code review 5545065: old/netchan: fix data race on clients hashmap

src/pkg/old/netchan/common.go

 // Copyright 2010 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package netchan
 
 import (
         "encoding/gob"
         "errors"
         "io"
@@ skipping 147 matching lines @@
                 time.Sleep(100 * time.Millisecond)
         }
         return nil
 }
 
 // See the comment for Exporter.Sync.
 func (cs *clientSet) sync(timeout time.Duration) error {
         deadline := time.Now().Add(timeout)
         // seq remembers the clients and their seqNum at point of entry.
         seq := make(map[unackedCounter]int64)
+        cs.mu.Lock()
         for client := range cs.clients {
                 seq[client] = client.seq()
         }
+        cs.mu.Unlock()
         for {
                 pending := false
                 cs.mu.Lock()
                 // Any unacknowledged messages?  Look only at clients that existed
                 // when we started and are still in this client set.
                 for client := range seq {
                         if _, ok := cs.clients[client]; ok {
                                 if client.ack() < seq[client] {
                                         pending = true
                                         break
@@ skipping 146 matching lines @@
                 panic("recv on wrong direction of channel")
         }
         select {
         case nch.ackCh <- true:
                 // ok
         default:
                 // TODO: should this be more resilient?
                 panic("netchan: remote receiver sent too many acks")
         }
 }