Provide a transport constructor for transporters with a cache.

cache.go

@@ -12,46 +12,39 @@ import (
 
 // Cache represents a token cacher.
 type Cache interface {
-	// Token returns the initial token retrieved from the cache,
-	// if there is no existing token nil value is returned.
-	Token() (token *Token)
+	// Reads a cached token.
+	// It may return a nil value if no token is cached.
+	Read() (token *Token, err error)
 	// Write writes a token to the specified file.
 	Write(token *Token)
 }
 
 // NewFileCache creates a new file cache.
-func NewFileCache(filename string) (cache *FileCache, err error) {
-	data, err := ioutil.ReadFile(filename)
-	if os.IsNotExist(err) {
-		// no token has cached before, skip reading
-		return &FileCache{filename: filename}, nil
-	}
-	if err != nil {
-		return
-	}
-	var token Token
-	if err = json.Unmarshal(data, &token); err != nil {
-		return
-	}
-	cache = &FileCache{filename: filename, initialToken: &token}
-	return
+func NewFileCache(filename string) (cache *FileCache) {
+	return &FileCache{filename: filename}
 }
 
 // FileCache represents a file based token cacher.
 type FileCache struct {
-	// Handler to be invoked if an error occurs
-	// during read or write operations.
+	// Handler to be invoked if an error occurs during writing.
 	ErrorHandler func(error)
 
-	initialToken *Token
 	filename string
 }
 
-// Token returns the initial token read from the cache. It should be used to
-// warm the authorization mechanism, token refreshes and later writes don't
-// change the returned value. If no token is cached before, returns nil.
-func (f *FileCache) Token() (token *Token) {
-	return f.initialToken
+func (f *FileCache) Read() (token *Token, err error) {
+	data, err := ioutil.ReadFile(f.filename)
+	if os.IsNotExist(err) {
+		// no token has cached before, skip reading
+		return nil, nil
+	}
+	if err != nil {
+		return
+	}
+	if err = json.Unmarshal(data, &token); err != nil {
+		return
+	}
+	return
 }
 
 // Write writes a token to the specified file.

cache_test.go

@@ -12,13 +12,14 @@ import (
 
 var tokenBody = `{"access_token":"abc123","token_type":"Bearer","refresh_token":"def789","expiry":"0001-01-01T00:00:00Z"}`
 
-func TestNewFileCacheNotExist(t *testing.T) {
-	cache, err := NewFileCache("/path/that/doesnt/exist")
+func TestFileCacheReadNotExist(t *testing.T) {
+	cache := NewFileCache("/path/that/doesnt/exist")
+	token, err := cache.Read()
 	if err != nil {
 		t.Fatalf("NewFileCache shouldn't return an error for if cache file doesn't exist, but returned %v", err)
 	}
-	if cache == nil {
-		t.Fatalf("A file cache should be inited with a non existing cache file")
+	if token != nil {
+		t.Fatalf("Token should be nil, but found %v", token)
 	}
 }
 
@@ -32,11 +33,12 @@ func TestNewFileCache(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	cache, err := NewFileCache(f.Name())
+	cache := NewFileCache(f.Name())
+	token, err := cache.Read()
 	if err != nil {
-		t.Fatalf("Cache should have read the file cache at %v, but recieved %v", f.Name(), err)
+		t.Fatal(err)
 	}
-	token := cache.Token()
+
 	if token.AccessToken != "abc123" {
 		t.Fatalf("Cached access token is %v, expected to be abc123", token.AccessToken)
 	}
@@ -51,15 +53,12 @@ func TestFileCacheWrite(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	cache, err := NewFileCache(path.Join(dirName, "cache-file"))
+	cache := NewFileCache(path.Join(dirName, "cache-file"))
 	cache.ErrorHandler = func(err error) {
 		if err != nil {
 			t.Fatalf("Cache write should have been succeeded succesfully, recieved %v", err)
 		}
 	}
-	if err != nil {
-		t.Fatal(err)
-	}
 
 	cache.Write(&Token{
 		AccessToken:  "abc123",

oauth2.go

@@ -188,6 +188,15 @@ func (c *Config) NewTransportWithCode(exchangeCode string) (Transport, error) {
 	return NewAuthorizedTransport(c, token), nil
 }
 
+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
+}
+
 // Exchange exchanges the exchange code with the OAuth 2.0 provider
 // to retrieve a new access token.
 func (c *Config) Exchange(exchangeCode string) (*Token, error) {

transport.go

@@ -95,13 +95,6 @@ func NewAuthorizedTransport(fetcher TokenFetcher, token *Token) Transport {
 func (t *authorizedTransport) RoundTrip(req *http.Request) (resp *http.Response, err error) {
 	token := t.Token()
 
-	// Try to initialize the token from the cache.
-	if token == nil {
-		if c := t.fetcher.Cache(); c != nil {
-			token = c.Token()
-		}
-	}
-
 	if token == nil || token.Expired() {
 		// Check if the token is refreshable.
 		// If token is refreshable, don't return an error,