itest+lntest: fix flake in MPP-related tests

itest/lnd_mpp_test.go

@@ -378,6 +378,32 @@ func (c *mppTestScenario) setupSendPaymentCase() btcutil.Amount {
 	// - 2nd attempt(75,000 sats): Alice->Dave->Bob: 155,000 sats.
 	// - 3rd attempt(75,000 sats): Alice->Carol->Eve->Bob: 155,000 sats.
 	//
+	// There is a case where the payment will fail due to the channel
+	// capacity not being updated in the graph, which has been seen many
+	// times:
+	// 1. the 1st attempt (150,000 sats) is sent via
+	//    Alice->Carol->Eve->Bob, after which the capacity in Carol->Eve
+	//    should decrease.
+	// 2. the 2nd attempt (75,000 sats) is sent via Alice->Carol->Eve->Bob,
+	//    which shouldn't happen because the capacity in Carol->Eve is
+	//    depleted. However, since the HTLCs are sent in parallel, the 2nd
+	//    attempt can be sent before the capacity is updated in the graph.
+	// 3. if the 2nd attempt succeeds, the 1st attempt will fail and be
+	//    split into two attempts, each holding 75,000 sats. At this point,
+	//    we have three attempts to send, but only two routes are
+	//    available, causing the payment to be failed.
+	// 4. In addition, with recent fee buffer addition, the attempts will
+	//    fail even earlier without being further split.
+	//
+	// To avoid this case, we now increase the channel capacity of the
+	// route Carol->Eve->Bob such that even the above case happened, we can
+	// still send the HTLCs.
+	//
+	// TODO(yy): we should properly fix this in the router. Atm we only
+	// perform this hack to unblock the CI.
+	req.amtEveBob = 285_000
+	req.amtCarolEve = 285_000
+
 	// Open the channels as described above.
 	c.openChannels(req)
 

lntest/harness_assertion.go

@@ -1513,13 +1513,14 @@ func (h *HarnessTest) AssertNumHTLCsAndStage(hn *node.HarnessNode,
 				lnutils.SpewLogClosure(target.PendingHtlcs)())
 		}
 
-		for i, htlc := range target.PendingHtlcs {
+		for _, htlc := range target.PendingHtlcs {
 			if htlc.Stage == stage {
 				continue
 			}
 
-			return fmt.Errorf("HTLC %d got stage: %v, "+
-				"want stage: %v", i, htlc.Stage, stage)
+			return fmt.Errorf("HTLC %s got stage: %v, "+
+				"want stage: %v", htlc.Outpoint, htlc.Stage,
+				stage)
 		}
 
 		return nil