lnwallet: update core coop close logic with custom payer

In this commit, we update the core coop close logic with the new custom
payer param. We also expand the existing unit tests to ensure that the
fee is deducted from the proper party.

lnwallet/channel.go

@@ -8203,6 +8203,8 @@ type chanCloseOpt struct {
 	customSequence fn.Option[uint32]
 
 	customLockTime fn.Option[uint32]
+
+	customPayer fn.Option[lntypes.ChannelParty]
 }
 
 // ChanCloseOpt is a closure type that cen be used to modify the set of default
@@ -8255,6 +8257,15 @@ func WithCustomLockTime(lockTime uint32) ChanCloseOpt {
 	}
 }
 
+// WithCustomPayer can be used to specify a custom payer for the closing
+// transaction. This overrides the default payer, which is the initiator of the
+// channel.
+func WithCustomPayer(payer lntypes.ChannelParty) ChanCloseOpt {
+	return func(opts *chanCloseOpt) {
+		opts.customPayer = fn.Some(payer)
+	}
+}
+
 // CreateCloseProposal is used by both parties in a cooperative channel close
 // workflow to generate proposed close transactions and signatures. This method
 // should only be executed once all pending HTLCs (if any) on the channel have
@@ -8270,16 +8281,17 @@ func (lc *LightningChannel) CreateCloseProposal(proposedFee btcutil.Amount,
 	lc.Lock()
 	defer lc.Unlock()
 
-	// If we're already closing the channel, then ignore this request.
-	if lc.isClosed {
-		return nil, nil, 0, ErrChanClosing
-	}
-
 	opts := defaultCloseOpts()
 	for _, optFunc := range closeOpts {
 		optFunc(opts)
 	}
 
+	// Unless there's a custom payer (sign of the RBF flow), if we're
+	// already closing the channel, then ignore this request.
+	if lc.isClosed && opts.customPayer.IsNone() {
+		return nil, nil, 0, ErrChanClosing
+	}
+
 	// 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.
@@ -8289,7 +8301,7 @@ func (lc *LightningChannel) CreateCloseProposal(proposedFee btcutil.Amount,
 		lc.channelState.LocalCommitment.LocalBalance.ToSatoshis(),
 		lc.channelState.LocalCommitment.RemoteBalance.ToSatoshis(),
 		lc.channelState.LocalCommitment.CommitFee,
-		fn.None[lntypes.ChannelParty](),
+		opts.customPayer,
 	)
 	if err != nil {
 		return nil, nil, 0, err
@@ -8385,17 +8397,17 @@ func (lc *LightningChannel) CompleteCooperativeClose(
 	lc.Lock()
 	defer lc.Unlock()
 
-	// If the channel is already closing, then ignore this request.
-	if lc.isClosed {
-		// TODO(roasbeef): check to ensure no pending payments
-		return nil, 0, ErrChanClosing
-	}
-
 	opts := defaultCloseOpts()
 	for _, optFunc := range closeOpts {
 		optFunc(opts)
 	}
 
+	// Unless there's a custom payer (sign of the RBF flow), if we're
+	// already closing the channel, then ignore this request.
+	if lc.isClosed && opts.customPayer.IsNone() {
+		return nil, 0, ErrChanClosing
+	}
+
 	// Get the final balances after subtracting the proposed fee.
 	ourBalance, theirBalance, err := CoopCloseBalance(
 		lc.channelState.ChanType, lc.channelState.IsInitiator,
@@ -8403,7 +8415,7 @@ func (lc *LightningChannel) CompleteCooperativeClose(
 		lc.channelState.LocalCommitment.LocalBalance.ToSatoshis(),
 		lc.channelState.LocalCommitment.RemoteBalance.ToSatoshis(),
 		lc.channelState.LocalCommitment.CommitFee,
-		fn.None[lntypes.ChannelParty](),
+		opts.customPayer,
 	)
 	if err != nil {
 		return nil, 0, err