blindedpath: allow zero amount paths with MinHTLC policies

When building blinded paths for zero amount payments (where the sender
decides the amount), skip MinHTLC validation during path construction.
Zero amount payments indicate the final amount will be determined by
the sender, so MinHTLC constraints should be validated at payment time
rather than path construction time.

This enables creation of blinded path invoices for zero amount payments
even when intermediate channels have MinHTLC policies greater than zero.

Author: a-mpch

routing/blindedpath/blinded_path.go

@@ -438,7 +438,11 @@ func collectRelayInfo(cfg *BuildBlindedPathCfg, path *candidatePath) (
 			}
 		}
 
-		if policy.MinHTLCMsat > cfg.ValueMsat {
+		// If the payment amount is 0, means the sender is deciding the
+		// amount, so we don't need to check the min HTLC value while
+		// building the path. Payment would fail if there is no
+		// route with the min HTLC value from the sender's perspective.
+		if policy.MinHTLCMsat > cfg.ValueMsat && cfg.ValueMsat > 0 {
 			return nil, 0, 0, fmt.Errorf("%w: minHTLC of hop "+
 				"policy larger than payment amt: sentAmt(%v), "+
 				"minHTLC(%v)", errInvalidBlindedPath,

routing/blindedpath/blinded_path_test.go

@@ -1059,3 +1059,109 @@ func decryptAndDecodeHopData(t *testing.T, priv *btcec.PrivateKey,
 
 	return routeData, nextEphem
 }
+
+// TestBuildBlindedPathZeroAmount tests that blinded paths can be built
+// successfully when ValueMsat is 0 (zero amount), even when channels have
+// MinHTLC policies > 0. This is important for building blinded paths invoices
+// for zero amount payments.
+func TestBuildBlindedPathZeroAmount(t *testing.T) {
+	// Alice chooses the following path to herself for blinded path
+	// construction:
+	//    	Carol -> Bob -> Alice.
+	var (
+		_, pkC = btcec.PrivKeyFromBytes([]byte{1})
+		_, pkB = btcec.PrivKeyFromBytes([]byte{2})
+		_, pkA = btcec.PrivKeyFromBytes([]byte{3})
+
+		carol = route.NewVertex(pkC)
+		bob   = route.NewVertex(pkB)
+		alice = route.NewVertex(pkA)
+
+		chanCB = uint64(1)
+		chanBA = uint64(2)
+	)
+
+	realRoute := &route.Route{
+		SourcePubKey: carol,
+		Hops: []*route.Hop{
+			{
+				PubKeyBytes: bob,
+				ChannelID:   chanCB,
+			},
+			{
+				PubKeyBytes: alice,
+				ChannelID:   chanBA,
+			},
+		},
+	}
+
+	realPolicies := map[uint64]*models.ChannelEdgePolicy{
+		chanCB: {
+			ChannelID:                 chanCB,
+			ToNode:                    bob,
+			MinHTLC:                   1000, // MinHTLC > 0
+			MaxHTLC:                   lnwire.MaxMilliSatoshi,
+			FeeBaseMSat:               100,
+			FeeProportionalMillionths: 500,
+			TimeLockDelta:             144,
+		},
+		chanBA: {
+			ChannelID:                 chanBA,
+			ToNode:                    alice,
+			MinHTLC:                   500, // MinHTLC > 0
+			MaxHTLC:                   lnwire.MaxMilliSatoshi,
+			FeeBaseMSat:               50,
+			FeeProportionalMillionths: 250,
+			TimeLockDelta:             72,
+		},
+	}
+
+	paths, err := BuildBlindedPaymentPaths(&BuildBlindedPathCfg{
+		FindRoutes: func(_ lnwire.MilliSatoshi) ([]*route.Route,
+			error) {
+
+			return []*route.Route{realRoute}, nil
+		},
+		FetchChannelEdgesByID: func(chanID uint64) (
+			*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
+			*models.ChannelEdgePolicy, error) {
+
+			return nil, realPolicies[chanID], nil, nil
+		},
+		BestHeight: func() (uint32, error) {
+			return 1000, nil
+		},
+		AddPolicyBuffer: func(policy *BlindedHopPolicy) (
+			*BlindedHopPolicy, error) {
+
+			return policy, nil
+		},
+		PathID:                  []byte{1, 2, 3},
+		ValueMsat:               0,
+		MinFinalCLTVExpiryDelta: 12,
+		BlocksUntilExpiry:       200,
+	})
+
+	require.NoError(t, err)
+	require.Len(t, paths, 1, "Should return exactly one blinded path")
+
+	path := paths[0]
+	require.Len(
+		t,
+		path.Hops, 3,
+		"Path should have 3 hops (intro + 2 relay hops)",
+	)
+
+	hop := path.Hops[0]
+	require.True(
+		t,
+		hop.BlindedNodePub.IsEqual(pkC),
+		"First hop should be Carol (introduction node)",
+	)
+	require.EqualValues(
+		t,
+		path.HTLCMinMsat,
+		uint64(1000),
+		"Path MinHTLC should be 1000 based on channel policies",
+	)
+}