peer+server: wait until peer is active to begin channel opening process

Without waiting, we would proceed to retrieve the remote peer's
supported features, which may have not been set due to not yet receiving
their Init message.

peer.go

@@ -121,6 +121,10 @@ type peer struct {
 	addr        *lnwire.NetAddress
 	pubKeyBytes [33]byte
 
+	// activeSignal when closed signals that the peer is now active and
+	// ready to process messages.
+	activeSignal chan struct{}
+
 	// startTime is the time this peer connection was successfully
 	// established. It will be zero for peers that did not successfully
 	// Start().
@@ -241,6 +245,8 @@ func newPeer(conn net.Conn, connReq *connmgr.ConnReq, server *server,
 		conn: conn,
 		addr: addr,
 
+		activeSignal: make(chan struct{}),
+
 		inbound: inbound,
 		connReq: connReq,
 
@@ -369,6 +375,9 @@ func (p *peer) Start() error {
 	go p.channelManager()
 	go p.pingHandler()
 
+	// Signal to any external processes that the peer is now active.
+	close(p.activeSignal)
+
 	// Now that the peer has started up, we send any channel sync messages
 	// that must be resent for borked channels.
 	if len(msgs) > 0 {

server.go

@@ -3207,6 +3207,18 @@ func (s *server) OpenChannel(
 	}
 	s.mu.RUnlock()
 
+	// We'll wait until the peer is active before beginning the channel
+	// opening process.
+	select {
+	case <-peer.activeSignal:
+	case <-peer.quit:
+		req.err <- fmt.Errorf("peer %x disconnected", pubKeyBytes)
+		return req.updates, req.err
+	case <-s.quit:
+		req.err <- ErrServerShuttingDown
+		return req.updates, req.err
+	}
+
 	// If the fee rate wasn't specified, then we'll use a default
 	// confirmation target.
 	if req.fundingFeePerKw == 0 {