Refuse circular payments

.. we previously got reports from users trying to pay their own JIT
invoice, which we currently don't support (and possibly never will).

In order to avoid entering any weird states, we just reject our own
circular payment.

Author: tnull

src/event.rs

@@ -468,6 +468,25 @@ where
 			} => {
 				let payment_id = PaymentId(payment_hash.0);
 				if let Some(info) = self.payment_store.get(&payment_id) {
+					if info.direction == PaymentDirection::Outbound {
+						log_info!(
+							self.logger,
+							"Refused inbound payment with ID {}: circular payments are unsupported.",
+							payment_id
+						);
+						self.channel_manager.fail_htlc_backwards(&payment_hash);
+
+						let update = PaymentDetailsUpdate {
+							status: Some(PaymentStatus::Failed),
+							..PaymentDetailsUpdate::new(payment_id)
+						};
+						self.payment_store.update(&update).unwrap_or_else(|e| {
+							log_error!(self.logger, "Failed to access payment store: {}", e);
+							panic!("Failed to access payment store");
+						});
+						return;
+					}
+
 					if info.status == PaymentStatus::Succeeded
 						|| matches!(info.kind, PaymentKind::Spontaneous { .. })
 					{