Add a Clear() function to generic sets

This is useful when a given set is shared through pointers, and
therefore can't be replaced with a new empty set. This replaces
set.Delete(set.UnsortedList()...), and allows the compiler to optimize
the function to a call to runtime.mapclear() when the set content type
is reflexive for ==. That optimization *is* currently accessible using

    s := Set[...]{}
    ...
    for i := range s {
        delete(s, i)
    }

but this circumvents the published API for sets; calling s.Delete(i)
instead can't be optimized in this fashion.

Alternatives considered but discarded include:

* turning sets into a pointer type (this isn't possible because
  pointer types can't be receivers)

* using a pointer receiver for the Clear() function, i.e.

    func (s *Set[T]) Clear() {
        *s = New[T]()
    }

  but this doesn't update shared references.

Signed-off-by: Stephen Kitt <skitt@redhat.com>

staging/src/k8s.io/apimachinery/pkg/util/sets/set.go

@@ -64,6 +64,20 @@ func (s Set[T]) Delete(items ...T) Set[T] {
 	return s
 }
 
+// Clear empties the set.
+// It is preferable to replace the set with a newly constructed set,
+// but not all callers can do that (when there are other references to the map).
+// In some cases the set *won't* be fully cleared, e.g. a Set[float32] containing NaN
+// can't be cleared because NaN can't be removed.
+// For sets containing items of a type that is reflexive for ==,
+// this is optimized to a single call to runtime.mapclear().
+func (s Set[T]) Clear() Set[T] {
+	for key := range s {
+		delete(s, key)
+	}
+	return s
+}
+
 // Has returns true if and only if item is contained in the set.
 func (s Set[T]) Has(item T) bool {
 	_, contained := s[item]

staging/src/k8s.io/apimachinery/pkg/util/sets/set_generic_test.go

@@ -84,6 +84,57 @@ func TestSetDeleteMultiples(t *testing.T) {
 
 }
 
+func TestSetClear(t *testing.T) {
+	s := sets.Set[string]{}
+	s.Insert("a", "b", "c")
+	if s.Len() != 3 {
+		t.Errorf("Expected len=3: %d", s.Len())
+	}
+
+	s.Clear()
+	if s.Len() != 0 {
+		t.Errorf("Expected len=0: %d", s.Len())
+	}
+}
+
+func TestSetClearWithSharedReference(t *testing.T) {
+	s := sets.Set[string]{}
+	s.Insert("a", "b", "c")
+	if s.Len() != 3 {
+		t.Errorf("Expected len=3: %d", s.Len())
+	}
+
+	m := s
+	s.Clear()
+	if s.Len() != 0 {
+		t.Errorf("Expected len=0 on the cleared set: %d", s.Len())
+	}
+	if m.Len() != 0 {
+		t.Errorf("Expected len=0 on the shared reference: %d", m.Len())
+	}
+}
+
+func TestSetClearInSeparateFunction(t *testing.T) {
+	s := sets.Set[string]{}
+	s.Insert("a", "b", "c")
+	if s.Len() != 3 {
+		t.Errorf("Expected len=3: %d", s.Len())
+	}
+
+	clearSetAndAdd(s, "d")
+	if s.Len() != 1 {
+		t.Errorf("Expected len=1: %d", s.Len())
+	}
+	if !s.Has("d") {
+		t.Errorf("Unexpected contents: %#v", s)
+	}
+}
+
+func clearSetAndAdd[T comparable](s sets.Set[T], a T) {
+	s.Clear()
+	s.Insert(a)
+}
+
 func TestNewSet(t *testing.T) {
 	s := sets.New("a", "b", "c")
 	if len(s) != 3 {