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详解Golang中select的使用与源码分析

胡大海 人气:0

背景

golang 中主推 channel 通信。单个 channel 的通信可以通过一个goroutine往 channel 发数据，另外一个从channel取数据进行。这是阻塞的，因为要想顺利执行完这个步骤，需要 channel 准备好才行，准备好的条件如下:

1.发送

2.接收

对channel实际使用中还有如下两个需求，这个时候就需要select了。

select 流程

1.空select。作用是阻塞当前goroutine。不要用for{}来阻塞goroutine，因为会占用cpu。而select{}不会，因为当前goroutine不会再被调度。

 if len(cases) == 0 {
         block()
 }

2.配置好poll的顺序。由于是同时监听多个channel的发送或者接收，所以需要按照一定的顺序查看哪个channel准备好了。如果每次采用select中的顺序查看channel是否准备好了，那么只要在前面的channel准备好的足够快，那么会造成后面的channel即使准备好了，也永远不会被执行。打乱顺序的逻辑如下，采用了洗牌算法\color{red}{洗牌算法}洗牌算法，注意此过程中会过滤掉channel为nil的case。\color{red}{注意此过程中会过滤掉 channel 为 nil 的 case。}注意此过程中会过滤掉channel为nil的case。

 // generate permuted order
 norder := 0
 for i := range scases {
         cas := &scases[i]

         // Omit cases without channels from the poll and lock orders.
         if cas.c == nil {
                 cas.elem = nil // allow GC
                 continue
         }

         j := fastrandn(uint32(norder + 1))
         pollorder[norder] = pollorder[j]
         pollorder[j] = uint16(i)
         norder++
 }

3.配置好lock的顺序。由于可能会修改channel中的数据，所以在打算往channel中发送数据或者从channel接收数据的时候，需要锁住 channel。而一个channel可能被多个select监听，如果两个select对两个channel A和B，分别按照顺序A, B和B,A上锁，是可能会造成死锁的，导致两个select都执行不下去。

所以select中锁住channel的顺序至关重要，解决方案是按照channel的地址的顺序锁住channel。因为在两个select中channel有交集的时候，都是按照交集中channel的地址顺序锁channel。

实际排序代码如下，采用堆排序算法\color{red}{堆排序算法}堆排序算法按照channel的地址从小到大对channel进行排序。

 // sort the cases by Hchan address to get the locking order.
 // simple heap sort, to guarantee n log n time and constant stack footprint.
 for i := range lockorder {
         j := i
         // Start with the pollorder to permute cases on the same channel.
         c := scases[pollorder[i]].c
         for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
                 k := (j - 1) / 2
                 lockorder[j] = lockorder[k]
                 j = k
         }
         lockorder[j] = pollorder[i]
 }
 for i := len(lockorder) - 1; i >= 0; i-- {
         o := lockorder[i]
         c := scases[o].c
         lockorder[i] = lockorder[0]
         j := 0
         for {
                 k := j*2 + 1
                 if k >= i {
                         break
                 }
                 if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
                         k++
                 }
                 if c.sortkey() < scases[lockorder[k]].c.sortkey() {
                         lockorder[j] = lockorder[k]
                         j = k
                         continue
                 }
                 break
         }
         lockorder[j] = o
 }

4.锁住select中的所有channel。要查看channel中的数据了。

 // lock all the channels involved in the select
 sellock(scases, lockorder)

5.第一轮查看是否已有准备好的channel。如果有直接发送数据到channel或者从channel接收数据。注意select的channel切片中，前面部分是从channel接收数据的case，后半部分是往channel发送数据的case。

按照pollorder顺序查看是否有channel准备好了。

 for _, casei := range pollorder {
         casi = int(casei)
         cas = &scases[casi]
         c = cas.c
         if casi >= nsends {
                 sg = c.sendq.dequeue()
                 if sg != nil {
                         goto recv
                 }
                 if c.qcount > 0 {
                         goto bufrecv
                 }
                 if c.closed != 0 {
                         goto rclose
                 }
         } else {
                 if raceenabled {
                         racereadpc(c.raceaddr(), casePC(casi), chansendpc)
                 }
                 if c.closed != 0 {
                         goto sclose
                 }
                 sg = c.recvq.dequeue()
                 if sg != nil {
                         goto send
                 }
                 if c.qcount < c.dataqsiz {
                         goto bufsend
                 }
         }
 }

6.直接执行default分支

 if !block {
         selunlock(scases, lockorder)
         casi = -1
         goto retc
 }

7.第二轮遍历channel。创建sudog把当前goroutine放到每个channel的等待列表中去，等待channel准备好时被唤醒。

 // pass 2 - enqueue on all chans
 gp = getg()
 if gp.waiting != nil {
         throw("gp.waiting != nil")
 }
 nextp = &gp.waiting
 for _, casei := range lockorder {
         casi = int(casei)
         cas = &scases[casi]
         c = cas.c
         sg := acquireSudog()
         sg.g = gp
         sg.isSelect = true
         // No stack splits between assigning elem and enqueuing
         // sg on gp.waiting where copystack can find it.
         sg.elem = cas.elem
         sg.releasetime = 0
         if t0 != 0 {
                 sg.releasetime = -1
         }
         sg.c = c
         // Construct waiting list in lock order.
         *nextp = sg
         nextp = &sg.waitlink

         if casi < nsends {
                 c.sendq.enqueue(sg)
         } else {
                 c.recvq.enqueue(sg)
         }
 }

8.等待被唤醒。其中gopark的时候会释放对所有channel占用的锁。

 // wait for someone to wake us up
 gp.param = nil
 // Signal to anyone trying to shrink our stack that we're about
 // to park on a channel. The window between when this G's status
 // changes and when we set gp.activeStackChans is not safe for
 // stack shrinking.
 atomic.Store8(&gp.parkingOnChan, 1)
 gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
 gp.activeStackChans = false

9.被唤醒

 sellock(scases, lockorder)

 gp.selectDone = 0
 sg = (*sudog)(gp.param)
 gp.param = nil

 // pass 3 - dequeue from unsuccessful chans
 // otherwise they stack up on quiet channels
 // record the successful case, if any.
 // We singly-linked up the SudoGs in lock order.
 casi = -1
 cas = nil
 caseSuccess = false
 sglist = gp.waiting
 // Clear all elem before unlinking from gp.waiting.
 for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
         sg1.isSelect = false
         sg1.elem = nil
         sg1.c = nil
 }
 gp.waiting = nil

 for _, casei := range lockorder {
         k = &scases[casei]
         if sg == sglist {
                 // sg has already been dequeued by the G that woke us up.
                 casi = int(casei)
                 cas = k
                 caseSuccess = sglist.success
                 if sglist.releasetime > 0 {
                         caseReleaseTime = sglist.releasetime
                 }
         } else {
                 c = k.c
                 if int(casei) < nsends {
                         c.sendq.dequeueSudoG(sglist)
                 } else {
                         c.recvq.dequeueSudoG(sglist)
                 }
         }
         sgnext = sglist.waitlink
         sglist.waitlink = nil
         releaseSudog(sglist)
         sglist = sgnext
 }

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