make GossipData immutable

The same piece of GossipData can be shared and subject to different
Merge operations. We either need to copy the data when we share it, or
create copies on modification, i.e. copy-on-write.

We choose the latter, since it is more efficient and there is just one
mutating operation.

Fixes #1697.

nameserver/entry.go

@@ -245,12 +245,18 @@ type GossipData struct {
 	Entries
 }
 
-func (g *GossipData) Merge(o mesh.GossipData) {
+func (g *GossipData) Merge(o mesh.GossipData) mesh.GossipData {
 	other := o.(*GossipData)
-	g.Entries.merge(other.Entries)
-	if g.Timestamp < other.Timestamp {
-		g.Timestamp = other.Timestamp
+	gossip := &GossipData{
+		Entries:   make(Entries, len(g.Entries)),
+		Timestamp: g.Timestamp,
 	}
+	copy(gossip.Entries, g.Entries)
+	gossip.Entries.merge(other.Entries)
+	if gossip.Timestamp < other.Timestamp {
+		gossip.Timestamp = other.Timestamp
+	}
+	return gossip
 }
 
 func (g *GossipData) Decode(msg []byte) error {

nameserver/entry_test.go

@@ -117,7 +117,7 @@ func TestGossipDataMerge(t *testing.T) {
 	g1 := GossipData{Entries: makeEntries("AcDf")}
 	g2 := GossipData{Entries: makeEntries("BEf")}
 
-	g1.Merge(&g2)
+	g3 := g1.Merge(&g2).(*GossipData)
 
-	require.Equal(t, GossipData{Entries: makeEntries("ABcDEf")}, g1)
+	require.Equal(t, GossipData{Entries: makeEntries("ABcDEf")}, *g3)
 }