browser-flag: wrapped bool type denotes browser on/off flag. (#3963)

Statically typed BrowserFlag prevents any arbitrary string value
usage. The wrapped bool marshals/unmarshals JSON according to the
typed value ie string value "on" represents boolean true and "off" as
boolean false.

cmd/browser-flag.go

@@ -0,0 +1,68 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// BrowserFlag - wrapper bool type.
+type BrowserFlag bool
+
+// String - returns string of BrowserFlag.
+func (bf BrowserFlag) String() string {
+	if bf {
+		return "on"
+	}
+
+	return "off"
+}
+
+// MarshalJSON - converts BrowserFlag into JSON data.
+func (bf BrowserFlag) MarshalJSON() ([]byte, error) {
+	return json.Marshal(bf.String())
+}
+
+// UnmarshalJSON - parses given data into BrowserFlag.
+func (bf *BrowserFlag) UnmarshalJSON(data []byte) (err error) {
+	var s string
+	if err = json.Unmarshal(data, &s); err == nil {
+		b := BrowserFlag(true)
+		if s == "" {
+			// Empty string is treated as valid.
+			*bf = b
+		} else if b, err = ParseBrowserFlag(s); err == nil {
+			*bf = b
+		}
+	}
+
+	return err
+}
+
+// ParseBrowserFlag - parses string into BrowserFlag.
+func ParseBrowserFlag(s string) (bf BrowserFlag, err error) {
+	if s == "on" {
+		bf = true
+	} else if s == "off" {
+		bf = false
+	} else {
+		err = fmt.Errorf("invalid value ‘%s’ for BrowserFlag", s)
+	}
+
+	return bf, err
+}