go并发编程sync.Cond使用场景及实现原理

type pipe struct {
   mu       sync.Mutex
   c        sync.Cond     // c.L lazily initialized to &p.mu
   b        pipeBuffer    // nil when done reading
   ...
}

// Read waits until data is available and copies bytes
// from the buffer into p.
func (p *pipe) Read(d []byte) (n int, err error) {
   p.mu.Lock()
   defer p.mu.Unlock()
   if p.c.L == nil {
      p.c.L = &p.mu
   }
   for {
      ...
      if p.b != nil && p.b.Len() > 0 {
         return p.b.Read(d)
      }
      ...
      p.c.Wait() // write未完成前调用Wait进入等待
   }
}

// Write copies bytes from p into the buffer and wakes a reader.
// It is an error to write more data than the buffer can hold.
func (p *pipe) Write(d []byte) (n int, err error) {
   p.mu.Lock()
   defer p.mu.Unlock()
   if p.c.L == nil {
      p.c.L = &p.mu
   }
   defer p.c.Signal() // 唤醒所有等待的goroutine
   if p.err != nil {
      return 0, errClosedPipeWrite
   }
   if p.breakErr != nil {
      p.unread += len(d)
      return len(d), nil // discard when there is no reader
   }
   return p.b.Write(d)
}

type Cond struct {
   noCopy noCopy       // 用来保证结构体无法在编译期间拷贝
   // L is held while observing or changing the condition
   L Locker             // 用来保证condition变更安全
   notify  notifyList   // 待通知的goutine列表
   checker copyChecker  // 用于禁止运行期间发生的拷贝
}

type notifyList struct {
   wait   uint32      // 正在等待的goroutine的ticket
   notify uint32      // 已经通知到的goroutine的ticket
   lock   uintptr // key field of the mutex
   head   unsafe.Pointer     // 链表头部
   tail   unsafe.Pointer     // 链表尾部
}

// copyChecker holds back pointer to itself to detect object copying.
type copyChecker uintptr
func (c *copyChecker) check() {
   if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
      !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
      uintptr(*c) != uintptr(unsafe.Pointer(c)) {
      panic("sync.Cond is copied")
   }
}

func (c *Cond) Wait() {
   c.checker.check()
   t := runtime_notifyListAdd(&c.notify)
   c.L.Unlock()
   runtime_notifyListWait(&c.notify, t)
   c.L.Lock()
}

func notifyListAdd(l *notifyList) uint32 {
    return atomic.Xadd(&l.wait, 1) - 1
}

func notifyListWait(l *notifyList, t uint32) {
    s := acquireSudog()
    s.g = getg()
    s.ticket = t
    if l.tail == nil {
       l.head = s
    } else {
       l.tail.next = s
    }
    l.tail = s
    goparkunlock(&l.lock, waitReasonSyncCondWait, traceEvGoBlockCond, 3)
    releaseSudog(s)
}

func (c *Cond) Signal() {
   c.checker.check()
   runtime_notifyListNotifyOne(&c.notify)
}

func notifyListNotifyOne(l *notifyList) {
   t := l.notify
   atomic.Store(&l.notify, t+1)
   for p, s := (*sudog)(nil), l.head; s != nil; p, s = s, s.next {
      if s.ticket == t {
         n := s.next
         if p != nil {
            p.next = n
         } else {
            l.head = n
         }
         if n == nil {
            l.tail = p
         }
         s.next = nil
         readyWithTime(s, 4)
         return
      }
   }
}

func (c *Cond) Broadcast() {
    c.checker.check()
    runtime_notifyListNotifyAll(&c.notify)
}

func notifyListNotifyAll(l *notifyList) {
   s := l.head
   l.head = nil
   l.tail = nil
   atomic.Store(&l.notify, atomic.Load(&l.wait))
   for s != nil {
      next := s.next
      s.next = nil
      readyWithTime(s, 4)
      s = next
   }
}