types/opt: add opt package for a new opt.Bool JSON type

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/opt/bool.go

@@ -0,0 +1,68 @@
+// 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 opt defines optional types.
+package opt
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// Bool represents an optional boolean to be JSON-encoded.
+// The string can be empty (for unknown or unspecified), or
+// "true" or "false".
+type Bool string
+
+func (b *Bool) Set(v bool) {
+	*b = Bool(strconv.FormatBool(v))
+}
+
+func (b *Bool) Clear() { *b = "" }
+
+func (b Bool) Get() (v bool, ok bool) {
+	if b == "" {
+		return
+	}
+	v, err := strconv.ParseBool(string(b))
+	return v, err == nil
+}
+
+var (
+	trueBytes  = []byte("true")
+	falseBytes = []byte("false")
+	nullBytes  = []byte("null")
+)
+
+func (b Bool) MarshalJSON() ([]byte, error) {
+	switch b {
+	case "true":
+		return trueBytes, nil
+	case "false":
+		return falseBytes, nil
+	case "":
+		return nullBytes, nil
+	}
+	return nil, fmt.Errorf("invalid opt.Bool value %q", string(b))
+}
+
+func (b *Bool) UnmarshalJSON(j []byte) error {
+	// Note: written with a bunch of ifs instead of a switch
+	// because I'm sure the Go compiler optimizes away these
+	// []byte->string allocations in an == comparison, but I'm too
+	// lazy to check whether that's true in a switch also.
+	if string(j) == "true" {
+		*b = "true"
+		return nil
+	}
+	if string(j) == "false" {
+		*b = "false"
+		return nil
+	}
+	if string(j) == "null" {
+		*b = ""
+		return nil
+	}
+	return fmt.Errorf("invalid opt.Bool value %q", j)
+}

types/opt/bool_test.go

@@ -0,0 +1,66 @@
+// 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 opt
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestBool(t *testing.T) {
+	tests := []struct {
+		name string
+		in   interface{}
+		want string // JSON
+	}{
+		{
+			name: "null_for_unset",
+			in: struct {
+				True  Bool
+				False Bool
+				Unset Bool
+			}{
+				True:  "true",
+				False: "false",
+			},
+			want: `{"True":true,"False":false,"Unset":null}`,
+		},
+		{
+			name: "omitempty_unset",
+			in: struct {
+				True  Bool
+				False Bool
+				Unset Bool `json:",omitempty"`
+			}{
+				True:  "true",
+				False: "false",
+			},
+			want: `{"True":true,"False":false}`,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			j, err := json.Marshal(tt.in)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(j) != tt.want {
+				t.Errorf("wrong JSON:\n got: %s\nwant: %s\n", j, tt.want)
+			}
+
+			// And back again:
+			newVal := reflect.New(reflect.TypeOf(tt.in))
+			out := newVal.Interface()
+			if err := json.Unmarshal(j, out); err != nil {
+				t.Fatalf("Unmarshal %#q: %v", j, err)
+			}
+			got := newVal.Elem().Interface()
+			if !reflect.DeepEqual(tt.in, got) {
+				t.Errorf("value mismatch\n got: %+v\nwant: %+v\n", got, tt.in)
+			}
+		})
+	}
+}