"Upgrade Required":                http.StatusUpgradeRequired,
	"Precondition Required":           http.StatusPreconditionRequired,
	"Too Many Requests":               http.StatusTooManyRequests,
	"Request Header Fields Too Large": http.StatusRequestHeaderFieldsTooLarge,
	"Unavailable For Legal Reasons":   http.StatusUnavailableForLegalReasons,
	"Internal Server Error":           http.StatusInternalServerError,

pkg net/http, const StatusPreconditionRequired ideal-int
pkg net/http, const StatusRequestHeaderFieldsTooLarge = 431
pkg net/http, const StatusRequestHeaderFieldsTooLarge ideal-int
pkg net/http, const StatusTooManyRequests = 429
pkg net/http, const StatusTooManyRequests ideal-int
pkg net/http, const StatusUnavailableForLegalReasons = 451
pkg net/http, const StatusUnavailableForLegalReasons ideal-int

// writeErrorResponse writes error headers
func writeErrorResponse(ctx context.Context, w http.ResponseWriter, err APIError, reqURL *url.URL) {
	switch err.HTTPStatusCode {
	case http.StatusServiceUnavailable, http.StatusTooManyRequests:
		// Set retry-after header to indicate user-agents to retry request after 60 seconds.
		// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
		w.Header().Set(xhttp.RetryAfter, "60")
	}