sweep: create new Input interface

This commit introduces a common interface for sweep
inputs. It eliminates the type checking from UtxoSweeper.

Also the formerly present Amount() getter is removed. It was redundant
because this value is present in SignDesc().Output.Value as well.

breacharbiter.go

@@ -824,9 +824,16 @@ func (bo *breachedOutput) BuildWitness(signer lnwallet.Signer, txn *wire.MsgTx,
 	return bo.witnessFunc(txn, hashCache, txinIdx)
 }
 
-// Add compile-time constraint ensuring breachedOutput implements
-// SpendableOutput.
-var _ sweep.SpendableOutput = (*breachedOutput)(nil)
+// BlocksToMaturity returns the relative timelock, as a number of blocks, that
+// must be built on top of the confirmation height before the output can be
+// spent.
+func (bo *breachedOutput) BlocksToMaturity() uint32 {
+	return 0
+}
+
+// Add compile-time constraint ensuring breachedOutput implements the Input
+// interface.
+var _ sweep.Input = (*breachedOutput)(nil)
 
 // retributionInfo encapsulates all the data needed to sweep all the contested
 // funds within a channel whose contract has been breached by the prior
@@ -937,13 +944,13 @@ func (b *breachArbiter) createJusticeTx(
 	// outputs, while simultaneously computing the estimated weight of the
 	// transaction.
 	var (
-		spendableOutputs []sweep.SpendableOutput
+		spendableOutputs []sweep.Input
 		weightEstimate   lnwallet.TxWeightEstimator
 	)
 
 	// Allocate enough space to potentially hold each of the breached
 	// outputs in the retribution info.
-	spendableOutputs = make([]sweep.SpendableOutput, 0, len(r.breachedOutputs))
+	spendableOutputs = make([]sweep.Input, 0, len(r.breachedOutputs))
 
 	// The justice transaction we construct will be a segwit transaction
 	// that pays to a p2wkh output. Components such as the version,
@@ -997,7 +1004,7 @@ func (b *breachArbiter) createJusticeTx(
 // sweepSpendableOutputsTxn creates a signed transaction from a sequence of
 // spendable outputs by sweeping the funds into a single p2wkh output.
 func (b *breachArbiter) sweepSpendableOutputsTxn(txWeight int64,
-	inputs ...sweep.SpendableOutput) (*wire.MsgTx, error) {
+	inputs ...sweep.Input) (*wire.MsgTx, error) {
 
 	// First, we obtain a new public key script from the wallet which we'll
 	// sweep the funds to.
@@ -1011,7 +1018,7 @@ func (b *breachArbiter) sweepSpendableOutputsTxn(txWeight int64,
 	// Compute the total amount contained in the inputs.
 	var totalAmt btcutil.Amount
 	for _, input := range inputs {
-		totalAmt += input.Amount()
+		totalAmt += btcutil.Amount(input.SignDesc().Output.Value)
 	}
 
 	// We'll actually attempt to target inclusion within the next two
@@ -1059,7 +1066,7 @@ func (b *breachArbiter) sweepSpendableOutputsTxn(txWeight int64,
 	// witness, and attaching it to the transaction. This function accepts
 	// an integer index representing the intended txin index, and the
 	// breached output from which it will spend.
-	addWitness := func(idx int, so sweep.SpendableOutput) error {
+	addWitness := func(idx int, so sweep.Input) error {
 		// First, we construct a valid witness for this outpoint and
 		// transaction using the SpendableOutput's witness generation
 		// function.