Cache read operation should be handled during construction.

cache.go

@@ -7,19 +7,34 @@ package oauth2
 import (
 	"encoding/json"
 	"io/ioutil"
+	"os"
 )
 
 // Cache represents a token cacher.
 type Cache interface {
-	// Read reads a cache token from the specified file.
-	Read() (token *Token)
+	// Token returns the initial token retrieved from the cache,
+	// if there is no existing token nil value is returned.
+	Token() (token *Token)
 	// Write writes a token to the specified file.
 	Write(token *Token)
 }
 
 // NewFileCache creates a new file cache.
-func NewFileCache(filename string) *FileCache {
-	return &FileCache{filename: filename}
+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
 }
 
 // FileCache represents a file based token cacher.
@@ -28,19 +43,15 @@ type FileCache struct {
 	// during read or write operations.
 	ErrorHandler func(error)
 
-	filename string
+	initialToken *Token
+	filename     string
 }
 
-// Read reads a cache token from the specified file.
-func (f *FileCache) Read() (token *Token) {
-	data, err := ioutil.ReadFile(f.filename)
-	if err == nil {
-		err = json.Unmarshal(data, token)
-	}
-	if f.ErrorHandler != nil {
-		f.ErrorHandler(err)
-	}
-	return
+// 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
 }
 
 // Write writes a token to the specified file.