types/opt: support an explicit "unset" value for Bool

Updates #4843

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

types/opt/bool.go

@@ -10,9 +10,13 @@ import (
 	"strconv"
 )
 
-// Bool represents an optional boolean to be JSON-encoded.
+// Bool represents an optional boolean to be JSON-encoded.  The string
-// The string can be empty (for unknown or unspecified), or
+// is either "true", "false", or the enmpty string to mean unset.
-// "true" or "false".
+//
+// As a special case, the underlying string may also be the string
+// "unset" as as a synonym for the empty string. This lets the
+// explicit unset value be exchanged over an encoding/json "omitempty"
+// field without it being dropped.
 type Bool string
 
 func (b *Bool) Set(v bool) {
@@ -22,11 +26,14 @@ func (b *Bool) Set(v bool) {
 func (b *Bool) Clear() { *b = "" }
 
 func (b Bool) Get() (v bool, ok bool) {
-	if b == "" {
+	switch b {
-		return
+	case "true":
+		return true, true
+	case "false":
+		return false, true
+	default:
+		return false, false
 	}
-	v, err := strconv.ParseBool(string(b))
-	return v, err == nil
 }
 
 // Scan implements database/sql.Scanner.
@@ -74,7 +81,7 @@ func (b Bool) MarshalJSON() ([]byte, error) {
 		return trueBytes, nil
 	case "false":
 		return falseBytes, nil
-	case "":
+	case "", "unset":
 		return nullBytes, nil
 	}
 	return nil, fmt.Errorf("invalid opt.Bool value %q", string(b))
@@ -94,7 +101,7 @@ func (b *Bool) UnmarshalJSON(j []byte) error {
 		return nil
 	}
 	if string(j) == "null" {
-		*b = ""
+		*b = "unset"
 		return nil
 	}
 	return fmt.Errorf("invalid opt.Bool value %q", j)

types/opt/bool_test.go

@@ -12,9 +12,10 @@ import (
 
 func TestBool(t *testing.T) {
 	tests := []struct {
-		name string
+		name     string
-		in   any
+		in       any
-		want string // JSON
+		want     string // JSON
+		wantBack any
 	}{
 		{
 			name: "null_for_unset",
@@ -27,6 +28,15 @@ func TestBool(t *testing.T) {
 				False: "false",
 			},
 			want: `{"True":true,"False":false,"Unset":null}`,
+			wantBack: struct {
+				True  Bool
+				False Bool
+				Unset Bool
+			}{
+				True:  "true",
+				False: "false",
+				Unset: "unset",
+			},
 		},
 		{
 			name: "omitempty_unset",
@@ -40,6 +50,24 @@ func TestBool(t *testing.T) {
 			},
 			want: `{"True":true,"False":false}`,
 		},
+		{
+			name: "unset_marshals_as_null",
+			in: struct {
+				True  Bool
+				False Bool
+				Foo   Bool
+			}{
+				True:  "true",
+				False: "false",
+				Foo:   "unset",
+			},
+			want: `{"True":true,"False":false,"Foo":null}`,
+			wantBack: struct {
+				True  Bool
+				False Bool
+				Foo   Bool
+			}{"true", "false", "unset"},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -51,6 +79,10 @@ func TestBool(t *testing.T) {
 				t.Errorf("wrong JSON:\n got: %s\nwant: %s\n", j, tt.want)
 			}
 
+			wantBack := tt.in
+			if tt.wantBack != nil {
+				wantBack = tt.wantBack
+			}
 			// And back again:
 			newVal := reflect.New(reflect.TypeOf(tt.in))
 			out := newVal.Interface()
@@ -58,8 +90,8 @@ func TestBool(t *testing.T) {
 				t.Fatalf("Unmarshal %#q: %v", j, err)
 			}
 			got := newVal.Elem().Interface()
-			if !reflect.DeepEqual(tt.in, got) {
+			if !reflect.DeepEqual(got, wantBack) {
-				t.Errorf("value mismatch\n got: %+v\nwant: %+v\n", got, tt.in)
+				t.Errorf("value mismatch\n got: %+v\nwant: %+v\n", got, wantBack)
 			}
 		})
 	}
@@ -79,11 +111,12 @@ func TestBoolEqualBool(t *testing.T) {
 		{"true", false, false},
 		{"false", true, false},
 		{"false", false, true},
+		{"1", true, false},    // "1" is not true; only "true" is
+		{"True", true, false}, // "True" is not true; only "true" is
 	}
 	for _, tt := range tests {
 		if got := tt.b.EqualBool(tt.v); got != tt.want {
 			t.Errorf("(%q).EqualBool(%v) = %v; want %v", string(tt.b), tt.v, got, tt.want)
 		}
 	}
-
 }