WaitGroup原理分析

背景

在实际业务开发中，我们会遇到以下场景：请求数据库，批量获取1000条数据记录后，处理数据
为了减少因一次批量获取的数据太多，导致的数据库延时增加，我们可以把一次请求拆分成多次请求，并发去处理，当所有的并发请求完成后，再继续处理这些返回的数据
golang中的WaitGroup，就可以帮助我们实现上述的场景

快速入门

背景：开启10个goroutine并发执行，等待所有goroutine执行完成后，当前goroutine打印执行完成

func TestWaitGroup(t *testing.T) {
   var wg sync.WaitGroup
   for i := 0; i < 10; i++ {
      index := i
      go func() {
         wg.Add(1)
         defer wg.Done()
         fmt.Println(fmt.Sprintf("%+v 正在执行", index))
      }()
   }
   wg.Wait()
   fmt.Println("TestWaitGroup method done")
}
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源码分析

golang版本：1.18.2

源码路径：src/sync/waitgroup.go

// A WaitGroup waits for a collection of goroutines to finish.
// The main goroutine calls Add to set the number of
// goroutines to wait for. Then each of the goroutines
// runs and calls Done when finished. At the same time,
// Wait can be used to block until all goroutines have finished.
//
// A WaitGroup must not be copied after first use.
// WaitGroup 等待 goroutine 集合完成
// 主 goroutine 调用 Add 设置等待的 goroutine 数量
// 然后每个 goroutine 运行并在完成时调用 Done
// 同时，Wait 可以用来阻塞，直到所有 goroutine 都完成
type WaitGroup struct {
   noCopy noCopy

   // 64-bit value: high 32 bits are counter, low 32 bits are waiter count.
   // 64-bit atomic operations require 64-bit alignment, but 32-bit
   // compilers only guarantee that 64-bit fields are 32-bit aligned.
   // For this reason on 32 bit architectures we need to check in state()
   // if state1 is aligned or not, and dynamically "swap" the field order if
   // needed.
   // 64位值：高32位是计数器，低32位是waiter计数
   // 64位原子操作需要64位对齐，但32位编译器仅保证64位字段是32位对齐的
   // 因此，在 32 位架构上，我们需要在 state() 中检查 state1 是否对齐，并在需要时动态“交换”字段顺序
   state1 uint64
   state2 uint32
}

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noCopy：WaitGroup在首次使用后，不能被复制
state1，state2：一共占用12字节，保存了三类信息：4字节保存goroutine计数，4字节保存waiter计数，4字节保存信号量
WaitGroup对外提供了以下三个方法：

// 设置等待的goroutine数量
func (wg *WaitGroup) Add(delta int)
// goroutine执行完成
func (wg *WaitGroup) Done()
// 阻塞等待所有的goroutine都执行完成
func (wg *WaitGroup) Wait()
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Add

// state returns pointers to the state and sema fields stored within wg.state*.
func (wg *WaitGroup) state() (statep *uint64, semap *uint32) {
   if unsafe.Alignof(wg.state1) == 8 || uintptr(unsafe.Pointer(&wg.state1))%8 == 0 {
      // state1 is 64-bit aligned: nothing to do.
      return &wg.state1, &wg.state2
   } else {
      // state1 is 32-bit aligned but not 64-bit aligned: this means that
      // (&state1)+4 is 64-bit aligned.
      state := (*[3]uint32)(unsafe.Pointer(&wg.state1))
      return (*uint64)(unsafe.Pointer(&state[1])), &state[0]
   }
}

