gee-rpc day6 add robin select for discovery

gee-rpc/day6-discovery/xclient/discovery.go

@@ -10,7 +10,7 @@ type SelectMode int
 
 const (
  RandomSelect SelectMode = iota // select randomly
- RobbinSelect // select using Robbin algorithm, not implemented
+ RobinSelect  // select using Robbin algorithm
 )
 
 type Discovery interface {
@@ -26,6 +26,7 @@ type MultiServersDiscovery struct {
  r *rand.Rand // generate random number
  mu sync.RWMutex // protect following
  servers []string
+ index int // record the selected position for robin algorithm
 }
 
 // Update the servers of discovery dynamically if needed
@@ -36,14 +37,18 @@ func (d *MultiServersDiscovery) Update(servers []string) {
 }
 
 func (d *MultiServersDiscovery) Get(mode SelectMode) string {
- d.mu.RLock()
- defer d.mu.RUnlock()
+ d.mu.Lock()
+ defer d.mu.Unlock()
  if len(d.servers) == 0 {
  return ""
  }
  switch mode {
  case RandomSelect:
  return d.servers[d.r.Intn(len(d.servers))]
+ case RobinSelect:
+ s := d.servers[d.index]
+ d.index = (d.index + 1) % len(d.servers)
+ return s
  default:
  return ""
  }