util/set: implement json.Marshaler/Unmarshaler (#10308)

Marshal as a JSON list instead of a map. Because set elements are
`comparable` and not `cmp.Ordered`, we cannot easily sort the items
before marshaling.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

util/set/set.go

@@ -5,6 +5,7 @@
 package set
 
 import (
+	"encoding/json"
 	"maps"
 )
 
@@ -66,3 +67,16 @@ func (s Set[T]) Len() int { return len(s) }
 func (s Set[T]) Equal(other Set[T]) bool {
 	return maps.Equal(s, other)
 }
+
+func (s Set[T]) MarshalJSON() ([]byte, error) {
+	return json.Marshal(s.Slice())
+}
+
+func (s *Set[T]) UnmarshalJSON(buf []byte) error {
+	var ss []T
+	if err := json.Unmarshal(buf, &ss); err != nil {
+		return err
+	}
+	*s = SetOf(ss)
+	return nil
+}

util/set/set_test.go

@@ -4,6 +4,7 @@
 package set
 
 import (
+	"encoding/json"
 	"slices"
 	"testing"
 )
@@ -112,3 +113,48 @@ func TestClone(t *testing.T) {
 		t.Error("clone is not distinct from original")
 	}
 }
+
+func TestSetJSONRoundTrip(t *testing.T) {
+	tests := []struct {
+		desc    string
+		strings Set[string]
+		ints    Set[int]
+	}{
+		{"empty", make(Set[string]), make(Set[int])},
+		{"nil", nil, nil},
+		{"one-item", SetOf([]string{"one"}), SetOf([]int{1})},
+		{"multiple-items", SetOf([]string{"one", "two", "three"}), SetOf([]int{1, 2, 3})},
+	}
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			t.Run("strings", func(t *testing.T) {
+				buf, err := json.Marshal(tt.strings)
+				if err != nil {
+					t.Fatalf("json.Marshal: %v", err)
+				}
+				t.Logf("marshaled: %s", buf)
+				var s Set[string]
+				if err := json.Unmarshal(buf, &s); err != nil {
+					t.Fatalf("json.Unmarshal: %v", err)
+				}
+				if !s.Equal(tt.strings) {
+					t.Errorf("set changed after JSON marshal/unmarshal, before: %v, after: %v", tt.strings, s)
+				}
+			})
+			t.Run("ints", func(t *testing.T) {
+				buf, err := json.Marshal(tt.ints)
+				if err != nil {
+					t.Fatalf("json.Marshal: %v", err)
+				}
+				t.Logf("marshaled: %s", buf)
+				var s Set[int]
+				if err := json.Unmarshal(buf, &s); err != nil {
+					t.Fatalf("json.Unmarshal: %v", err)
+				}
+				if !s.Equal(tt.ints) {
+					t.Errorf("set changed after JSON marshal/unmarshal, before: %v, after: %v", tt.ints, s)
+				}
+			})
+		})
+	}
+}