gistfile1.go

package oauth2Sample

imports (
    "fmt"
    "log"
    "net/http"

    "golang.org/x/oauth2"
)    

// UserOauthClient returns an oauth2 client for a specific user
func UserOAuthClient(ctx oauth2.Context, config auth2.Config, userId string) (client *http.Client, err error) {
	var userToken *oauth2.Token
	if userToken, err = getCachedToken(userId); err != nil {
	    // if token for user is not cached then go through oauth2 flow
	    if userToken, err = newUserToken(ctx, config, userId); err != nil {
	        return
	    } 
	}
	if  !userToken.Valid() { // if user token is expired
	    userToken = &oauth2.Token{ RefreshToken: userToken.RefreshToken }
	}
	return config.Client(ctx, userToken)
}

func getCachedToken(userId string) (*oauth2.Token, error) {
    var tk *oauth2.Token
    // logic to retrieve refresh token  from your datastore
    return tk, err
}

func newUserToken(ctx, config, userId) (*oauth2.Token, error) {
    stateBytes := make([]byte, 32)
	_, err := rand.Read(stateBytes)
	if err != nil {
		log.Fatalf("Unable to read random bytes: %v", err)
		return nil, err
	}
	state := fmt.Sprintf("%x", stateBytes)
	authUrl := config.AuthCodeURL(state)
	//... your logic for handing retrieving code for oauth exchange and matching state
	token, err := config.Exchange(oauth2.NoContext, authcode)
	if err != nil {
		log.Fatalf("Exchange error: %v", err)
		return nil, err
	}
	saveToken(userId, token) // save token to datastore
	
	return token, nil
}