shrink "improved" topology update

When an update contains an older version of a peer we know about,
there is no point in including our better version in the "improved"
update, since we will have told our neighbours about that already and
any race conditions are taken care off by the periodic sending of the
complete topology.

router/peers.go

@@ -62,16 +62,12 @@ func (peers *Peers) ForEach(fun func(PeerName, *Peer)) {
 	}
 }
 
-// Merge an incoming update with our own topology, and
-// create an "improved" update:
-// - elements which the original payload added to the receiver are
-//   included
-// - elements which the original payload updated in the receiver are
-//   included
-// - elements which are equal between the receiver and the payload are
-//   not included
-// - elements where the payload was older than the receiver's version
-//   are updated
+// Merge an incoming update with our own topology.
+//
+// We add peers hitherto unknown to us, and update peers for which the
+// update contains a more recent version than known to us. The return
+// value is an "improved" update containing just these new/updated
+// elements.
 func (peers *Peers) ApplyUpdate(update []byte) ([]byte, error) {
 	peers.Lock()
 
@@ -236,23 +232,12 @@ func (peers *Peers) applyUpdate(decodedUpdate []*Peer, decodedConns [][]byte) ma
 		name := newPeer.Name
 		// guaranteed to find peer in the peers.table
 		peer := peers.table[name]
-		if peer != newPeer {
-			if peer.Version() > newPeer.Version() {
-				// we know more about this one than the update. If
-				// peer is ourself, this is slightly racey (further
-				// changes could occur to ourself in the mean
-				// time). But it doesn't matter as we know that we're
-				// already > newPeer.Version() and that's not going to
-				// change.
-				newUpdate[name] = peer
-				continue
-			} else if peer == peers.ourself {
-				// nobody but us updates us
-				continue
-			} else if peer.Version() == newPeer.Version() {
-				// implication is that connections are equal too
-				continue
-			}
+		if peer != newPeer &&
+			(peer == peers.ourself || peer.Version() >= newPeer.Version()) {
+			// Nobody but us updates us. And if we know more about a
+			// peer than what's in the the update, we ignore the
+			// latter.
+			continue
 		}
 		// If we're here, either it was a new peer, or the update has
 		// more info about the peer than we do. Either case, we need

site/how-it-works.md

@@ -125,19 +125,11 @@ peers that have just connected
   * when a connection is deleted - the update contains just the peer
 that lost the connection
 
-The receiver merges it with its own topology model. If the payload is
-a subset of the receiver's topology, no further action is
-taken. Otherwise, the receiver sends out to all its connections an
-"improved" update:
-
- - elements which the original payload added to the receiver are
-   included
- - elements which the original payload updated in the receiver are
-   included
- - elements which are equal between the receiver and the payload are
-   not included
- - elements where the payload was older than the receiver's version
-   are updated
+The receiver merges the update with its own topology model, adding
+peers hitherto unknown to it, and updating peers for which the update
+contains a more recent version than known to it. If there were any
+such new/updated peers, then an improved update containing them is
+sent out on all connections.
 
 If the update mentions a peer that the receiver does not know, then
 the entire update is rejected and the receiver sends a FetchAll