lnwallet: add ability to specify custom sequence for co-op close tx

In this commit, we add the ability to specify a custom sequence for a
co-op close tx. This'll come in handy later as the new co-op close
process allows a party to set a custom sequence.

lnwallet/channel.go

@@ -8199,6 +8199,8 @@ type chanCloseOpt struct {
 	// transaction outputs. If this isn't set, then the default BIP-69
 	// sorting is used.
 	customSort CloseSortFunc
+
+	customSequence fn.Option[uint32]
 }
 
 // ChanCloseOpt is a closure type that cen be used to modify the set of default
@@ -8232,6 +8234,14 @@ func WithExtraCloseOutputs(extraOutputs []CloseOutput) ChanCloseOpt {
 func WithCustomCoopSort(sorter CloseSortFunc) ChanCloseOpt {
 	return func(opts *chanCloseOpt) {
 		opts.customSort = sorter
+    }
+}
+
+// WithCustomSequence can be used to specify a custom sequence number for the
+// co-op close process. Otherwise, a default non-final sequence will be used.
+func WithCustomSequence(sequence uint32) ChanCloseOpt {
+	return func(opts *chanCloseOpt) {
+		opts.customSequence = fn.Some(sequence)
 	}
 }
 
@@ -8294,6 +8304,12 @@ func (lc *LightningChannel) CreateCloseProposal(proposedFee btcutil.Amount,
 		)
 	}
 
+	opts.customSequence.WhenSome(func(sequence uint32) {
+		closeTxOpts = append(closeTxOpts, WithCustomTxInSequence(
+			sequence,
+		))
+	})
+
 	closeTx, err := CreateCooperativeCloseTx(
 		fundingTxIn(lc.channelState), lc.channelState.LocalChanCfg.DustLimit,
 		lc.channelState.RemoteChanCfg.DustLimit, ourBalance, theirBalance,
@@ -8395,6 +8411,12 @@ func (lc *LightningChannel) CompleteCooperativeClose(
 		)
 	}
 
+	opts.customSequence.WhenSome(func(sequence uint32) {
+		closeTxOpts = append(closeTxOpts, WithCustomTxInSequence(
+			sequence,
+		))
+	})
+
 	// Create the transaction used to return the current settled balance
 	// on this active channel back to both parties. In this current model,
 	// the initiator pays full fees for the cooperative close transaction.
@@ -9111,6 +9133,11 @@ type closeTxOpts struct {
 	// transaction outputs. If this isn't set, then the default BIP-69
 	// sorting is used.
 	customSort CloseSortFunc
+
+	// customSequence is an optional custom sequence to set on the co-op
+	// close transaction. This gives slightly more control compared to the
+	// enableRBF option.
+	customSequence fn.Option[uint32]
 }
 
 // defaultCloseTxOpts returns a closeTxOpts struct with default values.
@@ -9144,6 +9171,14 @@ func WithExtraTxCloseOutputs(extraOutputs []CloseOutput) CloseTxOpt {
 func WithCustomTxSort(sorter CloseSortFunc) CloseTxOpt {
 	return func(opts *closeTxOpts) {
 		opts.customSort = sorter
+    }
+}
+
+// WithCustomTxInSequence allows a caller to set a custom sequence on the sole
+// input of the co-op close tx.
+func WithCustomTxInSequence(sequence uint32) CloseTxOpt {
+	return func(o *closeTxOpts) {
+		o.customSequence = fn.Some(sequence)
 	}
 }
 
@@ -9169,6 +9204,11 @@ func CreateCooperativeCloseTx(fundingTxIn wire.TxIn,
 		fundingTxIn.Sequence = mempool.MaxRBFSequence
 	}
 
+	// Otherwise, a custom sequence might be specified.
+	opts.customSequence.WhenSome(func(sequence uint32) {
+		fundingTxIn.Sequence = sequence
+	})
+
 	// Construct the transaction to perform a cooperative closure of the
 	// channel. In the event that one side doesn't have any settled funds
 	// within the channel then a refund output for that particular side can
@@ -9176,6 +9216,8 @@ func CreateCooperativeCloseTx(fundingTxIn wire.TxIn,
 	closeTx := wire.NewMsgTx(2)
 	closeTx.AddTxIn(&fundingTxIn)
 
+	// TODO(roasbeef): needs support for dropping inputs
+
 	// Create both cooperative closure outputs, properly respecting the
 	// dust limits of both parties.
 	var localOutputIdx fn.Option[int]