kubernetes/staging/src/k8s.io/apiserver/pkg/util/notfoundhandler/not_found_handler.go

// New returns an HTTP handler that is meant to be executed at the end of the delegation chain.
// It checks if the request have been made before the server has installed all known HTTP paths.
// In that case it returns a 503 response otherwise it returns a 404.
//
// Note that we don't want to add additional checks to the readyz path as it might prevent fixing bricked clusters.
// This specific handler is meant to "protect" requests that arrive before the paths and handlers are fully initialized.
func New(serializer runtime.NegotiatedSerializer, isMuxAndDiscoveryCompleteFn func(ctx context.Context) bool) *Handler { … }

type Handler …

func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) { … }