sweep: make sweeper aware of unconfirmed parent transactions.

Extend the fee estimator to take into account parent transactions with
their weights and fees.

Do not try to cpfp parent transactions that have a higher fee rate than
the sweep tx fee rate.

sweep/sweeper.go

@@ -505,8 +505,11 @@ func (s *UtxoSweeper) collector(blockEpochs <-chan *chainntnfs.BlockEpoch) {
 				log.Debugf("Already pending input %v received",
 					outpoint)
 
-				// Update sweep parameters.
+				// Update input details and sweep parameters.
+				// The re-offered input details may contain a
+				// change to the unconfirmed parent tx info.
 				pendInput.params = input.params
+				pendInput.Input = input.input
 
 				// Add additional result channel to signal
 				// spend of this input.
@@ -763,6 +766,26 @@ func (s *UtxoSweeper) clusterBySweepFeeRate() []inputCluster {
 			log.Warnf("Skipping input %v: %v", op, err)
 			continue
 		}
+
+		// Only try to sweep inputs with an unconfirmed parent if the
+		// current sweep fee rate exceeds the parent tx fee rate. This
+		// assumes that such inputs are offered to the sweeper solely
+		// for the purpose of anchoring down the parent tx using cpfp.
+		parentTx := input.UnconfParent()
+		if parentTx != nil {
+			parentFeeRate :=
+				chainfee.SatPerKWeight(parentTx.Fee*1000) /
+					chainfee.SatPerKWeight(parentTx.Weight)
+
+			if parentFeeRate >= feeRate {
+				log.Debugf("Skipping cpfp input %v: fee_rate=%v, "+
+					"parent_fee_rate=%v", op, feeRate,
+					parentFeeRate)
+
+				continue
+			}
+		}
+
 		feeGroup := s.bucketForFeeRate(feeRate)
 
 		// Create a bucket list for this fee rate if there isn't one