From a48c369250831d8ca5ed3312091237c35acc56bc Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Mon, 24 Aug 2020 15:44:13 +0200
Subject: [PATCH] lnwallet: check coop close fee negative balance

Also modify the test to check for this condition.
---
 lnwallet/channel.go      | 10 ++++++++--
 lnwallet/channel_test.go | 23 +++++++++++++++++++----
 lnwallet/commitment.go   | 12 ++++++++++--
 3 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/lnwallet/channel.go b/lnwallet/channel.go
index 30deb5a2..ccdf5530 100644
--- a/lnwallet/channel.go
+++ b/lnwallet/channel.go
@@ -6182,10 +6182,13 @@ func (lc *LightningChannel) CreateCloseProposal(proposedFee btcutil.Amount,
 	// Get the final balances after subtracting the proposed fee, taking
 	// care not to persist the adjusted balance, as the feeRate may change
 	// during the channel closing process.
-	ourBalance, theirBalance := CoopCloseBalance(
+	ourBalance, theirBalance, err := CoopCloseBalance(
 		lc.channelState.ChanType, lc.channelState.IsInitiator,
 		proposedFee, lc.channelState.LocalCommitment,
 	)
+	if err != nil {
+		return nil, nil, 0, err
+	}
 
 	closeTx := CreateCooperativeCloseTx(
 		fundingTxIn(lc.channelState), lc.channelState.LocalChanCfg.DustLimit,
@@ -6241,10 +6244,13 @@ func (lc *LightningChannel) CompleteCooperativeClose(
 	}
 
 	// Get the final balances after subtracting the proposed fee.
-	ourBalance, theirBalance := CoopCloseBalance(
+	ourBalance, theirBalance, err := CoopCloseBalance(
 		lc.channelState.ChanType, lc.channelState.IsInitiator,
 		proposedFee, lc.channelState.LocalCommitment,
 	)
+	if err != nil {
+		return nil, 0, err
+	}
 
 	// Create the transaction used to return the current settled balance
 	// on this active channel back to both parties. In this current model,
diff --git a/lnwallet/channel_test.go b/lnwallet/channel_test.go
index 5347e132..190e31c1 100644
--- a/lnwallet/channel_test.go
+++ b/lnwallet/channel_test.go
@@ -2230,11 +2230,11 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 			"got %v", 2, len(closeTx.TxOut))
 	}
 
-	// We'll reset the channel states before proceeding to our nest test.
+	// We'll reset the channel states before proceeding to our next test.
 	resetChannelState()
 
 	// Next we'll modify the current balances and dust limits such that
-	// Bob's current balance is above _below_ his dust limit.
+	// Bob's current balance is _below_ his dust limit.
 	aliceBal := lnwire.NewMSatFromSatoshis(btcutil.SatoshiPerBitcoin)
 	bobBal := lnwire.NewMSatFromSatoshis(250)
 	setBalances(aliceBal, bobBal)
@@ -2277,11 +2277,26 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 			int64(closeTx.TxOut[0].Value))
 	}
 
-	// Finally, we'll modify the current balances and dust limits such that
-	// Alice's current balance is _below_ his her limit.
+	// We'll modify the current balances and dust limits such that
+	// Alice's current balance is too low to pay the proposed fee.
 	setBalances(bobBal, aliceBal)
 	resetChannelState()
 
+	// Attempting to close with this fee now should fail, since Alice
+	// cannot afford it.
+	_, _, _, err = aliceChannel.CreateCloseProposal(
+		aliceFee, aliceDeliveryScript, bobDeliveryScript,
+	)
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+
+	// Finally, we'll modify the current balances and dust limits such that
+	// Alice's balance after paying the coop fee is _below_ her dust limit.
+	lowBalance := lnwire.NewMSatFromSatoshis(aliceFee) + 1000
+	setBalances(lowBalance, aliceBal)
+	resetChannelState()
+
 	// Our final attempt at another cooperative channel closure. It should
 	// succeed without any issues.
 	aliceSig, _, _, err = aliceChannel.CreateCloseProposal(
diff --git a/lnwallet/commitment.go b/lnwallet/commitment.go
index 1039bbfc..44b3c81c 100644
--- a/lnwallet/commitment.go
+++ b/lnwallet/commitment.go
@@ -667,7 +667,7 @@ func CreateCommitTx(chanType channeldb.ChannelType,
 // the cooperative close tx, given the channel type and transaction fee.
 func CoopCloseBalance(chanType channeldb.ChannelType, isInitiator bool,
 	coopCloseFee btcutil.Amount, localCommit channeldb.ChannelCommitment) (
-	btcutil.Amount, btcutil.Amount) {
+	btcutil.Amount, btcutil.Amount, error) {
 
 	// Get both parties' balances from the latest commitment.
 	ourBalance := localCommit.LocalBalance.ToSatoshis()
@@ -693,7 +693,15 @@ func CoopCloseBalance(chanType channeldb.ChannelType, isInitiator bool,
 		theirBalance += initiatorDelta
 	}
 
-	return ourBalance, theirBalance
+	// During fee negotiation it should always be verified that the
+	// initiator can pay the proposed fee, but we do a sanity check just to
+	// be sure here.
+	if ourBalance < 0 || theirBalance < 0 {
+		return 0, 0, fmt.Errorf("initiator cannot afford proposed " +
+			"coop close fee")
+	}
+
+	return ourBalance, theirBalance, nil
 }
 
 // genHtlcScript generates the proper P2WSH public key scripts for the HTLC