rfq: add pure asset forwarding policy

If a payment is coming in through one asset channel and is supposed to
leave through another asset channel, we don't actually have to do any
conversion to BTC but instead to the outgoing asset directly (this would
be a single hop payment with asset channels on both sides).
To allow that forwarding mode, we add a new policy that contains both an
incoming and outgoing channel policy.

rfq/order.go

@@ -312,6 +312,118 @@ func (c *AssetPurchasePolicy) GenerateInterceptorResponse(
 // Ensure that AssetPurchasePolicy implements the Policy interface.
 var _ Policy = (*AssetPurchasePolicy)(nil)
 
+// AssetForwardPolicy is a struct that holds the terms which determine whether a
+// channel HTLC for an asset-to-asset forward is accepted or rejected.
+type AssetForwardPolicy struct {
+	incomingPolicy *AssetPurchasePolicy
+	outgoingPolicy *AssetSalePolicy
+}
+
+// NewAssetForwardPolicy creates a new asset forward policy.
+func NewAssetForwardPolicy(incoming, outgoing Policy) (*AssetForwardPolicy,
+	error) {
+
+	incomingPolicy, ok := incoming.(*AssetPurchasePolicy)
+	if !ok {
+		return nil, fmt.Errorf("incoming policy is not an asset "+
+			"purchase policy, but %T", incoming)
+	}
+
+	outgoingPolicy, ok := outgoing.(*AssetSalePolicy)
+	if !ok {
+		return nil, fmt.Errorf("outgoing policy is not an asset "+
+			"sale policy, but %T", outgoing)
+	}
+
+	return &AssetForwardPolicy{
+		incomingPolicy: incomingPolicy,
+		outgoingPolicy: outgoingPolicy,
+	}, nil
+}
+
+// CheckHtlcCompliance returns an error if the given HTLC intercept descriptor
+// does not satisfy the subject policy.
+func (a *AssetForwardPolicy) CheckHtlcCompliance(
+	htlc lndclient.InterceptedHtlc) error {
+
+	if err := a.incomingPolicy.CheckHtlcCompliance(htlc); err != nil {
+		return fmt.Errorf("error checking forward policy, inbound "+
+			"HTLC does not comply with policy: %w", err)
+	}
+
+	if err := a.outgoingPolicy.CheckHtlcCompliance(htlc); err != nil {
+		return fmt.Errorf("error checking forward policy, outbound "+
+			"HTLC does not comply with policy: %w", err)
+	}
+
+	return nil
+}
+
+// Expiry returns the policy's expiry time as a unix timestamp in seconds. The
+// returned expiry time is the earliest expiry time of the incoming and outgoing
+// policies.
+func (a *AssetForwardPolicy) Expiry() uint64 {
+	if a.incomingPolicy.Expiry() < a.outgoingPolicy.Expiry() {
+		return a.incomingPolicy.Expiry()
+	}
+
+	return a.outgoingPolicy.Expiry()
+}
+
+// HasExpired returns true if the policy has expired.
+func (a *AssetForwardPolicy) HasExpired() bool {
+	expireTime := time.Unix(int64(a.Expiry()), 0).UTC()
+
+	return time.Now().UTC().After(expireTime)
+}
+
+// Scid returns the serialised short channel ID (SCID) of the channel to which
+// the policy applies. This is the SCID of the incoming policy.
+func (a *AssetForwardPolicy) Scid() uint64 {
+	return a.incomingPolicy.Scid()
+}
+
+// GenerateInterceptorResponse generates an interceptor response for the policy.
+func (a *AssetForwardPolicy) GenerateInterceptorResponse(
+	htlc lndclient.InterceptedHtlc) (*lndclient.InterceptedHtlcResponse,
+	error) {
+
+	incomingResponse, err := a.incomingPolicy.GenerateInterceptorResponse(
+		htlc,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("error generating incoming interceptor "+
+			"response: %w", err)
+	}
+
+	outgoingResponse, err := a.outgoingPolicy.GenerateInterceptorResponse(
+		htlc,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("error generating outgoing interceptor "+
+			"response: %w", err)
+	}
+
+	return &lndclient.InterceptedHtlcResponse{
+		// Both incoming and outgoing policies will resume with
+		// modifications.
+		Action: lndclient.InterceptorActionResumeModified,
+
+		// The incoming policy will modify the incoming amount in order
+		// to satisfy the fee check in `lnd`.
+		IncomingAmount: incomingResponse.IncomingAmount,
+
+		// The outgoing policy will modify the outgoing amount and add
+		// custom records in order to satisfy the terms of the receiving
+		// node.
+		OutgoingAmount: outgoingResponse.OutgoingAmount,
+		CustomRecords:  outgoingResponse.CustomRecords,
+	}, nil
+}
+
+// Ensure that AssetForwardPolicy implements the Policy interface.
+var _ Policy = (*AssetForwardPolicy)(nil)
+
 // OrderHandlerCfg is a struct that holds the configuration parameters for the
 // order handler service.
 type OrderHandlerCfg struct {
@@ -552,6 +664,39 @@ func (h *OrderHandler) fetchPolicy(htlc lndclient.InterceptedHtlc) (Policy,
 		})
 	}
 
+	// Here we handle a special case where we both have an incoming and
+	// outgoing policy. In this case, we need to create a forward policy.
+	if foundPolicy != nil && haveOutPolicy {
+		incomingPolicy := *foundPolicy
+		outgoingPolicy := outPolicy
+
+		if incomingPolicy.HasExpired() {
+			scid := incomingPolicy.Scid()
+			h.policies.Delete(SerialisedScid(scid))
+		}
+		if outgoingPolicy.HasExpired() {
+			scid := outgoingPolicy.Scid()
+			h.policies.Delete(SerialisedScid(scid))
+		}
+
+		// If either the incoming or outgoing policy has expired, we
+		// return false, as if we didn't find a policy.
+		if incomingPolicy.HasExpired() || outgoingPolicy.HasExpired() {
+			return nil, false, nil
+		}
+
+		forwardPolicy, err := NewAssetForwardPolicy(
+			incomingPolicy, outgoingPolicy,
+		)
+		if err != nil {
+			return nil, false, fmt.Errorf("error creating forward "+
+				"policy: %w", err)
+		}
+
+		return forwardPolicy, true, nil
+
+	}
+
 	// If no policy has been found so far, we attempt to look up a policy by
 	// the outgoing channel SCID.
 	if foundPolicy == nil && haveOutPolicy {