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

@@ -2463,13 +2463,22 @@ 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(
+			20*len(inputsWithBlobs),
+		)
 	}
 
 	return lfn.Ok(extraBudget)