select是Go在语言层面提供的多路I/O复用机制.用于检测多个管道是否就绪(可读或
可写).特性与管道息息相关.
1.select特性:
1.1管道读写:
select只能作用于管道.包括数据的读取和写入.
func SelectChan(c chan string) {
var recv string
send := "hello"
select {
case recv = <-c:
fmt.Println("recv:", recv)
case c <- send:
fmt.Println("send:", send)
}
}
上面代码中有两个case语句.分别对应管道的写入和读取操作.至于执行哪个case操
作,取决于函数传入的管道.
情况1:管道没有缓冲区.
func main() {
no := make(chan string)
SelectChan(no)
}
此时管道既不能读也不能写.两个case语句都不执行.
情况2:管道有缓冲区.
func main() {
no := make(chan string, 1)
SelectChan(no)
}
此时管道可以写入.写操作对应的case语句得到执行.且执行结束后函数退出.
情况3:管道有缓冲区且缓冲区已放满数据.
func main() {
no := make(chan string, 1)
no <- "hello"
SelectChan(no)
}
此时管道可以读取.读操作对应的case语句得到执行.且执行结束后函数退出.
情况4:管道有缓冲区,缓冲区有部分数据且还可以存入数据.
func main() {
no := make(chan string, 2)
no <- "hello"
SelectChan(no)
}
此时管道既可以读取也可以写入.select随机挑选一个case语句执行.任意一个case语
句执行结束后函数退出.
总结:
select的每个case语句只能操作一个管道.要么写入数据.要么读取数据.鉴于管道的
特性.如果管道中没有数据读操作就会阻塞.如果管道没有空余的缓冲区写入.则会写阻
塞.当select的全部case语句的管道均阻塞,在没有default语句的前提下就会进入阻
塞.直到任意一个管道解除阻塞.如果多个case语句都没有阻塞将随机挑选一个执行结
束.
1.2返回值:
select为Go语言的预留关键字并非函数.其可以在case语句中声明变量并为变量赋
值.看上去就像一个函数.
case语句读取管道的时候.最多可以给两个变量赋值.
func SelectChan(c chan string) {
send := "hello"
select {
case recv,ok := <-c:
if ok {
fmt.Println("recv:", recv)
}
case c <- send:
fmt.Println("send:", send)
}
}
case语句中管道的操作有两个返回条件.一个是成功独到的数据.第二个是管道已没有
数据且已关闭.
1.3default:
select语句中的default不能处理管道的读写操作.当select中所有语句都阻塞
时.default语句才能执行.
func SelectChan(c chan string) {
send := "hello"
select {
case recv, ok := <-c:
if ok {
fmt.Println("recv:", recv)
}
case c <- send:
fmt.Println("send:", send)
default:
fmt.Println("默认执行")
}
}
default实际上是特殊的case.它能出现在select中的任意位置.但每个select语句中
只能出现一次.
2.实现原理:
2.1数据结构:
源码位于runtime/select.go中.
type scase struct {
c *hchan // chan
elem unsafe.Pointer // data element
}
c: 操作的管道.由于每个scase只能存放一个管道.这里直接决定了每个case语句只
能处理一个管道.编译器在处理case语句的时候.如果case语句中没有管道操作(不能
处理成sacse对象)则会给出编译错误.
elem: 数据存放的地址.
在类型为caseRecv的case中.elem表示从管道中读出的数据的存放地址.
在类型为caseSend的case中.elem表示写入管道的数据的存放地址.
2.2实现逻辑:
Go在运行的时候提供了selectgo()方法处理select语句.
func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool)
selectgo函数会从一组case语句中挑选一个case.并返回命中的case的下标.对于类
型为caseRecv的case.还会返回是否从管道中读取到数据.
2.2.1参数:
编译器会将select语句中的case语句存储在一个数组中.selectgo的第一个参数
case0就是这个参数的地址.参数ncases表示case的个数(包括default).即case0数
组的长度.
selectgo的第二个参数order0为一个整形数组的地址.长度为case个数的二
倍.order0数组是case执行随机性的关键.
2.2.2返回值:
当所有的case都不可能就绪时.selectgo()会陷入永久阻塞.此时函数不会返回.一旦
select()返回.就说明某个case语句就绪了.第一个返回值代表case的编号.这个编号
与代码出现的顺序一致.
第二个返回值代表是否从管道读取了数据.该值只针对类型为caseRecv的case有意
义.第二个返回值为true时.仅代表从管道读取了数据.对于已关闭通道也是一样的.
selectgo()源码:
func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
gp := getg()
if debugSelect {
print("select: cas0=", cas0, "\n")
}
// NOTE: In order to maintain a lean stack size, the number of scases
// is capped at 65536.
cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
ncases := nsends + nrecvs
//case数组切片
scases := cas1[:ncases:ncases]
//切取order0前半段.用于保存随机顺序.
pollorder := order1[:ncases:ncases]
lockorder := order1[ncases:][:ncases:ncases]
// NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler.
// Even when raceenabled is true, there might be select
// statements in packages compiled without -race (e.g.,
// ensureSigM in runtime/signal_unix.go).
var pcs []uintptr
if raceenabled && pc0 != nil {
pc1 := (*[1 << 16]uintptr)(unsafe.Pointer(pc0))
pcs = pc1[:ncases:ncases]
}
casePC := func(casi int) uintptr {
if pcs == nil {
return 0
}
return pcs[casi]
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
}
// The compiler rewrites selects that statically have
// only 0 or 1 cases plus default into simpler constructs.
