Add cancelSet flag HtlcModify interface

invoices/interface.go

@@ -247,6 +247,12 @@ type HtlcModifyResponse struct {
 	// HTLC was originally sent with, in case additional value is carried
 	// along with it (which might be the case in custom channels).
 	AmountPaid lnwire.MilliSatoshi
+
+	// CancelSet is a flag the interceptor client can set to force a
+	// cancellation of all HTLCs associated with the invoice that are
+	// currently accepted. Setting this field will ignore the AmountPaid
+	// field.
+	CancelSet bool
 }
 
 // HtlcModifyCallback is a function that is called when an invoice is

invoices/invoiceregistry.go

@@ -1049,6 +1049,8 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 		return nil, nil, err
 	}
 
+	var cancelSet bool
+
 	// Provide the invoice to the settlement interceptor to allow
 	// the interceptor's client an opportunity to manipulate the
 	// settlement process.
@@ -1066,6 +1068,8 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 		if resp.AmountPaid != 0 {
 			ctx.amtPaid = resp.AmountPaid
 		}
+
+		cancelSet = resp.CancelSet
 	})
 	if err != nil {
 		err := fmt.Errorf("error during invoice HTLC interception: %w",
@@ -1092,7 +1096,18 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 			updateDesc.State != nil
 
 		// Assign resolution to outer scope variable.
-		resolution = res
+		if cancelSet {
+			// If a cancel signal was set for the htlc set, we set
+			// the resolution as a failure with an underpayment
+			// indication. Something was wrong with this htlc, so
+			// we probably can't settle the invoice at all.
+			resolution = NewFailResolution(
+				ctx.circuitKey, ctx.currentHeight,
+				ResultAmountTooLow,
+			)
+		} else {
+			resolution = res
+		}
 
 		return updateDesc, nil
 	}

lnrpc/invoicesrpc/htlc_modifier.go

@@ -82,5 +82,6 @@ func (r *htlcModifier) onIntercept(
 
 	return &invoices.HtlcModifyResponse{
 		AmountPaid: amtPaid,
+		CancelSet:  resp.CancelSet,
 	}, nil
 }

lnrpc/invoicesrpc/invoices.proto

@@ -242,4 +242,10 @@ message HtlcModifyResponse {
     // HTLC carries other valuable items, as can be the case with custom channel
     // types.
     optional uint64 amt_paid = 2;
+
+    // This flag indicates whether the HTLCs associated with the invoices should
+    // be cancelled. The interceptor client may set this field if some
+    // unexpected behavior is encountered. Setting this will ignore the amt_paid
+    // field.
+    bool cancel_set = 3;
 }

lnrpc/invoicesrpc/invoices.swagger.json

@@ -423,6 +423,10 @@
           "type": "string",
           "format": "uint64",
           "description": "The modified amount in milli-satoshi that the exit HTLC is paying. This\nvalue can be different from the actual on-chain HTLC amount, in case the\nHTLC carries other valuable items, as can be the case with custom channel\ntypes."
+        },
+        "cancel_set": {
+          "type": "boolean",
+          "description": "This flag indicates whether the HTLCs associated with the invoices should\nbe cancelled. The interceptor client may set this field if some\nunexpected behavior is encountered. Setting this will ignore the amt_paid\nfield."
         }
       }
     },