routing: if MaxShardAmt is set, then use that as a ceiling for our splits

In this commit, we thread through the necessary state to allow users to
set a max shard amount. If this value is set, then this'll effectively
serve as a ceiling for all our split attempts. If we need to split,
we'll first try to use `paymentAmt/2`, if that's bigger than
`MaxShardAmt, then we'll use the latter instead.

Ideally in the future we have a dynamic way to automatically set both
the `MaxShardAmt` as well as `MaxParts` for users. Until then exposing
these two new fields will allow us to experiment with setting them
automatically using the RPC interface, and also give users a bit more
control over how we attempt to route payments, akin to coin control for
on-chain payments.

Fixes #4730

lntest/itest/lnd_multi-hop-error-propagation_test.go

@@ -204,6 +204,7 @@ out:
 		FinalCltvDelta: int32(carolPayReq.CltvExpiry),
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -240,6 +241,7 @@ out:
 		FinalCltvDelta: int32(carolPayReq.CltvExpiry),
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -300,6 +302,7 @@ out:
 				PaymentRequest: carolInvoice2.PaymentRequest,
 				TimeoutSeconds: 60,
 				FeeLimitMsat:   noFeeLimitMsat,
+				MaxParts:       1,
 			},
 		)
 
@@ -332,6 +335,7 @@ out:
 		PaymentRequest: carolInvoice3.PaymentRequest,
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -381,6 +385,7 @@ out:
 			PaymentRequest: carolInvoice.PaymentRequest,
 			TimeoutSeconds: 60,
 			FeeLimitMsat:   noFeeLimitMsat,
+			MaxParts:       1,
 		},
 		lnrpc.PaymentFailureReason_FAILURE_REASON_NO_ROUTE,
 	)

routing/integrated_routing_context_test.go

@@ -28,6 +28,7 @@ type integratedRoutingContext struct {
 	target *mockNode
 
 	amt         lnwire.MilliSatoshi
+	maxShardAmt *lnwire.MilliSatoshi
 	finalExpiry int32
 
 	mcCfg          MissionControlConfig
@@ -151,6 +152,10 @@ func (c *integratedRoutingContext) testPayment(maxParts uint32,
 		MaxParts:       maxParts,
 	}
 
+	if c.maxShardAmt != nil {
+		payment.MaxShardAmt = c.maxShardAmt
+	}
+
 	session, err := newPaymentSession(
 		&payment, getBandwidthHints,
 		func() (routingGraph, func(), error) {

routing/integrated_routing_test.go

@@ -89,6 +89,7 @@ type mppSendTestCase struct {
 	graph           func(g *mockGraph)
 	expectedFailure bool
 	maxParts        uint32
+	maxShardSize    btcutil.Amount
 }
 
 const (
@@ -208,6 +209,33 @@ var mppTestCases = []mppSendTestCase{
 		expectedFailure:  true,
 		maxParts:         10,
 	},
+
+	// Test that if maxShardSize is set, then all attempts are below the
+	// max shard size, yet still sum up to the total payment amount. A
+	// payment of 30k satoshis with a max shard size of 10k satoshis should
+	// produce 3 payments of 10k sats each.
+	{
+		name:             "max shard size clamping",
+		graph:            onePathGraph,
+		amt:              30_000,
+		expectedAttempts: 3,
+		expectedSuccesses: []expectedHtlcSuccess{
+			{
+				amt:   10_000,
+				chans: []uint64{chanSourceIm1, chanIm1Target},
+			},
+			{
+				amt:   10_000,
+				chans: []uint64{chanSourceIm1, chanIm1Target},
+			},
+			{
+				amt:   10_000,
+				chans: []uint64{chanSourceIm1, chanIm1Target},
+			},
+		},
+		maxParts:     1000,
+		maxShardSize: 10_000,
+	},
 }
 
 // TestMppSend tests that a payment can be completed using multiple shards.
@@ -229,6 +257,11 @@ func testMppSend(t *testing.T, testCase *mppSendTestCase) {
 
 	ctx.amt = lnwire.NewMSatFromSatoshis(testCase.amt)
 
+	if testCase.maxShardSize != 0 {
+		shardAmt := lnwire.NewMSatFromSatoshis(testCase.maxShardSize)
+		ctx.maxShardAmt = &shardAmt
+	}
+
 	attempts, err := ctx.testPayment(testCase.maxParts)
 	switch {
 	case err == nil && testCase.expectedFailure:

routing/payment_session.go

@@ -230,6 +230,18 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 
 	finalHtlcExpiry := int32(height) + int32(finalCltvDelta)
 
+	// Before we enter the loop below, we'll make sure to respect the max
+	// payment shard size (if it's set), which is effectively our
+	// client-side MTU that we'll attempt to respect at all times.
+	maxShardActive := p.payment.MaxShardAmt != nil
+	if maxShardActive && maxAmt > *p.payment.MaxShardAmt {
+		p.log.Debug("Clamping payment attempt from %v to %v due to "+
+			"max shard size of %v", maxAmt,
+			*p.payment.MaxShardAmt, maxAmt)
+
+		maxAmt = *p.payment.MaxShardAmt
+	}
+
 	for {
 		// We'll also obtain a set of bandwidthHints from the lower
 		// layer for each of our outbound channels. This will allow the
@@ -279,7 +291,8 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 			}
 
 			if !p.payment.DestFeatures.HasFeature(lnwire.MPPOptional) {
-				p.log.Debug("not splitting because destination doesn't declare MPP")
+				p.log.Debug("not splitting because " +
+					"destination doesn't declare MPP")
 
 				return nil, errNoPathFound
 			}