kubernetes/vendor/go.etcd.io/etcd/server/v3/etcdserver/api/membership/downgrade.go

type DowngradeInfo …

func (d *DowngradeInfo) GetTargetVersion() *semver.Version { … }

// isValidDowngrade verifies whether the cluster can be downgraded from verFrom to verTo
func isValidDowngrade(verFrom *semver.Version, verTo *semver.Version) bool { … }

// mustDetectDowngrade will detect unexpected downgrade when the local server is recovered.
func mustDetectDowngrade(lg *zap.Logger, cv *semver.Version, d *DowngradeInfo) { … }

func AllowedDowngradeVersion(ver *semver.Version) *semver.Version { … }