func main() {
  var wg sync.WaitGroup
  for i := 0; i < 5; i++ {
    wg.Add(1)
    go func() {
      defer wg.Done()
      result := getProductById("商品A")
      fmt.Printf("%v\n", result)
    }()
  }
  wg.Wait()
}
func getProductById(name string) string {
  fmt.Println("getProductById doing...")
  time.Sleep(time.Millisecond * 10) // 模擬一下耗時
  return name
}

$ go run main.go
getProductById doing...
getProductById doing...
getProductById doing...
getProductById doing...
getProductById doing...
商品A
商品A
商品A
商品A
商品A

import (
  "fmt"
  "golang.org/x/sync/singleflight"
  "sync"
  "time"
)
var g singleflight.Group
func main() {
  var wg sync.WaitGroup
  for i := 0; i < 5; i++ {
    wg.Add(1)
    go func() {
      defer wg.Done()
      resp, _, _ := g.Do("商品A", func() (interface{}, error) {
        result := getProductById("商品A")
        return result, nil
      })
      fmt.Printf("%v\n", resp)
    }()
  }
  wg.Wait()
}
func getProductById(name string) string {
  fmt.Println("getProductById doing...")
  time.Sleep(time.Millisecond * 10) // 模擬一下耗時
  return name
}

$ go run main.go
getProductById doing...
商品A
商品A
商品A
商品A
商品A

// call is an in-flight or completed singleflight.Do call
type call struct {
  wg sync.WaitGroup
  // These fields are written once before the WaitGroup is done
  // and are only read after the WaitGroup is done.
  val interface{}
  err error
  // These fields are read and written with the singleflight
  // mutex held before the WaitGroup is done, and are read but
  // not written after the WaitGroup is done.
  dups  int
  chans []chan<- Result
}
// Group represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type Group struct {
  mu sync.Mutex       // protects m
  m  map[string]*call // lazily initialized
}
// Result holds the results of Do, so they can be passed
// on a channel.
type Result struct {
  Val    interface{}
  Err    error
  Shared bool
}

// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
// The return value shared indicates whether v was given to multiple callers.
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
  g.mu.Lock()
  if g.m == nil {
    g.m = make(map[string]*call)
  }
  if c, ok := g.m[key]; ok {
    c.dups++
    g.mu.Unlock()
    c.wg.Wait()
    if e, ok := c.err.(*panicError); ok {
      panic(e)
    } else if c.err == errGoexit {
      runtime.Goexit()
    }
    return c.val, c.err, true
  }
  c := new(call)
  c.wg.Add(1)
  g.m[key] = c
  g.mu.Unlock()
  g.doCall(c, key, fn)
  return c.val, c.err, c.dups > 0
}

// DoChan is like Do but returns a channel that will receive the
// results when they are ready.
//
// The returned channel will not be closed.
func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
  ch := make(chan Result, 1)
  g.mu.Lock()
  if g.m == nil {
    g.m = make(map[string]*call)
  }
  if c, ok := g.m[key]; ok {
    c.dups++
    c.chans = append(c.chans, ch)
    g.mu.Unlock()
    return ch
  }
  c := &call{chans: []chan<- Result{ch}}
  c.wg.Add(1)
  g.m[key] = c
  g.mu.Unlock()
  go g.doCall(c, key, fn)
  return ch
}

// Forget tells the singleflight to forget about a key.  Future calls
// to Do for this key will call the function rather than waiting for
// an earlier call to complete.
func (g *Group) Forget(key string) {
  g.mu.Lock()
  delete(g.m, key)
  g.mu.Unlock()
}