Golang中channel的原理解读(推荐)

type hchan struct {

   qcount   uint           // 当前队列中剩余元素个数

   dataqsiz uint           // 环形队列长度，即可以存放的元素个数

   buf      unsafe.Pointer // 环形队列指针

   elemsize uint16         // 每个元素的大小

   closed   uint32         // 标记是否关闭

   elemtype *_type         // 元素类型

   sendx    uint           // 队列下标，指向元素写入时存放到队列中的位置

   recvx    uint           // 队列下标，指向元素从队列中读出的位置

   recvq    waitq          // 等待读消息的groutine队列

   sendq    waitq          // 等待写消息的groutine队列

   lock     mutex          // 互斥锁

}

type waitq struct {
   first *sudog
   last  *sudog
}

type sudog struct {
   g            *g
   next         *sudog
   prev         *sudog
   elem         unsafe.Pointer // data element (may point to stack)
   
   acquiretime  int64
   releasetime  int64
   ticket       uint32
   isSelect     bool
   
   parent       *sudog // semaRoot binary tree
   waitlink     *sudog // g.waiting list or semaRoot
   waittail     *sudog // semaRoot
   c            *hchan // channel
}

func makechan(t *chantype, size int) *hchan {
   elem := t.elem

   // compiler checks this but be safe.
   if elem.size >= 1<<16 {
      throw("makechan: invalid channel element type")
   }
   if hchanSize%maxAlign != 0 || elem.align > maxAlign {
      throw("makechan: bad alignment")
   }

   mem, overflow := math.MulUintptr(elem.size, uintptr(size))
   if overflow || mem > maxAlloc-hchanSize || size < 0 {
      panic(plainError("makechan: size out of range"))
   }
   var c *hchan
   switch {
   
   case mem == 0:
   // 如果当前 Channel 中不存在缓冲区，那么就只会为 runtime.hchan 分配一段内存空间；
      c = (*hchan)(mallocGC(hchanSize, nil, true))
      c.buf = c.raceaddr()
   case elem.ptrdata == 0:
   // 如果当前 Channel 中存储的类型不是指针类型，会为当前的 Channel 和底层的数组分配一块连续的内存空间；
      c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
      c.buf = add(unsafe.Pointer(c), hchanSize)
   default:
   //单独为 runtime.hchan 和缓冲区分配内存；
      c = new(hchan)
      c.buf = mallocgc(mem, elem, true)
   }

   c.elemsize = uint16(elem.size)
   c.elemtype = elem
   c.dataqsiz = uint(size)
   lockInit(&c.lock, lockRankHchan)
   // 在函数的最后会统一更新elemsize、elemtype 和 dataqsiz 几个字段;
   if debugChan {
      print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
   }
   return c
}

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
   .....
   lock(&c.lock)

   if c.closed != 0 {
      unlock(&c.lock)
      panic(plainError("send on closed channel"))
   }

   // 如果Channel 没有被关闭并且已经有处于读等待的 Goroutine，
   // 那么从接收队列 recvq 中取出最先陷入等待的 Goroutine 并直接向它发送数据
   if sg := c.recvq.dequeue(); sg != nil {
      send(c, sg, ep, func() { unlock(&c.lock) }, 3)
      return true
   }
   
   // 如果recvq为空且缓冲区中还有剩余空间
   if c.qcount < c.dataqsiz {
   // 计算出下一个可以存储数据的位置，
      qp := chanbuf(c, c.sendx)
      // raceenabled: 是否启用数据竞争检测，在编译时指定，默认为false
      if raceenabled {
      // 发出数据竞争警告
         raceacquire(qp)
         racerelease(qp)
      }
      // 将发送的数据拷贝到缓冲区中，产生内存拷贝
      typedmemmove(c.elemtype, qp, ep)
      // 增加 sendx 索引
      c.sendx++
      if c.sendx == c.dataqsiz {
         c.sendx = 0
      }
      // 增加计数器
      c.qcount++
      unlock(&c.lock)
      return true
   }
   
   if !block {
      unlock(&c.lock)
      return false
   }

   // 将channel数据绑定到当前groutine并使groutine休眠
   // 获取发送数据使用的 Goroutine
   gp := getg()
   // 获取 runtime.sudog 结构并设置这一次阻塞发送的相关信息，
   // 例如发送的 Channel、是否在 select 中和待发送数据的内存地址等
   mysg := acquireSudog()
   mysg.releasetime = 0
   if t0 != 0 {
      mysg.releasetime = -1
   }
   // 将刚刚创建并初始化的 mysg 加入发送等待队列，并设置到当前 Goroutine的waiting上，
   // 表示 Goroutine 正在等待该sudog准备就绪
   mysg.elem = ep
   mysg.waitlink = nil
   mysg.g = gp
   mysg.isSelect = false
   mysg.c = c
   gp.waiting = mysg
   gp.param = nil
   c.sendq.enqueue(mysg)
   // 休眠groutine
   gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)
   // 保证传入的数据不被GC
   KeepAlive(ep)

