invoices: update cancel logic for AMP invoices to be more efficient

By passing in the set ID here, we ensure that we don't need to read ALL
the invoices to update the state of a set of them.

invoices/invoiceregistry.go

@@ -672,14 +672,39 @@ func (i *InvoiceRegistry) cancelSingleHtlc(invoiceRef channeldb.InvoiceRef,
 		}
 
 		// Lookup the current status of the htlc in the database.
+		var (
+			htlcState channeldb.HtlcState
+			setID     *channeldb.SetID
+		)
 		htlc, ok := invoice.Htlcs[key]
 		if !ok {
-			return nil, fmt.Errorf("htlc %v not found", key)
+			// If this is an AMP invoice, then all the HTLCs won't
+			// be read out, so we'll consult the other mapping to
+			// try to find the HTLC state in question here.
+			var found bool
+			for ampSetID, htlcSet := range invoice.AMPState {
+				ampSetID := ampSetID
+				for htlcKey := range htlcSet.InvoiceKeys {
+					if htlcKey == key {
+						htlcState = htlcSet.State
+						setID = &ampSetID
+
+						found = true
+						break
+					}
+				}
+			}
+
+			if !found {
+				return nil, fmt.Errorf("htlc %v not found", key)
+			}
+		} else {
+			htlcState = htlc.State
 		}
 
 		// Cancelation is only possible if the htlc wasn't already
 		// resolved.
-		if htlc.State != channeldb.HtlcStateAccepted {
+		if htlcState != channeldb.HtlcStateAccepted {
 			log.Debugf("cancelSingleHtlc: htlc %v on invoice %v "+
 				"is already resolved", key, invoiceRef)
 
@@ -697,6 +722,7 @@ func (i *InvoiceRegistry) cancelSingleHtlc(invoiceRef channeldb.InvoiceRef,
 
 		return &channeldb.InvoiceUpdateDesc{
 			CancelHtlcs: canceledHtlcs,
+			SetID:       setID,
 		}, nil
 	}