tstime/rate: implement Value.{Marshal,Unmarshal}JSON (#8481)

Implement support for marshaling and unmarshaling a Value.

Updates tailscale/corp#8427

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tstime/rate/value.go

@@ -4,6 +4,7 @@
 package rate
 
 import (
+	"encoding/json"
 	"fmt"
 	"math"
 	"sync"
@@ -181,3 +182,41 @@ func (r *Value) rateNow(now mono.Time) float64 {
 func (r *Value) normalizedIntegral() float64 {
 	return r.halfLife() / math.Ln2
 }
+
+type jsonValue struct {
+	// TODO: Use v2 "encoding/json" for native time.Duration formatting.
+	HalfLife string    `json:"halfLife,omitempty,omitzero"`
+	Value    float64   `json:"value,omitempty,omitzero"`
+	Updated  mono.Time `json:"updated,omitempty,omitzero"`
+}
+
+func (r *Value) MarshalJSON() ([]byte, error) {
+	if r == nil {
+		return []byte("null"), nil
+	}
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	v := jsonValue{Value: r.value, Updated: r.updated}
+	if r.HalfLife > 0 {
+		v.HalfLife = r.HalfLife.String()
+	}
+	return json.Marshal(v)
+}
+
+func (r *Value) UnmarshalJSON(b []byte) error {
+	var v jsonValue
+	if err := json.Unmarshal(b, &v); err != nil {
+		return err
+	}
+	halfLife, err := time.ParseDuration(v.HalfLife)
+	if err != nil && v.HalfLife != "" {
+		return fmt.Errorf("invalid halfLife: %w", err)
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.HalfLife = halfLife
+	r.value = v.Value
+	r.updated = v.Updated
+	return nil
+}

tstime/rate/value_test.go

@@ -6,12 +6,14 @@
 import (
 	"flag"
 	"math"
+	"reflect"
 	"testing"
 	"time"
 
 	qt "github.com/frankban/quicktest"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"tailscale.com/tstime/mono"
+	"tailscale.com/util/must"
 )
 
 const (
@@ -234,3 +236,26 @@ func BenchmarkValue(b *testing.B) {
 		v.Add(1)
 	}
 }
+
+func TestValueMarshal(t *testing.T) {
+	now := mono.Now()
+	tests := []struct {
+		val *Value
+		str string
+	}{
+		{val: &Value{}, str: `{}`},
+		{val: &Value{HalfLife: 5 * time.Minute}, str: `{"halfLife":"` + (5 * time.Minute).String() + `"}`},
+		{val: &Value{value: 12345, updated: now}, str: `{"value":12345,"updated":` + string(must.Get(now.MarshalJSON())) + `}`},
+	}
+	for _, tt := range tests {
+		str := string(must.Get(tt.val.MarshalJSON()))
+		if str != tt.str {
+			t.Errorf("string mismatch: got %v, want %v", str, tt.str)
+		}
+		var val Value
+		must.Do(val.UnmarshalJSON([]byte(str)))
+		if !reflect.DeepEqual(&val, tt.val) {
+			t.Errorf("value mismatch: %+v, want %+v", &val, tt.val)
+		}
+	}
+}