Add Custom TLVs for payment::ReceiveTlvs

- Building on the previous commit, this update allows users to
  include their own custom TLVs within the reply path of a sent
  onion message.
- With this, users can attach custom data to the message, which
  will be returned in the response, providing more flexibility for
  custom use cases.

Author: shaavan

fuzz/src/invoice_request_deser.rs

@@ -99,6 +99,7 @@ fn build_response<T: secp256k1::Signing + secp256k1::Verification>(
 			htlc_minimum_msat: 1,
 		},
 		payment_context,
+		custom_data: Vec::new(),
 	};
 	let intermediate_nodes = [PaymentForwardNode {
 		tlvs: ForwardTlvs {

fuzz/src/refund_deser.rs

@@ -76,6 +76,7 @@ fn build_response<T: secp256k1::Signing + secp256k1::Verification>(
 			htlc_minimum_msat: 1,
 		},
 		payment_context,
+		custom_data: Vec::new(),
 	};
 	let intermediate_nodes = [PaymentForwardNode {
 		tlvs: ForwardTlvs {

lightning/src/blinded_path/payment.rs

@@ -260,6 +260,15 @@ pub struct ReceiveTlvs {
 	pub payment_constraints: PaymentConstraints,
 	/// Context for the receiver of this payment.
 	pub payment_context: PaymentContext,
+	/// Custom data set by the user. And is returned back when the blinded path is used.
+	///
+	/// ## Note on Forward Compatibility:
+	/// Users can encode any kind of data into the `Vec<u8>` bytes here. However, they should ensure
+	/// that the data is structured in a forward-compatible manner. This is especially important as
+	/// `ReceiveTlvs` created in one version of the software may still appear in payments received
+	/// shortly after a software upgrade. Proper forward compatibility helps prevent data loss or
+	/// misinterpretation in future versions.
+	pub custom_data: Vec<u8>,
 }
 
 /// Data to construct a [`BlindedHop`] for sending a payment over.
@@ -404,7 +413,8 @@ impl Writeable for ReceiveTlvs {
 		encode_tlv_stream!(w, {
 			(12, self.payment_constraints, required),
 			(65536, self.payment_secret, required),
-			(65537, self.payment_context, required)
+			(65537, self.payment_context, required),
+			(65539, self.custom_data, (default_value, Vec::new())),
 		});
 		Ok(())
 	}
@@ -432,6 +442,7 @@ impl Readable for BlindedPaymentTlvs {
 			(14, features, (option, encoding: (BlindedHopFeatures, WithoutLength))),
 			(65536, payment_secret, option),
 			(65537, payment_context, (default_value, PaymentContext::unknown())),
+			(65539, custom_data, (default_value, Vec::new()))
 		});
 		let _padding: Option<utils::Padding> = _padding;
 
@@ -452,6 +463,7 @@ impl Readable for BlindedPaymentTlvs {
 				payment_secret: payment_secret.ok_or(DecodeError::InvalidValue)?,
 				payment_constraints: payment_constraints.0.unwrap(),
 				payment_context: payment_context.0.unwrap(),
+				custom_data: custom_data.0.unwrap(),
 			}))
 		}
 	}
@@ -683,6 +695,7 @@ mod tests {
 				htlc_minimum_msat: 1,
 			},
 			payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 		};
 		let htlc_maximum_msat = 100_000;
 		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_maximum_msat, 12).unwrap();
@@ -702,6 +715,7 @@ mod tests {
 				htlc_minimum_msat: 1,
 			},
 			payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 		};
 		let blinded_payinfo = super::compute_payinfo(&[], &recv_tlvs, 4242, TEST_FINAL_CLTV as u16).unwrap();
 		assert_eq!(blinded_payinfo.fee_base_msat, 0);
@@ -758,6 +772,7 @@ mod tests {
 				htlc_minimum_msat: 3,
 			},
 			payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 		};
 		let htlc_maximum_msat = 100_000;
 		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_maximum_msat, TEST_FINAL_CLTV as u16).unwrap();
@@ -811,6 +826,7 @@ mod tests {
 				htlc_minimum_msat: 1,
 			},
 			payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 		};
 		let htlc_minimum_msat = 3798;
 		assert!(super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_minimum_msat - 1, TEST_FINAL_CLTV as u16).is_err());
@@ -868,6 +884,7 @@ mod tests {
 				htlc_minimum_msat: 1,
 			},
 			payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 		};
 
