Score PaymentPathFailed w/o scid as ProbeSuccess

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

Author: valentinewallace

lightning-background-processor/src/lib.rs

@@ -225,10 +225,17 @@ fn update_scorer<'a, S: 'static + Deref<Target = SC> + Send + Sync, SC: 'a + Wri
 	scorer: &'a S, event: &Event
 ) {
 	match event {
-		Event::PaymentPathFailed { ref path, short_channel_id: Some(scid), .. } => {
+		Event::PaymentPathFailed { ref path, short_channel_id, .. } => {
 			let path = path.iter().collect::<Vec<_>>();
 			let mut score = scorer.lock();
-			score.payment_path_failed(&path, *scid);
+			if let Some(scid) = short_channel_id {
+				score.payment_path_failed(&path, *scid);
+			} else {
+				// `short_channel_id` will be None when 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<_>>();