Expand TestCustomMessage enum to support arbitrary number of message exchanges.

- The current test suite architecture is limited as it allows only one request
  followed by one response and doesn't support arbitrary number of back-and-forth
  messages.
- Expand the TestCustomMessage enum to introduce this functionality within the
  test suite.
- These two variants will serve as responses to each other, allowing unlimited
  back-and-forth communication to test the response mechanism.

Author: shaavan

lightning/src/onion_message/functional_tests.rs

@@ -82,18 +82,28 @@ impl OffersMessageHandler for TestOffersMessageHandler {
 enum TestCustomMessage {
 	Request,
 	Response,
+	/// Variants to allow reponse ping-pong.
+	ResponseA,
+	ResponseB,
 }
 
 const CUSTOM_REQUEST_MESSAGE_TYPE: u64 = 4242;
 const CUSTOM_RESPONSE_MESSAGE_TYPE: u64 = 4343;
+const CUSTOM_RESPONSE_A_MESSAGE_TYPE: u64 = 4344;
+const CUSTOM_RESPONSE_B_MESSAGE_TYPE: u64 = 4345;
+
 const CUSTOM_REQUEST_MESSAGE_CONTENTS: [u8; 32] = [42; 32];
 const CUSTOM_RESPONSE_MESSAGE_CONTENTS: [u8; 32] = [43; 32];
+const CUSTOM_RESPONSE_A_MESSAGE_CONTENTS: [u8; 32] = [44; 32];
+const CUSTOM_RESPONSE_B_MESSAGE_CONTENTS: [u8; 32] = [45; 32];
 
 impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
 		match self {
 			TestCustomMessage::Request => CUSTOM_REQUEST_MESSAGE_TYPE,
 			TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
+			TestCustomMessage::ResponseA => CUSTOM_RESPONSE_A_MESSAGE_TYPE,
+			TestCustomMessage::ResponseB => CUSTOM_RESPONSE_B_MESSAGE_TYPE,
 		}
 	}
 	fn msg_type(&self) -> &'static str {
@@ -106,6 +116,8 @@ impl Writeable for TestCustomMessage {
 		match self {
 			TestCustomMessage::Request => Ok(CUSTOM_REQUEST_MESSAGE_CONTENTS.write(w)?),
 			TestCustomMessage::Response => Ok(CUSTOM_RESPONSE_MESSAGE_CONTENTS.write(w)?),
+			TestCustomMessage::ResponseA => Ok(CUSTOM_RESPONSE_A_MESSAGE_CONTENTS.write(w)?),
+			TestCustomMessage::ResponseB => Ok(CUSTOM_RESPONSE_B_MESSAGE_CONTENTS.write(w)?),
 		}
 	}
 }
@@ -145,6 +157,8 @@ impl CustomOnionMessageHandler for TestCustomMessageHandler {
 		let response_option = match msg {
 			TestCustomMessage::Request => Some(TestCustomMessage::Response),
 			TestCustomMessage::Response => None,
+			TestCustomMessage::ResponseA => Some(TestCustomMessage::ResponseB),
+			TestCustomMessage::ResponseB => Some(TestCustomMessage::ResponseA),
 		};
 		if let (Some(response), Some(responder)) = (response_option, responder) {
 			responder.respond(response)
@@ -164,6 +178,16 @@ impl CustomOnionMessageHandler for TestCustomMessageHandler {
 				assert_eq!(buf, CUSTOM_RESPONSE_MESSAGE_CONTENTS);
 				Ok(Some(TestCustomMessage::Response))
 			},
+			CUSTOM_RESPONSE_A_MESSAGE_TYPE => {
+				let buf = read_to_end(buffer)?;
+				assert_eq!(buf, CUSTOM_RESPONSE_A_MESSAGE_CONTENTS);
+				Ok(Some(TestCustomMessage::ResponseA))
+			},
+			CUSTOM_RESPONSE_B_MESSAGE_TYPE => {
+				let buf = read_to_end(buffer)?;
+				assert_eq!(buf, CUSTOM_RESPONSE_B_MESSAGE_CONTENTS);
+				Ok(Some(TestCustomMessage::ResponseB))
+			}
 			_ => Ok(None),
 		}
 	}