Score PaymentPathFailed with payment_failed_permanently as ProbeSuccess

Prior to this commit, we wouldn't score PaymentPathFailed events when the
destination explicitly failed the payment back when the blamed scid is None.
Instead, we should treat this as a ProbeSuccess because the payment reached the
destination.

Author: valentinewallace

lightning-background-processor/src/lib.rs

@@ -226,9 +226,15 @@ fn update_scorer<'a, S: 'static + Deref<Target = SC> + Send + Sync, SC: 'a + Wri
 ) {
 	let mut score = scorer.lock();
 	match event {
-		Event::PaymentPathFailed { ref path, short_channel_id: Some(scid), .. } => {
+		Event::PaymentPathFailed { ref path, short_channel_id, payment_failed_permanently, .. } => {
 			let path = path.iter().collect::<Vec<_>>();
-			score.payment_path_failed(&path, *scid);
+			if let Some(scid) = short_channel_id {
+				score.payment_path_failed(&path, *scid);
+			} else if *payment_failed_permanently {
+				// Reached if the destination explicitly failed it back. We treat this as a successful probe
+				// because the payment made it all the way to the destination with sufficient liquidity.
+				score.probe_successful(&path);
+			}
 		},
 		Event::PaymentPathSuccessful { path, .. } => {
 			let path = path.iter().collect::<Vec<_>>();