Optimize turning a Set into a Selector

Fixes 112647

Benchmark:

```
$ benchstat /tmp/{old,new}
name                         old time/op    new time/op    delta
SelectorFromValidatedSet-48     397ns ± 1%     123ns ± 0%   -68.87%  (p=0.008 n=5+5)

name                         old alloc/op   new alloc/op   delta
SelectorFromValidatedSet-48      192B ± 0%        0B       -100.00%  (p=0.008 n=5+5)

name                         old allocs/op  new allocs/op  delta
SelectorFromValidatedSet-48      5.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
```

pkg/scheduler/framework/plugins/podtopologyspread/filtering_test.go

@@ -505,7 +505,7 @@ func TestPreFilterState(t *testing.T) {
 					{
 						MaxSkew:            3,
 						TopologyKey:        "node",
-						Selector:           mustConvertLabelSelectorAsSelector(t, st.MakeLabelSelector().Label("foo", "bar").Obj()),
+						Selector:           labels.SelectorFromValidatedSet(labels.Set{"foo": "bar"}),
 						MinDomains:         1,
 						NodeAffinityPolicy: v1.NodeInclusionPolicyHonor,
 						NodeTaintsPolicy:   v1.NodeInclusionPolicyIgnore,
@@ -513,7 +513,7 @@ func TestPreFilterState(t *testing.T) {
 					{
 						MaxSkew:            5,
 						TopologyKey:        "rack",
-						Selector:           mustConvertLabelSelectorAsSelector(t, st.MakeLabelSelector().Label("foo", "bar").Obj()),
+						Selector:           labels.SelectorFromValidatedSet(labels.Set{"foo": "bar"}),
 						MinDomains:         1,
 						NodeAffinityPolicy: v1.NodeInclusionPolicyHonor,
 						NodeTaintsPolicy:   v1.NodeInclusionPolicyIgnore,
@@ -1283,7 +1283,7 @@ func TestPreFilterState(t *testing.T) {
 					{
 						MaxSkew:            1,
 						TopologyKey:        "zone",
-						Selector:           mustConvertLabelSelectorAsSelector(t, st.MakeLabelSelector().Label("foo", "a").Obj()),
+						Selector:           labels.SelectorFromValidatedSet(labels.Set{"foo": "a"}),
 						MinDomains:         1,
 						NodeAffinityPolicy: v1.NodeInclusionPolicyHonor,
 						NodeTaintsPolicy:   v1.NodeInclusionPolicyIgnore,

staging/src/k8s.io/apimachinery/pkg/labels/selector.go

@@ -919,6 +919,14 @@ func SelectorFromSet(ls Set) Selector {
 	return SelectorFromValidatedSet(ls)
 }
 
+var (
+	// Create constants for hot-path selectors
+	value0Path, keyPath = func() (*field.Path, *field.Path) {
+		path := field.ToPath()
+		return path.Child("values").Index(0), path.Child("key")
+	}()
+)
+
 // ValidatedSelectorFromSet returns a Selector which will match exactly the given Set. A
 // nil and empty Sets are considered equivalent to Everything().
 // The Set is validated client-side, which allows to catch errors early.
@@ -926,17 +934,19 @@ func ValidatedSelectorFromSet(ls Set) (Selector, error) {
 	if ls == nil || len(ls) == 0 {
 		return internalSelector{}, nil
 	}
-	requirements := make([]Requirement, 0, len(ls))
-	for label, value := range ls {
-		r, err := NewRequirement(label, selection.Equals, []string{value})
-		if err != nil {
-			return nil, err
+
+	var allErrs field.ErrorList
+	for k, v := range ls {
+		if err := validateLabelKey(k, keyPath); err != nil {
+			allErrs = append(allErrs, err)
+		}
+		if err := validateLabelValue(k, v, value0Path); err != nil {
+			allErrs = append(allErrs, err)
 		}
-		requirements = append(requirements, *r)
 	}
-	// sort to have deterministic string representation
-	sort.Sort(ByKey(requirements))
-	return internalSelector(requirements), nil
+
+	// TODO: validate labels are valid label values
+	return setSelector(ls), allErrs.ToAggregate()
 }
 
 // SelectorFromValidatedSet returns a Selector which will match exactly the given Set.
@@ -946,13 +956,7 @@ func SelectorFromValidatedSet(ls Set) Selector {
 	if ls == nil || len(ls) == 0 {
 		return internalSelector{}
 	}
-	requirements := make([]Requirement, 0, len(ls))
-	for label, value := range ls {
-		requirements = append(requirements, Requirement{key: label, operator: selection.Equals, strValues: []string{value}})
-	}
-	// sort to have deterministic string representation
-	sort.Sort(ByKey(requirements))
-	return internalSelector(requirements)
+	return setSelector(ls)
 }
 
