everywhere else, an empty labels map selects everything ...

// SelectorFromSet returns a Selector which will match exactly the given Set. A
// nil Set is considered equivalent to Everything().
func SelectorFromSet(ls Set) Selector {

making this switch so that an empty set selects nothing is really confusing and kind of breaking

not exactly
e.g.

a nil *LabelSelector means "match nothing"

an nil map[string]string label selector means "match everything"

Comparing an interface instance to a concrete value to determine if it is the special-case "match nothing" is a bit odd. I'm not sure whether it is better to add a new method to the labels.Selector interface so implementations can identify this case, or add a helper func MatchesNothing(selector labels.Selector) bool function to the labels package so we can keep the special comparison to the nothingSelector type local to the package.

cc @deads2k for an opinion

cc @munnerz who has spent some time thinking about optimizing / canonicalizing selectors (so something like matchExpressions: [{"key":"foo","operator":"Exists"}, {"key":"foo","operator":"DoesNotExist"}] would also correctly identify as matching nothing)

External callers might also need to check whether the selector is labels.Nothing(). Would it be ok to add a new function to the interface, similar to Empty()?

I prefer func MatchesNothing(selector labels.Selector) with an implementation we can improve over time.

Thank you all for the code review. The helper function has been added.