From 84ee6b6b8665f0819bb8155a90546ad741892849 Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Mon, 7 Dec 2020 09:34:13 +0100
Subject: [PATCH] sweep/txgenerator: fix input witness ordering

This commit fixes an issue that would arise if inputs without required
TxOuts would be swept together with inputs with required TxOuts. In this
case we would add the required ones first to the transaction, but did
not change the order we signed the inputs, resulting in signing the
wrong input index.
---
 sweep/txgenerator.go | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/sweep/txgenerator.go b/sweep/txgenerator.go
index 670a2255..ec58b414 100644
--- a/sweep/txgenerator.go
+++ b/sweep/txgenerator.go
@@ -137,28 +137,35 @@ func createSweepTx(inputs []input.Input, outputPkScript []byte,
 	dustLimit btcutil.Amount, signer input.Signer) (*wire.MsgTx, error) {
 
 	inputs, estimator := getWeightEstimate(inputs, feePerKw)
-
 	txFee := estimator.fee()
 
-	// Create the sweep transaction that we will be building. We use
-	// version 2 as it is required for CSV.
-	sweepTx := wire.NewMsgTx(2)
+	var (
+		// Create the sweep transaction that we will be building. We
+		// use version 2 as it is required for CSV.
+		sweepTx = wire.NewMsgTx(2)
 
-	// Track whether any of the inputs require a certain locktime.
-	locktime := int32(-1)
+		// Track whether any of the inputs require a certain locktime.
+		locktime = int32(-1)
+
+		// We keep track of total input amount, and required output
+		// amount to use for calculating the change amount below.
+		totalInput     btcutil.Amount
+		requiredOutput btcutil.Amount
+
+		// We'll add the inputs as we go so we know the final ordering
+		// of inputs to sign.
+		idxs []input.Input
+	)
 
 	// We start by adding all inputs that commit to an output. We do this
 	// since the input and output index must stay the same for the
 	// signatures to be valid.
-	var (
-		totalInput     btcutil.Amount
-		requiredOutput btcutil.Amount
-	)
 	for _, o := range inputs {
 		if o.RequiredTxOut() == nil {
 			continue
 		}
 
+		idxs = append(idxs, o)
 		sweepTx.AddTxIn(&wire.TxIn{
 			PreviousOutPoint: *o.OutPoint(),
 			Sequence:         o.BlocksToMaturity(),
@@ -186,6 +193,7 @@ func createSweepTx(inputs []input.Input, outputPkScript []byte,
 			continue
 		}
 
+		idxs = append(idxs, o)
 		sweepTx.AddTxIn(&wire.TxIn{
 			PreviousOutPoint: *o.OutPoint(),
 			Sequence:         o.BlocksToMaturity(),
@@ -255,10 +263,8 @@ func createSweepTx(inputs []input.Input, outputPkScript []byte,
 		return nil
 	}
 
-	// Finally we'll attach a valid input script to each csv and cltv input
-	// within the sweeping transaction.
-	for i, input := range inputs {
-		if err := addInputScript(i, input); err != nil {
+	for idx, inp := range idxs {
+		if err := addInputScript(idx, inp); err != nil {
 			return nil, err
 		}
 	}