Allow field/label users to get info from fields

Allows a consumer to get at the information stored in the field selector
for querying an underlying data store.  Not generic, but offers a
simple start.

pkg/labels/selector_test.go

@@ -158,6 +158,9 @@ func TestSetIsEmpty(t *testing.T) {
 	if (&hasTerm{}).Empty() {
 		t.Errorf("hasTerm should not be empty")
 	}
+	if (&notHasTerm{}).Empty() {
+		t.Errorf("notHasTerm should not be empty")
+	}
 	if !(andTerm{andTerm{}}).Empty() {
 		t.Errorf("Nested andTerm should be empty")
 	}
@@ -166,6 +169,36 @@ func TestSetIsEmpty(t *testing.T) {
 	}
 }
 
+func TestRequiresExactMatch(t *testing.T) {
+	testCases := map[string]struct {
+		S     Selector
+		Label string
+		Value string
+		Found bool
+	}{
+		"empty set":                 {Set{}.AsSelector(), "test", "", false},
+		"nil andTerm":               {andTerm(nil), "test", "", false},
+		"empty hasTerm":             {&hasTerm{}, "test", "", false},
+		"skipped hasTerm":           {&hasTerm{"a", "b"}, "test", "", false},
+		"valid hasTerm":             {&hasTerm{"test", "b"}, "test", "b", true},
+		"valid hasTerm no value":    {&hasTerm{"test", ""}, "test", "", true},
+		"valid notHasTerm":          {&notHasTerm{"test", "b"}, "test", "", false},
+		"valid notHasTerm no value": {&notHasTerm{"test", ""}, "test", "", false},
+		"nested andTerm":            {andTerm{andTerm{}}, "test", "", false},
+		"nested andTerm matches":    {andTerm{&hasTerm{"test", "b"}}, "test", "b", true},
+		"andTerm with non-match":    {andTerm{&hasTerm{}, &hasTerm{"test", "b"}}, "test", "b", true},
+	}
+	for k, v := range testCases {
+		value, found := v.S.RequiresExactMatch(v.Label)
+		if value != v.Value {
+			t.Errorf("%s: expected value %s, got %s", k, v.Value, value)
+		}
+		if found != v.Found {
+			t.Errorf("%s: expected found %s, got %s", k, v.Found, found)
+		}
+	}
+}
+
 func expectMatchRequirement(t *testing.T, req Requirement, ls Set) {
 	if !req.Matches(ls) {
 		t.Errorf("Wanted '%+v' to match '%s', but it did not.\n", req, ls)