Move node related stuff to node.go

The cornerstone for exchanging data is the node
structure and associated metadata. Moving it to a
separate file (maybe later a separate module) will help
decoupling.

node.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"bytes"
+	"encoding/gob"
+	"net"
+
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+)
+
+// nodeMeta holds metadata sent over the cluster
+type nodeMeta struct {
+	OverlayAddr net.IPNet
+	PubKey      string
+}
+
+// Node holds the memberlist node structure
+type node struct {
+	Name string
+	Addr net.IP
+	nodeMeta
+}
+
+func (n *node) String() string {
+	return n.Addr.String()
+}
+
+func (c *cluster) NodeMeta(limit int) []byte {
+	buf := &bytes.Buffer{}
+	if err := gob.NewEncoder(buf).Encode(nodeMeta{
+		OverlayAddr: c.wg.OverlayAddr,
+		PubKey:      c.wg.PubKey.String(),
+	}); err != nil {
+		logrus.Errorf("could not encode local state: %s", err)
+		return nil
+	}
+	if buf.Len() > limit {
+		logrus.Errorf("could not fit node metadata into %d bytes", limit)
+		return nil
+	}
+	return buf.Bytes()
+}
+
+func decodeNodeMeta(b []byte) (nodeMeta, error) {
+	// TODO: we blindly trust the info we get from the peers; We should be more defensive to limit the damage a leaked
+	// PSK can cause.
+	nm := nodeMeta{}
+	if err := gob.NewDecoder(bytes.NewReader(b)).Decode(&nm); err != nil {
+		return nm, errors.Wrap(err, "could not decode node meta")
+	}
+	return nm, nil
+}