Access JSONGateResolverBuilder.resolvers under a lock

Signed-off-by: Henry Robinson <hrobinson@slack-corp.com>

go/vt/vtgateproxy/discovery.go

@@ -210,13 +210,15 @@ func (b *JSONGateResolverBuilder) start() error {
  }
  parseCount.Add("changed", 1)
 
+ b.mu.RLock()
  // notify all the resolvers that the targets changed
  for _, r := range b.resolvers {
  err = b.update(r)
  if err != nil {
  log.Errorf("Failed to update resolver: %v", err)
  }
  }
+ b.mu.RUnlock()
  }
  }()
 
@@ -422,7 +424,10 @@ func (b *JSONGateResolverBuilder) Build(target resolver.Target, cc resolver.Clie
  if err != nil {
  return nil, err
  }
+
+ b.mu.Lock()
  b.resolvers = append(b.resolvers, r)
+ b.mu.Unlock()
 
  return r, nil
 }