// The only way we can end up with such small sel.ncase
// values here is for a larger select in which most channels
// have been nilled out. The general code handles those
// cases correctly, and they are rare enough not to bother
// optimizing (and needing to test).
// generate permuted order
norder := 0
allSynctest := true
//过滤掉管道为nil的case.
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
}
if cas.c.synctest {
if getg().syncGroup == nil {
panic(plainError("select on synctest channel from outside bubble"))
}
} else {
allSynctest = false
}
if cas.c.timer != nil {
cas.c.timer.maybeRunChan()
}
j := cheaprandn(uint32(norder + 1))
pollorder[norder] = pollorder[j]
pollorder[j] = uint16(i)
norder++
}
pollorder = pollorder[:norder]
lockorder = lockorder[:norder]
waitReason := waitReasonSelect
if gp.syncGroup != nil && allSynctest {
// Every channel selected on is in a synctest bubble,
// so this goroutine will count as idle while selecting.
waitReason = waitReasonSynctestSelect
}
// 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
}
if debugSelect {
for i := 0; i+1 < len(lockorder); i++ {
if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
throw("select: broken sort")
}
}
}
// lock all the channels involved in the select
sellock(scases, lockorder)
var (
sg *sudog
c *hchan
k *scase
sglist *sudog
sgnext *sudog
qp unsafe.Pointer
nextp **sudog
)
// pass 1 - look for something already waiting
var casi int
var cas *scase
var caseSuccess bool
var caseReleaseTime int64 = -1
var recvOK bool
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
}
}
}
if !block {
selunlock(scases, lockorder)
casi = -1
goto retc
}
// pass 2 - enqueue on all chans
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)
}
if c.timer != nil {
blockTimerChan(c)
}
}
// 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.
gp.parkingOnChan.Store(true)
gopark(selparkcommit, nil, waitReason, traceBlockSelect, 1)
gp.activeStackChans = false
sellock(scases, lockorder)
gp.selectDone.Store(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 k.c.timer != nil {
unblockTimerChan(k.c)
}
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
}
if cas == nil {
throw("selectgo: bad wakeup")
}
c = cas.c
if debugSelect {
print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
}
if casi < nsends {
if !caseSuccess {
goto sclose
}
} else {
recvOK = caseSuccess
}
if raceenabled {
if casi < nsends {
raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
} else if cas.elem != nil {
raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
}
}
if msanenabled {
if casi < nsends {
msanread(cas.elem, c.elemtype.Size_)
} else if cas.elem != nil {
msanwrite(cas.elem, c.elemtype.Size_)
}
}
if asanenabled {
if casi < nsends {
asanread(cas.elem, c.elemtype.Size_)
} else if cas.elem != nil {
asanwrite(cas.elem, c.elemtype.Size_)
}
}
selunlock(scases, lockorder)
goto retc
bufrecv:
// can receive from buffer
if raceenabled {
if cas.elem != nil {
raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
}
racenotify(c, c.recvx, nil)
}
if msanenabled && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.Size_)
}
if asanenabled && cas.elem != nil {
asanwrite(cas.elem, c.elemtype.Size_)
}
recvOK = true
qp = chanbuf(c, c.recvx)
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, qp)
}
typedmemclr(c.elemtype, qp)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
selunlock(scases, lockorder)
goto retc
bufsend:
// can send to buffer
if raceenabled {
racenotify(c, c.sendx, nil)
raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
}
if msanenabled {
msanread(cas.elem, c.elemtype.Size_)
}
if asanenabled {
asanread(cas.elem, c.elemtype.Size_)
}
typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
c.qcount++
selunlock(scases, lockorder)
goto retc
recv:
// can receive from sleeping sender (sg)
recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
if debugSelect {
print("syncrecv: cas0=", cas0, " c=", c, "\n")
}
recvOK = true
goto retc
rclose:
// read at end of closed channel
selunlock(scases, lockorder)
recvOK = false
if cas.elem != nil {
typedmemclr(c.elemtype, cas.elem)
}
if raceenabled {
raceacquire(c.raceaddr())
}
goto retc
send:
// can send to a sleeping receiver (sg)
if raceenabled {
raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
}
if msanenabled {
msanread(cas.elem, c.elemtype.Size_)
}
if asanenabled {
asanread(cas.elem, c.elemtype.Size_)
}
send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
if debugSelect {
print("syncsend: cas0=", cas0, " c=", c, "\n")
}
goto retc
retc:
if caseReleaseTime > 0 {
blockevent(caseReleaseTime-t0, 1)
}
return casi, recvOK
sclose:
// send on closed channel
selunlock(scases, lockorder)
panic(plainError("send on closed channel"))
}
selectgo函数要点:
通过随机函数fastrandn()将原始的case顺序打乱.在遍历各个case时使用打乱后的
顺序就会表现出随机性.
循环遍历各个case时.如果发现某个case就绪(管道可读或可写).则直接跳出循环进行
管道操作并返回.
循环遍历各个case时.循环能正常结束.(没有跳转).说明所有case都没有就绪.如果有
default语句则命中default.
如果所有case都未命中且没有default.selectgo()将阻塞等待所有通道.任一管道就
绪后.都将开始新的循环.
***山水迢迢太匆忙.
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