   // someone woke us up.
   if mysg != gp.waiting {
      throw("G waiting list is corrupted")
   }
   gp.waiting = nil
   gp.activeStackChans = false
   if gp.param == nil {
      if c.closed == 0 {
         throw("chansend: spurious wakeup")
      }
      panic(plainError("send on closed channel"))
   }
   gp.param = nil
   if mysg.releasetime > 0 {
      blockevent(mysg.releasetime-t0, 2)
   }
   mysg.c = nil
   releaseSudog(mysg)
   return true
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
 // block：这次接收是否阻塞
   if debugChan {
      print("chanrecv: chan=", c, "\n")
   }

   if c == nil {
      if !block {
         return
      }
      // 从一个空 Channel 接收数据时会直接让出处理器的使用权
      gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
      throw("unreachable")
   }

   // Fast path: check for failed non-blocking operation without acquiring the lock.
   if !block && empty(c) {
     // 如果channel为空并且未关闭，直接返回
      if atomic.Load(&c.closed) == 0 {
         return
      }

      if empty(c) {
         // The channel is irreversibly closed and empty.
         if raceenabled {
            raceacquire(c.raceaddr())
         }
         if ep != nil {
         // 手动标记清楚对象
            typedmemclr(c.elemtype, ep)
         }
         return true, false
      }
   }

   var t0 int64
   if blockprofilerate > 0 {
      t0 = cputicks()
   }

   lock(&c.lock)
    //如果channel为空，并且已关闭，说明对象不可达
   if c.closed != 0 && c.qcount == 0 {
      if raceenabled {
         raceacquire(c.raceaddr())
      }
      unlock(&c.lock)
      if ep != nil {
      // 手动标记清除
         typedmemclr(c.elemtype, ep)
      }
      return true, false
   }
    // 如果sendq不为空，直接消费，避免sendq --> queue --> recvx的过程
   if sg := c.sendq.dequeue(); sg != nil {
      recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
      return true, true
   }
    
    // 当 Channel 的缓冲区中已经包含数据时，从 Channel 中接收数据会直接从缓冲区中 
    // recvx 的索引位置中取出数据进行处理
   if c.qcount > 0 {
      // Receive directly from queue
      qp := chanbuf(c, c.recvx)
      if raceenabled {
         raceacquire(qp)
         racerelease(qp)
      }
      // 如果接收数据的内存地址不为空，那么会使用 runtime.typedmemmove将缓冲区中的数据拷贝到内存中
      if ep != nil {
         typedmemmove(c.elemtype, ep, qp)
      }
      // 使用 runtime.typedmemclr清除队列中的数据并完成收尾工作
      typedmemclr(c.elemtype, qp)
      c.recvx++
      // recvx位置归零
      if c.recvx == c.dataqsiz {
         c.recvx = 0
      }
      c.qcount-- // 计数减一
      unlock(&c.lock) 
      return true, true
   }

   if !block {
      unlock(&c.lock)
      return false, false
   }

   // 当 sendq不为空 并且缓冲区中也不存在任何数据时，阻塞并休眠当前groutine
   gp := getg()
   mysg := acquireSudog()
   mysg.releasetime = 0
   if t0 != 0 {
      mysg.releasetime = -1
   }
   // No stack splits between assigning elem and enqueuing mysg
   // on gp.waiting where copystack can find it.
   mysg.elem = ep
   mysg.waitlink = nil
   gp.waiting = mysg
   mysg.g = gp
   mysg.isSelect = false
   mysg.c = c
   gp.param = nil
   c.recvq.enqueue(mysg)
   gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)

   // someone woke us up
   if mysg != gp.waiting {
      throw("G waiting list is corrupted")
   }
   gp.waiting = nil
   gp.activeStackChans = false
   if mysg.releasetime > 0 {
      blockevent(mysg.releasetime-t0, 2)
   }
   closed := gp.param == nil
   gp.param = nil
   mysg.c = nil
   releaseSudog(mysg)
   return true, !closed
}