go get golang.org/x/sync

// 方法作用：保證并發(fā)請(qǐng)求只會(huì)執(zhí)行一次函數(shù)，并共享實(shí)際響應(yīng)
// 請(qǐng)求參數(shù)
// key：請(qǐng)求的唯一標(biāo)識(shí)，相同的key會(huì)被視為并發(fā)請(qǐng)求
// fn：實(shí)際需要執(zhí)行的函數(shù)
// 響應(yīng)參數(shù)
// v：實(shí)際執(zhí)行函數(shù)的返回值
// err：實(shí)際執(zhí)行函數(shù)的錯(cuò)誤
// shared：返回值v是否被共享，若存在并發(fā)請(qǐng)求，則為true；若不存在并發(fā)請(qǐng)求則為false
func (g *Group) Do(key string, fn func() (any, error)) (v any, err error, shared bool)
// 方法作用：和Do類似，不過方法返回的是chan
func (g *Group) DoChan(key string, fn func() (any, error)) (<-chan Result, bool)
// 方法作用：刪除key，一般來說不會(huì)直接使用這個(gè)方法
func (g *Group) ForgetUnshared(key string) bool

package main
import (
   "fmt"
   "sync"
   "testing"
   "time"
)
var (
   mx        sync.Mutex
   wg        sync.WaitGroup
   cacheData = make(map[string]string, 0)
)
func TestSingleFlight(t *testing.T) {
   // 添加10個(gè)任務(wù)，模擬并發(fā)請(qǐng)求
   wg.Add(10)
   for i := 0; i < 10; i++ {
      go getData("demo")
   }
   // 等待所有任務(wù)完成
   wg.Wait()
}
func getData(key string) {
   data, _ := getDataFromCache(key)
   if len(data) == 0 {
      // 緩存沒有找到，則進(jìn)行回源
      data, _ = getDataFromDB(key)
      // 設(shè)置緩存
      mx.Lock()
      cacheData[key] = data
      mx.Unlock()
   }
   fmt.Println(data)
   // 任務(wù)完成
   wg.Done()
}
func getDataFromCache(key string) (string, error) {
   return cacheData[key], nil
}
func getDataFromDB(key string) (string, error) {
   fmt.Println("getDataFromDB key: ", key)
   // 模擬訪問db的耗時(shí)
   time.Sleep(10 * time.Millisecond)
   return "db data", nil
}

package main
import (
   "fmt"
   "golang.org/x/sync/singleflight"
   "sync"
   "testing"
   "time"
)
var (
   mx        sync.Mutex
   wg        sync.WaitGroup
   g         singleflight.Group
   cacheData = make(map[string]string, 0)
)
func TestSingleFlight(t *testing.T) {
   // 添加10個(gè)任務(wù)
   wg.Add(10)
   for i := 0; i < 10; i++ {
      go getDataSingleWarp("demo")
   }
   // 等待所有任務(wù)完成
   wg.Wait()
}
func getDataSingleWarp(key string) {
   data, _ := getDataFromCache(key)
   if len(data) == 0 {
      // 使用singleflight來避免并發(fā)請(qǐng)求，實(shí)際改動(dòng)就這一行
      d, _, shared := g.Do(key, func() (interface{}, error) {
         return getDataFromDB(key)
      })
      fmt.Println(shared)
      data = d.(string)
      // 設(shè)置緩存
      mx.Lock()
      cacheData[key] = data
      mx.Unlock()
   }
   fmt.Println(data)
   wg.Done()
}
func getDataFromCache(key string) (string, error) {
   return cacheData[key], nil
}
func getDataFromDB(key string) (string, error) {
   fmt.Println("getDataFromDB key: ", key)
   // 模擬訪問db的耗時(shí)
   time.Sleep(10 * time.Millisecond)
   return "db data", nil
}

