tapchannel: add an extra budget for each input w/ a blob

In this commit, we fix an issue that would cause lnd to not broadcast
sweeps of HTLCs due to their small value. Before we didn't add enough
budget to actually convince lnd to init the fee function and broadcast
the sweep.

In the future, we should do a more careful calculation here based on the
current BTC value of the asset, to make an economical decision. For now,
we just increase the amt based on the amt of inputs we have. This gives
lnd a fee budget to use for bumping.

Author: Roasbeef

tapchannel/aux_sweeper.go

@@ -38,6 +38,14 @@ import (
 	"golang.org/x/exp/maps"
 )
 
+const (
+	// sweeperBudgetMultiplier is the multiplier used to determine the
+	// budget expressed in sats, report to lnd's sweeper. As we have small
+	// outputs on chain, we'll need an increased budget (the amount we
+	// should spend on fees) to make sure the outputs are always swept.
+	sweeperBudgetMultiplier = 20
+)
+
 // resolutionReq carries a request to resolve a contract output along with a
 // response channel of the result.
 type resolutionReq struct {
@@ -2445,13 +2453,22 @@ func (a *AuxSweeper) DeriveSweepAddr(inputs []input.Input,
 func (a *AuxSweeper) ExtraBudgetForInputs(
 	inputs []input.Input) lfn.Result[btcutil.Amount] {
 
-	hasResolutionBlob := fn.Any(inputs, func(i input.Input) bool {
+	inputsWithBlobs := fn.Filter(inputs, func(i input.Input) bool {
 		return i.ResolutionBlob().IsSome()
 	})
 
 	var extraBudget btcutil.Amount
-	if hasResolutionBlob {
-		extraBudget = tapsend.DummyAmtSats
+	if len(inputsWithBlobs) != 0 {
+		// In this case, just 1k sats (tapsend.DummyAmtSats) may not be
+		// enough budget to pay for sweeping. So instead, we'll use a
+		// multiple of this to ensure that any time we care about an
+		// output, we're pretty much always able to sweep it.
+		//
+		// TODO(roasbeef): return the sats equiv budget of the asset
+		// amount
+		extraBudget = tapsend.DummyAmtSats * btcutil.Amount(
+			sweeperBudgetMultiplier*len(inputsWithBlobs),
+		)
 	}
 
 	return lfn.Ok(extraBudget)