 		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, 10_000, TEST_FINAL_CLTV as u16).unwrap();

lightning/src/ln/blinded_payment_tests.rs

@@ -77,6 +77,7 @@ fn blinded_payment_path(
 				intro_node_min_htlc_opt.unwrap_or_else(|| channel_upds.last().unwrap().htlc_minimum_msat),
 		},
 		payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 	};
 	let mut secp_ctx = Secp256k1::new();
 	BlindedPaymentPath::new(
@@ -123,6 +124,7 @@ fn do_one_hop_blinded_path(success: bool) {
 			htlc_minimum_msat: chan_upd.htlc_minimum_msat,
 		},
 		payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 	};
 	let mut secp_ctx = Secp256k1::new();
 	let blinded_path = BlindedPaymentPath::new(
@@ -167,6 +169,7 @@ fn mpp_to_one_hop_blinded_path() {
 			htlc_minimum_msat: chan_upd_1_3.htlc_minimum_msat,
 		},
 		payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 	};
 	let blinded_path = BlindedPaymentPath::new(
 		&[], nodes[3].node.get_our_node_id(), payee_tlvs, u64::MAX, TEST_FINAL_CLTV as u16,
@@ -1376,6 +1379,7 @@ fn sender_custom_tlvs_to_blinded_path() {
 			htlc_minimum_msat: chan_upd.htlc_minimum_msat,
 		},
 		payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 	};
 	let mut secp_ctx = Secp256k1::new();
 	let blinded_path = BlindedPaymentPath::new(

lightning/src/ln/channelmanager.rs

@@ -10193,7 +10193,7 @@ where
 			Ok((payment_hash, payment_secret)) => {
 				let payment_context = PaymentContext::Bolt12Refund(Bolt12RefundContext {});
 				let payment_paths = self.create_blinded_payment_paths(
-					amount_msats, payment_secret, payment_context
+					amount_msats, payment_secret, payment_context, None
 				)
 					.map_err(|_| Bolt12SemanticError::MissingPaths)?;
 
@@ -10512,7 +10512,7 @@ where
 	/// Creates multi-hop blinded payment paths for the given `amount_msats` by delegating to
 	/// [`Router::create_blinded_payment_paths`].
 	fn create_blinded_payment_paths(
-		&self, amount_msats: u64, payment_secret: PaymentSecret, payment_context: PaymentContext
+		&self, amount_msats: u64, payment_secret: PaymentSecret, payment_context: PaymentContext, custom_data: Option<Vec<u8>>
 	) -> Result<Vec<BlindedPaymentPath>, ()> {
 		let secp_ctx = &self.secp_ctx;
 
@@ -10527,6 +10527,7 @@ where
 				htlc_minimum_msat: 1,
 			},
 			payment_context,
+			custom_data: custom_data.unwrap_or_default()
 		};
 		self.router.create_blinded_payment_paths(
 			payee_node_id, first_hops, payee_tlvs, amount_msats, secp_ctx
@@ -12066,7 +12067,7 @@ where
 					invoice_request: invoice_request.fields(),
 				});
 				let payment_paths = match self.create_blinded_payment_paths(
-					amount_msats, payment_secret, payment_context
+					amount_msats, payment_secret, payment_context, None
 				) {
 					Ok(payment_paths) => payment_paths,
 					Err(()) => {

lightning/src/ln/max_payment_path_len_tests.rs

@@ -165,6 +165,7 @@ fn one_hop_blinded_path_with_custom_tlv() {
 			htlc_minimum_msat: chan_upd_1_2.htlc_minimum_msat,
 		},
 		payment_context: PaymentContext::unknown(),
+			custom_data: Vec::new(),
 	};
 	let mut secp_ctx = Secp256k1::new();
 	let blinded_path = BlindedPaymentPath::new(

lightning/src/ln/msgs.rs

@@ -2925,9 +2925,11 @@ impl<NS: Deref> ReadableArgs<(Option<PublicKey>, NS)> for InboundOnionPayload wh
 						next_blinding_override,
 					})
 				},
+				// Note: The custom data in the receive tlvs is not used here.
 				ChaChaPolyReadAdapter { readable: BlindedPaymentTlvs::Receive(ReceiveTlvs {
-					payment_secret, payment_constraints, payment_context
+					payment_secret, payment_constraints, payment_context, custom_data
 				})} => {
+					debug_assert_eq!(custom_data, user_custom_data, "The custom TLVs in ReceiveTlvs must match the ones read from serialization.");
 					if total_msat.unwrap_or(0) > MAX_VALUE_MSAT { return Err(DecodeError::InvalidValue) }
 					Ok(Self::BlindedReceive {
 						sender_intended_htlc_amt_msat: amt.ok_or(DecodeError::InvalidValue)?,