var _ http.RoundTripper = (*Transport)(nil)

type Transport struct {

Base http.RoundTripper

Base http.RoundTripper

//Token is a OAuth token (which has a refresh token) that should be used during roundtrip to automatically

//refresh the OAuth access token once the current one has expired or is soon to expire

Token *Token

//mu is a mutex that should be acquired whenever token used

mu sync.Mutex

}

if err := t.refreshToken(ctx); err != nil {

return nil, err

}

token := t.getToken()

req2 := req.Clone(req.Context())

req2.Header.Set("Authorization", "Bearer "+t.Token.AccessToken)

req2.Header.Set("Authorization", "Bearer "+token.AccessToken)

if t.Base != nil {

return t.Base.RoundTrip(req2)

}

return http.DefaultTransport.RoundTrip(req2)

}

// getToken returns a value copy of token and is guarded by a mutex

func (t *Transport) getToken() Token {

t.mu.Lock()

defer t.mu.Unlock()

return *t.Token

}

// refreshToken checks if the token has expired or expiring soon and refreshes it. Once the token is

// refreshed, the in-memory token is updated and a best effort is made to store the token.

// If storing the token fails, no error is returned.