oauth2: perform caching ops at the transport level

oauth2.go

@@ -64,8 +64,6 @@ type TokenFetcher interface {
 	// If the implementation doesn't know how to retrieve a new token,
 	// it returns an error.
 	FetchToken(existing *Token) (*Token, error)
-	// Cache returns the Cache implementation to read/persist user tokens.
-	Cache() Cache
 }
 
 // Options represents options to provide OAuth 2.0 client credentials
@@ -130,8 +128,6 @@ type Config struct {
 	authURL string
 	// TokenURL is the URL used to retrieve OAuth tokens.
 	tokenURL string
-
-	cache Cache
 }
 
 // AuthCodeURL returns a URL to OAuth 2.0 provider's consent page
@@ -187,12 +183,7 @@ func (c *Config) NewTransportWithCode(exchangeCode string) (Transport, error) {
 // token from the provided cache. If a token refreshing occurs, it
 // writes the newly fetched token back to the cache.
 func (c *Config) NewTransportWithCache(cache Cache) (Transport, error) {
-	token, err := cache.Read()
-	if err != nil {
-		return nil, err
-	}
-	c.cache = cache
-	return NewAuthorizedTransport(c, token), nil
+	return NewAuthorizedTransportWithCache(c, cache)
 }
 
 // Exchange exchanges the exchange code with the OAuth 2.0 provider
@@ -225,11 +216,6 @@ func (c *Config) FetchToken(existing *Token) (*Token, error) {
 	return existing, err
 }
 
-// Cache returns a cache if specified, otherwise nil.
-func (c *Config) Cache() Cache {
-	return c.cache
-}
-
 // Checks if all required configuration fields have non-zero values.
 func (c *Config) validate() error {
 	if c.opts.ClientID == "" {