// Copyright 2013 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 singleflight provides a duplicate function call suppression
// mechanism.
package singleflight // import "golang.org/x/sync/singleflight"
import (
   "bytes"
   "errors"
   "fmt"
   "runtime"
   "runtime/debug"
   "sync"
)
// errGoexit indicates the runtime.Goexit was called in
// the user given function.
var errGoexit = errors.New("runtime.Goexit was called")
// A panicError is an arbitrary value recovered from a panic
// with the stack trace during the execution of given function.
type panicError struct {
   value interface{}
   stack []byte
}
// Error implements error interface.
func (p *panicError) Error() string {
   return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
}
func newPanicError(v interface{}) error {
   stack := debug.Stack()
   // The first line of the stack trace is of the form "goroutine N [status]:"
   // but by the time the panic reaches Do the goroutine may no longer exist
   // and its status will have changed. Trim out the misleading line.
   if line := bytes.IndexByte(stack[:], '\n'); line >= 0 {
      stack = stack[line+1:]
   }
   return &panicError{value: v, stack: stack}
}
// call is an in-flight or completed singleflight.Do call
type call struct {
   // 保證相同key，只會(huì)進(jìn)行一次實(shí)際請(qǐng)求
   // 相同key的并發(fā)請(qǐng)求會(huì)共享返回
   wg sync.WaitGroup
   // These fields are written once before the WaitGroup is done
   // and are only read after the WaitGroup is done.
   // 實(shí)際執(zhí)行函數(shù)的返回值和錯(cuò)誤
   val interface{}
   err error
   // forgotten indicates whether Forget was called with this call's key
   // while the call was still in flight.
   // 是否已刪除當(dāng)前并發(fā)請(qǐng)求的key
   forgotten bool
   // 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.
   // 并發(fā)請(qǐng)求的次數(shù)
   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
   // key代表請(qǐng)求的唯一標(biāo)識(shí)，相同的key會(huì)被視為并發(fā)請(qǐng)求
   // value代表實(shí)際請(qǐng)求，每一個(gè)實(shí)際請(qǐng)求都會(huì)被封裝為call  
   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) {
   // 加鎖
   g.mu.Lock()
   // 懶加載
   if g.m == nil {
      g.m = make(map[string]*call)
   }
   // 判斷是否有并發(fā)請(qǐng)求，如果key已經(jīng)存在，則說明存在并發(fā)請(qǐng)求
   if c, ok := g.m[key]; ok {
      // 并發(fā)請(qǐng)求次數(shù)+1
      c.dups++
      // 解鎖
      g.mu.Unlock()
      // 等待實(shí)際請(qǐng)求執(zhí)行完
      c.wg.Wait()
      if e, ok := c.err.(*panicError); ok {
         panic(e)
      } else if c.err == errGoexit {
         runtime.Goexit()
      }
      // 共享響應(yīng)
      return c.val, c.err, true
   }
   c := new(call)
   c.wg.Add(1)
   // 添加并發(fā)請(qǐng)求key
   g.m[key] = c
   // 解鎖
   g.mu.Unlock()
   // 進(jìn)行實(shí)際請(qǐng)求
   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
}
// doCall handles the single call for a key.
func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
   // 正常返回標(biāo)識(shí)
   normalReturn := false
   // 是否執(zhí)行了recover標(biāo)識(shí)
   recovered := false
   // use double-defer to distinguish panic from runtime.Goexit,
   // more details see https://golang.org/cl/134395
   defer func() {
      // the given function invoked runtime.Goexit
      if !normalReturn && !recovered {
         c.err = errGoexit
      }
      // 實(shí)際請(qǐng)求執(zhí)行完成
      c.wg.Done()
      // 加鎖
      g.mu.Lock()
      defer g.mu.Unlock()
      // 刪除并發(fā)請(qǐng)求key
      if !c.forgotten {
         delete(g.m, key)
      }
      if e, ok := c.err.(*panicError); ok {
         // In order to prevent the waiting channels from being blocked forever,
         // needs to ensure that this panic cannot be recovered.
         if len(c.chans) > 0 {
            go panic(e)
            select {} // Keep this goroutine around so that it will appear in the crash dump.
         } else {
            panic(e)
         }
      } else if c.err == errGoexit {
         // Already in the process of goexit, no need to call again
      } else {
         // Normal return
         for _, ch := range c.chans {
            ch <- Result{c.val, c.err, c.dups > 0}
         }
      }
   }()
   // 匿名函數(shù)立即執(zhí)行
   func() {
      defer func() {
         if !normalReturn {
            // Ideally, we would wait to take a stack trace until we've determined
            // whether this is a panic or a runtime.Goexit.
            //
            // Unfortunately, the only way we can distinguish the two is to see
            // whether the recover stopped the goroutine from terminating, and by
            // the time we know that, the part of the stack trace relevant to the
            // panic has been discarded.
            if r := recover(); r != nil {
               c.err = newPanicError(r)
            }
         }
      }()
      // 執(zhí)行實(shí)際函數(shù)
      c.val, c.err = fn()
      // 正常返回
      normalReturn = true
   }()
   if !normalReturn {
      recovered = true
   }
}
// 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()
   if c, ok := g.m[key]; ok {
      c.forgotten = true
   }
   delete(g.m, key)
   g.mu.Unlock()
}