func checkModSum(mod module.Version, h string) error {
	// We lock goSum when manipulating it,
	// but we arrange to release the lock when calling checkSumDB,
	// so that parallel calls to checkModHash can execute parallel calls
	// to checkSumDB.

	// Check whether mod+h is listed in go.sum already. If so, we're done.
	goSum.mu.Lock()
	inited, err := initGoSum()
	if err != nil {
		goSum.mu.Unlock()