Provide payment retry data when an MPP payment failed partially

This will allow `InvoicePayer` to properly retry payments that only
partially failed to send.

fuzz/src/chanmon_consistency.rs

@@ -271,8 +271,8 @@ fn check_payment_err(send_err: PaymentSendFailure) {
 		PaymentSendFailure::AllFailedRetrySafe(per_path_results) => {
 			for api_err in per_path_results { check_api_err(api_err); }
 		},
-		PaymentSendFailure::PartialFailure(per_path_results) => {
-			for res in per_path_results { if let Err(api_err) = res { check_api_err(api_err); } }
+		PaymentSendFailure::PartialFailure { results, .. } => {
+			for res in results { if let Err(api_err) = res { check_api_err(api_err); } }
 		},
 	}
 }

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -1950,7 +1950,7 @@ fn test_path_paused_mpp() {
 	// Now check that we get the right return value, indicating that the first path succeeded but
 	// the second got a MonitorUpdateFailed err. This implies PaymentSendFailure::PartialFailure as
 	// some paths succeeded, preventing retry.
-	if let Err(PaymentSendFailure::PartialFailure(results)) = nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret)) {
+	if let Err(PaymentSendFailure::PartialFailure { results, ..}) = nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret)) {
 		assert_eq!(results.len(), 2);
 		if let Ok(()) = results[0] {} else { panic!(); }
 		if let Err(APIError::MonitorUpdateFailed) = results[1] {} else { panic!(); }

lightning/src/ln/channelmanager.rs

@@ -946,7 +946,16 @@ pub enum PaymentSendFailure {
 	/// as they will result in over-/re-payment. These HTLCs all either successfully sent (in the
 	/// case of Ok(())) or will send once channel_monitor_updated is called on the next-hop channel
 	/// with the latest update_id.
-	PartialFailure(Vec<Result<(), APIError>>),
+	PartialFailure {
+		/// The errors themselves, in the same order as the route hops.
+		results: Vec<Result<(), APIError>>,
+		/// If some paths failed without irrevocably committing to the new HTLC(s), this will
+		/// contain a [`RouteParameters`] object which can be used to calculate a new route that
+		/// will pay all remaining unpaid balance.
+		failed_paths_retry: Option<RouteParameters>,
+		/// The payment id for the payment, which is now at least partially pending.
+		payment_id: PaymentId,
+	},
 }
 
 macro_rules! handle_error {
@@ -2236,7 +2245,9 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 		}
 		let mut has_ok = false;
 		let mut has_err = false;
-		for res in results.iter() {
+		let mut pending_amt_unsent = 0;
+		let mut max_unsent_cltv_delta = 0;
+		for (res, path) in results.iter().zip(route.paths.iter()) {
 			if res.is_ok() { has_ok = true; }
 			if res.is_err() { has_err = true; }
 			if let &Err(APIError::MonitorUpdateFailed) = res {
@@ -2244,11 +2255,25 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				// PartialFailure.
 				has_err = true;
 				has_ok = true;
-				break;
+			} else if res.is_err() {
+				pending_amt_unsent += path.last().unwrap().fee_msat;
+				max_unsent_cltv_delta = cmp::max(max_unsent_cltv_delta, path.last().unwrap().cltv_expiry_delta);
 			}
 		}
 		if has_err && has_ok {
-			Err(PaymentSendFailure::PartialFailure(results))
+			Err(PaymentSendFailure::PartialFailure {
+				results,
+				payment_id,
+				failed_paths_retry: if pending_amt_unsent != 0 {
+					if let Some(payee) = &route.payee {
+						Some(RouteParameters {
+							payee: payee.clone(),
+							final_value_msat: pending_amt_unsent,
+							final_cltv_expiry_delta: max_unsent_cltv_delta,
+						})
+					} else { None }
+				} else { None },
+			})
 		} else if has_err {
 			Err(PaymentSendFailure::AllFailedRetrySafe(results.drain(..).map(|r| r.unwrap_err()).collect()))
 		} else {

lightning/src/ln/functional_test_utils.rs

@@ -482,9 +482,9 @@ macro_rules! unwrap_send_err {
 					_ => panic!(),
 				}
 			},
-			&Err(PaymentSendFailure::PartialFailure(ref fails)) if !$all_failed => {
-				assert_eq!(fails.len(), 1);
-				match fails[0] {
+			&Err(PaymentSendFailure::PartialFailure { ref results, .. }) if !$all_failed => {
+				assert_eq!(results.len(), 1);
+				match results[0] {
 					Err($type) => { $check },
 					_ => panic!(),
 				}