robustsession: move server to front of list in prefer() instead of duplicating

In the prefer() function, move the specified preferred server to the top of the server list instead of prepending another copy of it. This keeps the list from ending up with duplicate entries, which can throw off the selection frequency.

Author: tkgle

robustsession/robustsession.go

@@ -214,14 +214,21 @@ func (n *Network) setServers(servers []string) {
 	n.servers = servers
 }
 
-// prefer adds the specified server to the front of the servers list, thereby
-// trying to prefer it over other servers for the next request. Note that
-// exponential backoff overrides this, so this is only a hint, not a guarantee.
+// prefer moves (or adds, if it doesn't already exist) the specified server to
+// the top of the servers list, thereby trying to prefer it over other servers
+// for the next request. Note that exponential backoff overrides this, so this
+// is only a hint, not a guarantee.
 func (n *Network) prefer(server string) {
 	n.mu.Lock()
 	defer n.mu.Unlock()
 
-	n.servers = append([]string{server}, n.servers...)
+	servers := []string{server}
+	for i := 0; i < len(n.servers); i++ {
+		if n.servers[i] != server {
+			servers = append(servers, n.servers[i])
+		}
+	}
+	n.servers = servers
 }
 
 func (n *Network) failed(server string) {