 // ParseToRequirements takes a string representing a selector and returns a list of
@@ -963,3 +967,75 @@ func SelectorFromValidatedSet(ls Set) Selector {
 func ParseToRequirements(selector string, opts ...field.PathOption) ([]Requirement, error) {
 	return parse(selector, field.ToPath(opts...))
 }
+
+type setSelector Set
+
+func (s setSelector) Matches(labels Labels) bool {
+	for k, v := range s {
+		if !labels.Has(k) || v != labels.Get(k) {
+			return false
+		}
+	}
+	return true
+}
+
+func (s setSelector) Empty() bool {
+	return len(s) == 0
+}
+
+func (s setSelector) String() string {
+	keys := make([]string, 0, len(s))
+	for k := range s {
+		keys = append(keys, k)
+	}
+	// Ensure deterministic output
+	sort.Strings(keys)
+	b := strings.Builder{}
+	for i, key := range keys {
+		last := i == len(keys)-1
+		v := s[key]
+		b.WriteString(key)
+		b.WriteString("=")
+		b.WriteString(v)
+		if !last {
+			b.WriteString(",")
+		}
+	}
+	return b.String()
+}
+
+func (s setSelector) Add(r ...Requirement) Selector {
+	return s.toFullSelector().Add(r...)
+}
+
+func (s setSelector) Requirements() (requirements Requirements, selectable bool) {
+	return s.toFullSelector().Requirements()
+}
+
+func (s setSelector) DeepCopySelector() Selector {
+	res := make(setSelector, len(s))
+	for k, v := range s {
+		res[k] = v
+	}
+	return res
+}
+
+func (s setSelector) RequiresExactMatch(label string) (value string, found bool) {
+	v, f := s[label]
+	return v, f
+}
+
+func (s setSelector) toFullSelector() Selector {
+	if s == nil || len(s) == 0 {
+		return internalSelector{}
+	}
+	requirements := make([]Requirement, 0, len(s))
+	for label, value := range s {
+		requirements = append(requirements, Requirement{key: label, operator: selection.Equals, strValues: []string{value}})
+	}
+	// sort to have deterministic string representation
+	sort.Sort(ByKey(requirements))
+	return internalSelector(requirements)
+}
+
+var _ Selector = setSelector{}

staging/src/k8s.io/apimachinery/pkg/labels/selector_test.go

@@ -809,11 +809,48 @@ func BenchmarkSelectorFromValidatedSet(b *testing.B) {
 		"foo": "foo",
 		"bar": "bar",
 	}
+	matchee := Set(map[string]string{
+		"foo":   "foo",
+		"bar":   "bar",
+		"extra": "label",
+	})
 
 	for i := 0; i < b.N; i++ {
-		if SelectorFromValidatedSet(set).Empty() {
+		s := SelectorFromValidatedSet(set)
+		if s.Empty() {
 			b.Errorf("Unexpected selector")
 		}
+		if !s.Matches(matchee) {
+			b.Errorf("Unexpected match")
+		}
+	}
+}
+
+func TestSetSelectorString(t *testing.T) {
+	cases := []struct {
+		set Set
+		out string
+	}{
+		{
+			Set{},
+			"",
+		},
+		{
+			Set{"app": "foo"},
+			"app=foo",
+		},
+		{
+			Set{"app": "foo", "a": "b"},
+			"a=b,app=foo",
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.out, func(t *testing.T) {
+			if got := setSelector(tt.set).String(); tt.out != got {
+				t.Fatalf("expected %v, got %v", tt.out, got)
+			}
+		})
 	}
 }
 
@@ -899,19 +936,13 @@ func TestValidatedSelectorFromSet(t *testing.T) {
 	tests := []struct {
 		name             string
 		input            Set
-		expectedSelector internalSelector
+		expectedSelector Selector
 		expectedError    field.ErrorList
 	}{
 		{
-			name:  "Simple Set, no error",
-			input: Set{"key": "val"},
-			expectedSelector: internalSelector{
-				Requirement{
-					key:       "key",
-					operator:  selection.Equals,
-					strValues: []string{"val"},
-				},
-			},
+			name:             "Simple Set, no error",
+			input:            Set{"key": "val"},
+			expectedSelector: setSelector{"key": "val"},
 		},
 		{
 			name:  "Invalid Set, value too long",