rpcserver: add breathing room to AddInvoice quote max amt

Use the rfq system's configured tolerance to apply it to the max units
we'll use with the actual negotiated rfq quote with the peer. These
units along with the peer negotiated price need to be sufficient to
route the requested amount in sats/msats.

Author: GeorgeTsagk

rpcserver.go

@@ -7730,6 +7730,34 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 		maxUnits = assetUnits.ToUint64()
 	}
 
+	// Since we used a different oracle price query above calculate the max
+	// amount of units, we want to add some breathing room to account for
+	// price fluctuations caused by the small time delay, plus the fact that
+	// the agreed upon quote may be different. If we don't add this safety
+	// window the peer may allow a routable amount that evaluates to less
+	// than what we ask for.
+	tolerance := rfqmath.NewBigIntFromUint64(
+		r.cfg.RfqManager.GetPriceDeviationPpm(),
+	)
+
+	// Parse the max asset units to the rfqmath type.
+	maxMathUnits := rfqmath.NewBigIntFromUint64(maxUnits)
+
+	// Calculate the tolerance margin.
+	toleranceUnits := maxMathUnits.Mul(tolerance).Div(
+		rfqmath.NewBigIntFromUint64(1_000_000),
+	)
+
+	// Apply the tolerance margin twice. Once due to the ask/bid price
+	// deviation that may occur during rfq negotiation, and once for the
+	// price movement that may occur between querying the oracle and
+	// acquiring the quote. We don't really care about this margin being too
+	// big, this only affects the max units our peer agrees to route.
+	maxMathUnits = maxMathUnits.Add(toleranceUnits).Add(toleranceUnits)
+
+	// Now parse the result back to uint64.
+	maxUnits = maxMathUnits.ToUint64()
+
 	resp, err := r.AddAssetBuyOrder(ctx, &rfqrpc.AddAssetBuyOrderRequest{
 		AssetSpecifier: &rfqrpc.AssetSpecifier{
 			Id: &rfqrpc.AssetSpecifier_AssetId{
@@ -7801,9 +7829,6 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 
 	switch {
 	case satsMode:
-		amtMsat := lnwire.MilliSatoshi(req.InvoiceRequest.ValueMsat +
-			req.InvoiceRequest.Value*(1000))
-
 		iReq.ValueMsat = int64(amtMsat)
 
 	default: