types/key: make Public implement TextMarshaler, TextUnmarshaler

So it can be a map key with encoding/json

types/key/key.go

@@ -7,6 +7,8 @@
 
 import (
 	"encoding/base64"
+	"errors"
+	"fmt"
 
 	"golang.org/x/crypto/curve25519"
 )
@@ -35,6 +37,26 @@ func (p Public) ShortString() string {
 	return "[" + base64.StdEncoding.EncodeToString(p[:])[:5] + "]"
 }
 
+func (p Public) MarshalText() ([]byte, error) {
+	buf := make([]byte, base64.StdEncoding.EncodedLen(len(p)))
+	base64.StdEncoding.Encode(buf, p[:])
+	return buf, nil
+}
+
+func (p *Public) UnmarshalText(txt []byte) error {
+	if *p != (Public{}) {
+		return errors.New("refusing to unmarshal into non-zero key.Public")
+	}
+	n, err := base64.StdEncoding.Decode(p[:], txt)
+	if err != nil {
+		return err
+	}
+	if n != 32 {
+		return fmt.Errorf("short decode of %d; want 32", n)
+	}
+	return nil
+}
+
 // B32 returns k as the *[32]byte type that's used by the
 // golang.org/x/crypto packages. This allocates; it might
 // not be appropriate for performance-sensitive paths.

types/key/key_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package key
+
+import (
+	"testing"
+)
+
+func TestTextUnmarshal(t *testing.T) {
+	p := Public{1, 2}
+	text, err := p.MarshalText()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var p2 Public
+	if err := p2.UnmarshalText(text); err != nil {
+		t.Fatal(err)
+	}
+	if p != p2 {
+		t.Fatalf("mismatch; got %x want %x", p2, p)
+	}
+}