokenCache) StandardToken() (*oauth2.Token, error) {
 	// The file token.json stores the user's access and refresh tokens, and is
 	// created automatically when the authorization flow completes for the first
 	// time.
+	log.Printf("Getting token from secret file %s with cache in %s", c.ConfigPath, c.TokenPath)
 	tok, err := c.CachedToken()
 	if err == nil && tok.Valid() {
 		return tok, nil
@@ -142,6 +185,7 @@ func (c *TokenCache) SaveToken(token *oauth2.Token) error {
 
 func main() {
 	tokenCacheSource := TokenCache{
+		CredentialScopes: []string{drive.DriveScope},
 		ConfigData: oauth2.Config{
 			Endpoint:    google.Endpoint,
 			RedirectURL: "urn:ietf:wg:oauth:2.0:oob",
@@ -149,15 +193,20 @@ func main() {
 		},
 	}
 	flag.StringVar(
-		&tokenCacheSource.ConfigPath,
-		"secrets",
-		"./secrets.json",
-		"The path to the json file containing the client ID and secret.")
+		&tokenCacheSource.CredentialPath,
+		"credentials",
+		"",
+		"If set, overrides secrets.json and token.json. The path where the credentials file should be read from.")
 	flag.StringVar(
 		&tokenCacheSource.TokenPath,
 		"token",
 		"./token.json",
 		"The path where the cached token should be stored and read from.")
+	flag.StringVar(
+		&tokenCacheSource.ConfigPath,
+		"secrets",
+		"./secrets.json",
+		"The path to the json file containing the client ID and secret.")
 	fileId := flag.String(
 		"file-id",
 		"",