types/wgkey: use value receiver with MarshalJSON

Pointer receivers used with MarshalJSON are code rakes.

https://github.com/golang/go/issues/22967
https://github.com/dominikh/go-tools/issues/911

I just stepped on one, and it hurt. Turn it over.
While we're here, optimize the code a bit.

name           old time/op    new time/op    delta
MarshalJSON-8     184ns ± 0%      44ns ± 0%  -76.03%  (p=0.000 n=20+19)

name           old alloc/op   new alloc/op   delta
MarshalJSON-8      184B ± 0%       80B ± 0%  -56.52%  (p=0.000 n=20+20)

name           old allocs/op  new allocs/op  delta
MarshalJSON-8      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.000 n=20+20)

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

types/wgkey/key.go

@@ -90,14 +90,12 @@ func (k *Key) IsZero() bool {
 	return subtle.ConstantTimeCompare(zeros[:], k[:]) == 1
 }
 
-func (k *Key) MarshalJSON() ([]byte, error) {
-	if k == nil {
-		return []byte("null"), nil
-	}
-	// TODO(josharian): use encoding/hex instead?
-	buf := new(bytes.Buffer)
-	fmt.Fprintf(buf, `"%x"`, k[:])
-	return buf.Bytes(), nil
+func (k Key) MarshalJSON() ([]byte, error) {
+	buf := make([]byte, 2+len(k)*2)
+	buf[0] = '"'
+	hex.Encode(buf[1:], k[:])
+	buf[len(buf)-1] = '"'
+	return buf, nil
 }
 
 func (k *Key) UnmarshalJSON(b []byte) error {

types/wgkey/key_test.go

@@ -6,6 +6,7 @@
 
 import (
 	"bytes"
+	"encoding/json"
 	"testing"
 )
 
@@ -20,7 +21,7 @@ func TestKeyBasics(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	t.Run("JSON round-trip", func(t *testing.T) {
+	t.Run("JSON round-trip (pointer)", func(t *testing.T) {
 		// should preserve the keys
 		k2 := new(Key)
 		if err := k2.UnmarshalJSON(b); err != nil {
@@ -55,6 +56,27 @@ func TestKeyBasics(t *testing.T) {
 			t.Fatalf("base64-encoded keys match: %s, %s", b1, b2)
 		}
 	})
+
+	t.Run("JSON round-trip (value)", func(t *testing.T) {
+		type T struct {
+			K Key
+		}
+		v := T{K: *k1}
+		b, err := json.Marshal(v)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var u T
+		if err := json.Unmarshal(b, &u); err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(v.K[:], u.K[:]) {
+			t.Fatalf("v.K %v != u.K %v", v.K[:], u.K[:])
+		}
+		if b1, b2 := v.K.String(), u.K.String(); b1 != b2 {
+			t.Fatalf("base64-encoded keys do not match: %s, %s", b1, b2)
+		}
+	})
 }
 func TestPrivateKeyBasics(t *testing.T) {
 	pri, err := NewPrivate()
@@ -109,3 +131,28 @@ func TestPrivateKeyBasics(t *testing.T) {
 		}
 	})
 }
+
+func TestMarshalJSONAllocs(t *testing.T) {
+	var k Key
+	f := testing.AllocsPerRun(100, func() {
+		k.MarshalJSON()
+	})
+	n := int(f)
+	if n != 1 {
+		t.Fatalf("max one alloc per Key.MarshalJSON, got %d", n)
+	}
+}
+
+var sink []byte
+
+func BenchmarkMarshalJSON(b *testing.B) {
+	b.ReportAllocs()
+	var k Key
+	for i := 0; i < b.N; i++ {
+		var err error
+		sink, err = k.MarshalJSON()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}