multi: make sure CPFP won't exceed max allowed fee rate

This commit updates the `fee()` method in `weightEstimator` to make sure
when doing CPFP we are not exceeding the max allowed fee rate. In order
to use the max fee rate, we need to modify several methods to pass the
configured value to the estimator.

sweep/weight_estimator.go

@@ -16,13 +16,19 @@ type weightEstimator struct {
 	parents       map[chainhash.Hash]struct{}
 	parentsFee    btcutil.Amount
 	parentsWeight int64
+
+	// maxFeeRate is the max allowed fee rate configured by the user.
+	maxFeeRate chainfee.SatPerKWeight
 }
 
 // newWeightEstimator instantiates a new sweeper weight estimator.
-func newWeightEstimator(feeRate chainfee.SatPerKWeight) *weightEstimator {
+func newWeightEstimator(
+	feeRate, maxFeeRate chainfee.SatPerKWeight) *weightEstimator {
+
 	return &weightEstimator{
-		feeRate: feeRate,
-		parents: make(map[chainhash.Hash]struct{}),
+		feeRate:    feeRate,
+		maxFeeRate: maxFeeRate,
+		parents:    make(map[chainhash.Hash]struct{}),
 	}
 }
 
@@ -119,5 +125,24 @@ func (w *weightEstimator) fee() btcutil.Amount {
 		fee = childFee
 	}
 
+	// Exit early if maxFeeRate is not set.
+	if w.maxFeeRate == 0 {
+		return fee
+	}
+
+	// Clamp the fee to the max fee rate.
+	maxFee := w.maxFeeRate.FeeForWeight(childWeight)
+	if fee > maxFee {
+		// Calculate the effective fee rate for logging.
+		childFeeRate := chainfee.SatPerKWeight(
+			fee * 1000 / btcutil.Amount(childWeight),
+		)
+		log.Warnf("Child fee rate %v exceeds max allowed fee rate %v, "+
+			"returning fee %v instead of %v", childFeeRate,
+			w.maxFeeRate, maxFee, fee)
+
+		fee = maxFee
+	}
+
 	return fee
 }