lnwallet: check coop close fee negative balance

Also modify the test to check for this condition.

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,

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(

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