multi: add zeroconfacceptor that default rejects if no rpc acceptors

This is a safety mechanism so that zero-conf channels are not accepted
by default if no rpc acceptor exists.

chanacceptor/zeroconfacceptor.go

@@ -0,0 +1,68 @@
+package chanacceptor
+
+import "github.com/lightningnetwork/lnd/lnwire"
+
+// ZeroConfAcceptor wraps a regular ChainedAcceptor. If no acceptors are in the
+// ChainedAcceptor, then Accept will reject all channel open requests. This
+// should only be enabled when the zero-conf feature bit is set and is used to
+// protect users from a malicious counter-party double-spending the zero-conf
+// funding tx.
+type ZeroConfAcceptor struct {
+	chainedAcceptor *ChainedAcceptor
+}
+
+// NewZeroConfAcceptor initializes a ZeroConfAcceptor.
+func NewZeroConfAcceptor() *ZeroConfAcceptor {
+	return &ZeroConfAcceptor{
+		chainedAcceptor: NewChainedAcceptor(),
+	}
+}
+
+// AddAcceptor adds a sub-ChannelAcceptor to the internal ChainedAcceptor.
+func (z *ZeroConfAcceptor) AddAcceptor(acceptor ChannelAcceptor) uint64 {
+	return z.chainedAcceptor.AddAcceptor(acceptor)
+}
+
+// RemoveAcceptor removes a sub-ChannelAcceptor from the internal
+// ChainedAcceptor.
+func (z *ZeroConfAcceptor) RemoveAcceptor(id uint64) {
+	z.chainedAcceptor.RemoveAcceptor(id)
+}
+
+// Accept will deny the channel open request if the internal ChainedAcceptor is
+// empty. If the internal ChainedAcceptor has any acceptors, then Accept will
+// instead be called on it.
+//
+// NOTE: Part of the ChannelAcceptor interface.
+func (z *ZeroConfAcceptor) Accept(
+	req *ChannelAcceptRequest) *ChannelAcceptResponse {
+
+	// Alias for less verbosity.
+	channelType := req.OpenChanMsg.ChannelType
+
+	// Check if the channel type sets the zero-conf bit.
+	var zeroConfSet bool
+
+	if channelType != nil {
+		channelFeatures := lnwire.RawFeatureVector(*channelType)
+		zeroConfSet = channelFeatures.IsSet(lnwire.ZeroConfRequired)
+	}
+
+	// If there are no acceptors and the counter-party is requesting a zero
+	// conf channel, reject the attempt.
+	if z.chainedAcceptor.numAcceptors() == 0 && zeroConfSet {
+		// Deny the channel open request.
+		rejectChannel := NewChannelAcceptResponse(
+			false, nil, nil, 0, 0, 0, 0, 0, 0, false,
+		)
+		return rejectChannel
+	}
+
+	// Otherwise, the ChainedAcceptor has sub-acceptors, so call Accept on
+	// it.
+	return z.chainedAcceptor.Accept(req)
+}
+
+// A compile-time constraint to ensure ZeroConfAcceptor implements the
+// MultiplexAcceptor interface.
+var _ MultiplexAcceptor = (*ZeroConfAcceptor)(nil)