message: implement String()

This provides a convenient way to visualize message data when debugging,
logging, etc. While the format is inspired by swanctl.conf, String() is
not guaranteed to produce valid output for use in swanctl.conf directly.

Exercise this new method in the message_test.go examples.

vici/message.go

@@ -257,6 +257,30 @@ func (m *Message) Err() error {
 	return nil
 }
 
+func (m *Message) stringIndent(prefix, indent string) string {
+	var str string
+
+	for k, v := range m.elements() {
+		switch v := v.(type) {
+		case string:
+			str += fmt.Sprintf("%s%s%s = %s\n", prefix, indent, k, v)
+		case []string:
+			str += fmt.Sprintf("%s%s%s = %s\n", prefix, indent, k, strings.Join(v, ","))
+		case *Message:
+			str += fmt.Sprintf("%s%s%s %s", prefix, indent, k, v.stringIndent(prefix+indent, indent))
+		}
+	}
+	str = fmt.Sprintf("{\n%s%s}\n", str, prefix)
+
+	return str
+}
+
+// String returns the string form of m. For readability, the output format is similar to
+// swanctl.conf configuration format.
+func (m *Message) String() string {
+	return m.stringIndent("", "  ")
+}
+
 // packetIsNamed returns a bool indicating the packet is a named type
 func (m *Message) packetIsNamed() bool {
 	if m.header == nil {

vici/message_test.go

@@ -333,16 +333,28 @@ func ExampleMarshalMessage() {
 		return
 	}
 
-	fmt.Println(m.Get("proposals"))
+	fmt.Println(m)
+	// Output: {
+	//   local_addrs = 192.168.0.1
+	//   local {
+	//     auth = pubkey
+	//     certs = moonCert.pem
+	//     id = moon.strongswan.org
+	//   }
+	//   remote {
+	//     auth = pubkey
+	//   }
+	//   children {
+	//     net {
+	//       local_ts = 10.1.0.0/16
+	//       updown = /usr/local/libexec/ipsec/_updown iptables
+	//       esp_proposals = aes128gcm128-x25519
+	//     }
+	//   }
+	//   version = 2
+	//   proposals = aes128-sha256-x25519
+	// }
 
-	net := m.Get("children").(*Message).Get("net").(*Message)
-	for _, k := range net.Keys() {
-		fmt.Printf("%s: %s\n", k, net.Get(k))
-	}
-	// Output: [aes128-sha256-x25519]
-	// local_ts: [10.1.0.0/16]
-	// updown: /usr/local/libexec/ipsec/_updown iptables
-	// esp_proposals: [aes128gcm128-x25519]
 }
 
 func ExampleUnmarshalMessage() {
@@ -737,8 +749,12 @@ func ExampleMessage_Set() {
 		return
 	}
 
-	fmt.Printf("%v, %v, %v\n", m.Get("version"), m.Get("mobike"), m.Get("local_addrs"))
-	// Output: 2, no, [192.168.0.1/24]
+	fmt.Println(m)
+	// Output: {
+	//   version = 2
+	//   mobike = no
+	//   local_addrs = 192.168.0.1/24
+	// }
 }
 
 func TestEmptyMessageElement(t *testing.T) {