multi: Use user provided close address for cooperative closes

This commit is adapted from @Bluetegu's original
pull request lightningnetwork#1462.

This commit reads an optional address to pay funds out to
from a user iniitiated close channel address. If the channel
already has a shutdown script set, the request will fail if
an address is provided. Otherwise, the cooperative close will
pay out to the address provided.

chancloser.go

@@ -28,10 +28,10 @@ var (
 	// a message while it is in an unknown state.
 	ErrInvalidState = fmt.Errorf("invalid state")
 
-	// errUpfrontShutdownScriptMismatch is returned when our peer sends us a
-	// shutdown message with a script that does not match the upfront shutdown
-	// script previously set.
-	errUpfrontShutdownScriptMismatch = fmt.Errorf("peer's shutdown " +
+	// errUpfrontShutdownScriptMismatch is returned when a peer or end user
+	// provides a script to cooperatively close out to which does not match
+	// the upfront shutdown script previously set for that party.
+	errUpfrontShutdownScriptMismatch = fmt.Errorf("shutdown " +
 		"script does not match upfront shutdown script")
 )
 

htlcswitch/switch.go

@@ -105,6 +105,9 @@ type ChanClose struct {
 	// process for the cooperative closure transaction kicks off.
 	TargetFeePerKw chainfee.SatPerKWeight
 
+	// DeliveryScript is an optional delivery script to pay funds out to.
+	DeliveryScript lnwire.DeliveryAddress
+
 	// Updates is used by request creator to receive the notifications about
 	// execution of the close channel request.
 	Updates chan interface{}
@@ -1365,12 +1368,13 @@ func (s *Switch) teardownCircuit(pkt *htlcPacket) error {
 }
 
 // CloseLink creates and sends the close channel command to the target link
-// directing the specified closure type. If the closure type if CloseRegular,
-// then the last parameter should be the ideal fee-per-kw that will be used as
-// a starting point for close negotiation.
-func (s *Switch) CloseLink(chanPoint *wire.OutPoint, closeType ChannelCloseType,
-	targetFeePerKw chainfee.SatPerKWeight) (chan interface{},
-	chan error) {
+// directing the specified closure type. If the closure type is CloseRegular,
+// targetFeePerKw parameter should be the ideal fee-per-kw that will be used as
+// a starting point for close negotiation. The deliveryScript parameter is an
+// optional parameter which sets a user specified script to close out to.
+func (s *Switch) CloseLink(chanPoint *wire.OutPoint,
+	closeType ChannelCloseType, targetFeePerKw chainfee.SatPerKWeight,
+	deliveryScript lnwire.DeliveryAddress) (chan interface{}, chan error) {
 
 	// TODO(roasbeef) abstract out the close updates.
 	updateChan := make(chan interface{}, 2)
@@ -1381,6 +1385,7 @@ func (s *Switch) CloseLink(chanPoint *wire.OutPoint, closeType ChannelCloseType,
 		ChanPoint:      chanPoint,
 		Updates:        updateChan,
 		TargetFeePerKw: targetFeePerKw,
+		DeliveryScript: deliveryScript,
 		Err:            errChan,
 	}
 

peer.go

@@ -2122,6 +2122,37 @@ func (p *peer) fetchActiveChanCloser(chanID lnwire.ChannelID) (*channelCloser, e
 	return chanCloser, nil
 }
 
+// chooseDeliveryScript takes two optionally set shutdown scripts and returns
+// a suitable script to close out to. This may be nil if neither script is
+// set. If both scripts are set, this function will error if they do not match.
+func chooseDeliveryScript(upfront,
+	requested lnwire.DeliveryAddress) (lnwire.DeliveryAddress, error) {
+
+	// If no upfront upfront shutdown script was provided, return the user
+	// requested address (which may be nil).
+	if len(upfront) == 0 {
+		return requested, nil
+	}
+
+	// If an upfront shutdown script was provided, and the user did not request
+	// a custom shutdown script, return the upfront address.
+	if len(requested) == 0 {
+		return upfront, nil
+	}
+
+	// If both an upfront shutdown script and a custom close script were
+	// provided, error if the user provided shutdown script does not match
+	// the upfront shutdown script (because closing out to a different script
+	// would violate upfront shutdown).
+	if !bytes.Equal(upfront, requested) {
+		return nil, errUpfrontShutdownScriptMismatch
+	}
+
+	// The user requested script matches the upfront shutdown script, so we
+	// can return it without error.
+	return upfront, nil
+}
+
 // handleLocalCloseReq kicks-off the workflow to execute a cooperative or
 // forced unilateral closure of the channel initiated by a local subsystem.
 func (p *peer) handleLocalCloseReq(req *htlcswitch.ChanClose) {
@@ -2144,13 +2175,26 @@ func (p *peer) handleLocalCloseReq(req *htlcswitch.ChanClose) {
 	// out this channel on-chain, so we execute the cooperative channel
 	// closure workflow.
 	case htlcswitch.CloseRegular:
-		// First, we'll fetch a  delivery script that we'll use to send the
-		// funds to in the case of a successful negotiation. If an upfront
-		// shutdown script was set, we will use it. Otherwise, we get a fresh
-		// delivery script.
-		deliveryScript := channel.LocalUpfrontShutdownScript()
+		// First, we'll choose a delivery address that we'll use to send the
+		// funds to in the case of a successful negotiation.
+
+		// An upfront shutdown and user provided script are both optional,
+		// but must be equal if both set  (because we cannot serve a request
+		// to close out to a script which violates upfront shutdown). Get the
+		// appropriate address to close out to (which may be nil if neither
+		// are set) and error if they are both set and do not match.
+		deliveryScript, err := chooseDeliveryScript(
+			channel.LocalUpfrontShutdownScript(), req.DeliveryScript,
+		)
+		if err != nil {
+			peerLog.Errorf("cannot close channel %v: %v", req.ChanPoint, err)
+			req.Err <- err
+			return
+		}
+
+		// If neither an upfront address or a user set address was
+		// provided, generate a fresh script.
 		if len(deliveryScript) == 0 {
-			var err error
 			deliveryScript, err = p.genDeliveryScript()
 			if err != nil {
 				peerLog.Errorf(err.Error())