config: add option to disable incoming push amounts in OpenChannel

This is useful for merchant-side prevention of accidental pushes
during channel opening.

config.go

@@ -235,6 +235,8 @@ type config struct {
 
 	NoChanUpdates bool `long:"nochanupdates" description:"If specified, lnd will not request real-time channel updates from connected peers. This option should be used by routing nodes to save bandwidth."`
 
+	RejectPush bool `long:"rejectpush" description:"If true, lnd will not accept channel opening requests with non-zero push amounts. This should prevent accidental pushes to merchant nodes."`
+
 	net tor.Net
 
 	Routing *routing.Conf `group:"routing" namespace:"routing"`

fundingmanager.go

@@ -1032,6 +1032,15 @@ func (f *fundingManager) handleFundingOpen(fmsg *fundingOpenMsg) {
 		return
 	}
 
+	// If request specifies non-zero push amount and 'rejectpush' is set,
+	// signal an error.
+	if cfg.RejectPush && msg.PushAmount > 0 {
+		f.failFundingFlow(
+			fmsg.peer, fmsg.msg.PendingChannelID,
+			lnwallet.ErrNonZeroPushAmount())
+		return
+	}
+
 	fndgLog.Infof("Recv'd fundingRequest(amt=%v, push=%v, delay=%v, "+
 		"pendingId=%x) from peer(%x)", amt, msg.PushAmount,
 		msg.CsvDelay, msg.PendingChannelID,

lnwallet/errors.go

@@ -78,6 +78,13 @@ func ErrChanReserveTooLarge(reserve,
 	}
 }
 
+// ErrNonZeroPushAmount is returned by a remote peer that receives a
+// FundingOpen request for a channel with non-zero push amount while
+// they have 'rejectpush' enabled.
+func ErrNonZeroPushAmount() ReservationError {
+	return ReservationError{errors.New("Non-zero push amounts are disabled")}
+}
+
 // ErrMinHtlcTooLarge returns an error indicating that the MinHTLC value the
 // remote required is too large to be accepted.
 func ErrMinHtlcTooLarge(minHtlc,