// Add adds delta, which may be negative, to the WaitGroup counter.
// If the counter becomes zero, all goroutines blocked on Wait are released.
// If the counter goes negative, Add panics.
//
// Note that calls with a positive delta that occur when the counter is zero
// must happen before a Wait. Calls with a negative delta, or calls with a
// positive delta that start when the counter is greater than zero, may happen
// at any time.
// Typically this means the calls to Add should execute before the statement
// creating the goroutine or other event to be waited for.
// If a WaitGroup is reused to wait for several independent sets of events,
// new Add calls must happen after all previous Wait calls have returned.
// See the WaitGroup example.
// Add 将 delta（可能为负）添加到 WaitGroup 计数器。
// 如果计数器变为零，则所有在 Wait 上阻塞的 goroutine 都会被释放。
// 如果计数器变为负数，则添加panic。
// 请注意，计数器为零时发生的具有正增量的调用必须在等待之前发生。 
// 具有负增量的调用或在计数器大于零时开始的具有正增量的调用可能随时发生。
// 通常，这意味着对 Add 的调用应该在创建 goroutine 或其他要等待的事件的语句之前执行。
// 如果重用一个 WaitGroup 来等待几个独立的事件集，新的 Add 调用必须在所有先前的 Wait 调用返回后发生。
func (wg *WaitGroup) Add(delta int) {
   statep, semap := wg.state()
   if race.Enabled {
      _ = *statep // trigger nil deref early
      if delta < 0 {
         // Synchronize decrements with Wait.
         race.ReleaseMerge(unsafe.Pointer(wg))
      }
      race.Disable()
      defer race.Enable()
   }
   // 记录goroutine计数
   state := atomic.AddUint64(statep, uint64(delta)<<32)
   // 获取goroutine计数
   v := int32(state >> 32)
   // 获取waiter计数
   w := uint32(state)
   if race.Enabled && delta > 0 && v == int32(delta) {
      // The first increment must be synchronized with Wait.
      // Need to model this as a read, because there can be
      // several concurrent wg.counter transitions from 0.
      race.Read(unsafe.Pointer(semap))
   }
   // goroutine计数小于0
   if v < 0 {
      panic("sync: negative WaitGroup counter")
   }
   // w != 0说明已经执行了Wait且还有阻塞等待的goroutine，此时不允许在执行Add
   if w != 0 && delta > 0 && v == int32(delta) {
      panic("sync: WaitGroup misuse: Add called concurrently with Wait")
   }
   // 存在没有执行完成的goroutine，或者当前没有waiter，直接返回
   if v > 0 || w == 0 {
      return
   }
   // This goroutine has set counter to 0 when waiters > 0.
   // Now there can't be concurrent mutations of state:
   // - Adds must not happen concurrently with Wait,
   // - Wait does not increment waiters if it sees counter == 0.
   // Still do a cheap sanity check to detect WaitGroup misuse.
   // 此时goroutine计数为0，且waiter计数大于0，不然上一步就返回了
   // 现在以下状态不能同时发生：
   // 1. 并发调用Add和Wait
   // 2. 当goroutine计数为0时，Wait不会继续增加waiter计数
   // 仍然做一个廉价的健全性检查来检测 WaitGroup 的滥用，防止以上情况发生
   if *statep != state {
      panic("sync: WaitGroup misuse: Add called concurrently with Wait")
   }
   // Reset waiters count to 0.
   // 重置waiter计数
   *statep = 0
   // 唤醒所有的waiter
   for ; w != 0; w-- {
      runtime_Semrelease(semap, false, 0)
   }
}
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delta代表本次需要记录的goroutine计数，可能为负数
64位原子操作需要64位对齐，但32位编译器仅保证64位字段是32位对齐的
当state1是64位对齐时，state1高32位是goroutine计数，低32位是waiter计数
当state1不是64位对齐时，动态“交换”字段顺序
记录goroutine计数的变化delta
如果goroutine计数小于0，则直接panic
如果已经执行了Wait且还有阻塞等待的goroutine，此时不允许在执行Add
如果存在没有执行完成的goroutine，或者当前没有waiter，直接返回
当goroutine计数为0，且waiter计数大于0时，现在以下状态不能同时发生：

并发调用Add和Wait
当goroutine计数为0时，Wait不会继续增加waiter计数

简单校验通过后，重置waiter计数为0，唤醒所有阻塞等待的waiter

Done

// Done decrements the WaitGroup counter by one.
func (wg *WaitGroup) Done() {
   wg.Add(-1)
}
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调用Add，delta = -1，代表goroutine计数-1

Wait

// Wait blocks until the WaitGroup counter is zero.
func (wg *WaitGroup) Wait() {
   statep, semap := wg.state()
   if race.Enabled {
      _ = *statep // trigger nil deref early
      race.Disable()
   }
   for {
      state := atomic.LoadUint64(statep)
      // 获取goroutine计数
      v := int32(state >> 32)
      // 获取waiter计数
      w := uint32(state)
      // goroutine计数为0，不需要等待，直接返回
      if v == 0 {
         // Counter is 0, no need to wait.
         if race.Enabled {
            race.Enable()
            race.Acquire(unsafe.Pointer(wg))
         }
         return
      }
      // Increment waiters count.
      // waiter计数+1
      if atomic.CompareAndSwapUint64(statep, state, state+1) {
         if race.Enabled && w == 0 {
            // Wait must be synchronized with the first Add.
            // Need to model this is as a write to race with the read in Add.
            // As a consequence, can do the write only for the first waiter,
            // otherwise concurrent Waits will race with each other.
            race.Write(unsafe.Pointer(semap))
         }
         // 阻塞，等待goroutine计数为0后唤醒继续执行
         runtime_Semacquire(semap)
         // Wait还没有执行完成，就开始复用WaitGroup
         if *statep != 0 {
            panic("sync: WaitGroup is reused before previous Wait has returned")
         }
         if race.Enabled {
            race.Enable()
            race.Acquire(unsafe.Pointer(wg))
         }
         return
      }
   }
}
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调用state()，保证字段内存对齐
如果goroutine计数为0，不需要等待，直接返回
尝试对waiter计数+1，若失败，则继续下一轮重试
对waiter计数+1成功，则阻塞当前goroutine，等待goroutine计数为0后唤醒继续执行
唤醒继续执行后，简单判断是否存在Wait还没有执行完成，就开始复用WaitGroup的情况，如果有，则panic；如果没有，则直接返回