标签：缓存 false chanrecv chansend send closed sg channel
前言
本篇聚集select 2个case(1个send/recv case、1个default case)场景时send、recv的具体处理。
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chansend
select {
case c <- v:
... foo
default:
... bar
}
其底层对应func为selectnbsend。
selectnbsend
// compiler implements
//
// select {
// case c <- v:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if selectnbsend(c, v) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
return chansend(c, elem, false, getcallerpc())
}
从注释中我们可以知道，select 2个case(1个send/recv case、1个default case)会被编译成if...else的形式进行处理。
注意：此时block默认为false。
chansend具体实现
以下源码中省略了block为true的逻辑及部分debug及race的逻辑。
/*
* generic single channel send/recv
* If block is not nil,
* then the protocol will not
* sleep but return if it could
* not complete.
*
* sleep can wake up with g.param == nil
* when a channel involved in the sleep has
* been closed. it is easiest to loop and re-run
* the operation; we'll see that it's now closed.
*/
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
if c == nil {
if !block { // 对于select chan，向nil的chan发送消息会直接返回false
return false
}
...
}
...
// Fast path: check for failed non-blocking operation without acquiring the lock.
//
// After observing that the channel is not closed, we observe that the channel is
// not ready for sending. Each of these observations is a single word-sized read
// (first c.closed and second full()).
// Because a closed channel cannot transition from 'ready for sending' to
// 'not ready for sending', even if the channel is closed between the two observations,
// they imply a moment between the two when the channel was both not yet closed
// and not ready for sending. We behave as if we observed the channel at that moment,
// and report that the send cannot proceed.
//
// It is okay if the reads are reordered here: if we observe that the channel is not
// ready for sending and then observe that it is not closed, that implies that the
// channel wasn't closed during the first observation. However, nothing here
// guarantees forward progress. We rely on the side effects of lock release in
// chanrecv() and closechan() to update this thread's view of c.closed and full().
if !block && c.closed == 0 && full(c) { // 对于select chan，未close时，缓存已满直接返回false
return false
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
}
lock(&c.lock) // 获取锁
if c.closed != 0 { // 如果chan已closed，则释放锁并panic
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
if sg := c.recvq.dequeue(); sg != nil {// 如果接收等待队列中有等待的接收者，直接发送到接收者。
// Found a waiting receiver. We pass the value we want to send
// directly to the receiver, bypassing the channel buffer (if any).
send(c, sg, ep, func() { unlock(&c.lock) }, 3)
return true
}
// 以下是没有接收者的处理
if c.qcount < c.dataqsiz {// 如果当前数据量少于缓存，即缓存还有剩余，存入发送缓存队列
// Space is available in the channel buffer. Enqueue the element to send.
qp := chanbuf(c, c.sendx)// 获取数据存入缓存的位置
if raceenabled {
racenotify(c, c.sendx, nil)
}
typedmemmove(c.elemtype, qp, ep)// 将数据存入缓存的指定位置
c.sendx++// 指向下一个位置
if c.sendx == c.dataqsiz {// 如果已达到队列大小，说明已满，重新指向开头位置
c.sendx = 0
}
c.qcount++ // 数据量+1
unlock(&c.lock)// 释放锁
return true // 发送成功
}
// 当缓存已满时
if !block {// select channel直接返回失败
unlock(&c.lock)
return false
}
...
return true // 发送成功
}
// send processes a send operation on an empty channel c.
// The value ep sent by the sender is copied to the receiver sg.
// The receiver is then woken up to go on its merry way.
// Channel c must be empty and locked. send unlocks c with unlockf.
// sg must already be dequeued from c.
// ep must be non-nil and point to the heap or the caller's stack.
func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
if raceenabled {
if c.dataqsiz == 0 {
racesync(c, sg)
} else {
// Pretend we go through the buffer, even though
// we copy directly. Note that we need to increment
// the head/tail locations only when raceenabled.
racenotify(c, c.recvx, nil)
racenotify(c, c.recvx, sg)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
}
if sg.elem != nil {// sg已存储发送内容
sendDirect(c.elemtype, sg, ep)// 直接将内存发送至接收者，实际是将数据拷贝至接收者
sg.elem = nil // 清空
}
gp := sg.g // 获取接收goroutine gp
unlockf()
gp.param = unsafe.Pointer(sg) // 设置唤醒参数为等待列表中的sg
sg.success = true // 设置sg的状态为成功
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, skip+1) // 接收goroutine准备运行
}
// Sends and receives on unbuffered or empty-buffered channels are the
// only operations where one running goroutine writes to the stack of
// another running goroutine. The GC assumes that stack writes only
// happen when the goroutine is running and are only done by that
// goroutine. Using a write barrier is sufficient to make up for
// violating that assumption, but the write barrier has to work.
// typedmemmove will call bulkBarrierPreWrite, but the target bytes
// are not in the heap, so that will not help. We arrange to call
// memmove and typeBitsBulkBarrier instead.
func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
// src is on our stack, dst is a slot on another stack. 跨栈拷贝
// Once we read sg.elem out of sg, it will no longer
// be updated if the destination's stack gets copied (shrunk).
// So make sure that no preemption points can happen between read & use.
dst := sg.elem
typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)// 拷贝数据前添加写屏障，确保在读取和使用之间不会发生抢占点。
// No need for cgo write barrier checks because dst is always
// Go memory.
memmove(dst, src, t.size) //将数据拷贝至dst，即sg.elem中
}
此场景下使用send时，需要注意：
block为false，不会发生阻塞- 向
nil的chansend，直接返回false - 若chan
没有closed且缓存已满，直接返回false - 向
closed的chansend会发送panic - 若已经有等待的接收者，会直接发送至接收者（直接拷贝数据到接收者）
- 若缓存未满，则继续缓存
- 若缓存已满，直接返回false
recv/close操作均可以唤醒sendgoroutine，区别在于close后当前goroutine会panic，recv后返回true
chanrecv
// recv1
select {
case v = <-c:
... foo
default:
... bar
}
// recv2
select {
case v, ok = <-c:
... foo
default:
... bar
}
以上两种场景recv处理方式一致，recv1、recv2分别对应底层func selectnbrecv、 selectnbrecv2。
selectnbrecv/selectnbrecv2
// compiler implements
//
// select {
// case v = <-c:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if selectnbrecv(&v, c) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
selected, _ = chanrecv(c, elem, false)
return
}
// compiler implements
//
// select {
// case v, ok = <-c:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if c != nil && selectnbrecv2(&v, &ok, c) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbrecv2(elem unsafe.Pointer, received *bool, c *hchan) (selected bool) {
// TODO(khr): just return 2 values from this function, now that it is in Go.
selected, *received = chanrecv(c, elem, false)
return
}å
selectnbrecv、selectnbrecv2区别在于是否有received，注意block的默认值为false。
chanrecv具体实现
以下源码中省略了block为true的逻辑及部分debug及race的逻辑。
// chanrecv receives on channel c and writes the received data to ep.
// ep may be nil, in which case received data is ignored.
// If block == false and no elements are available, returns (false, false).
// Otherwise, if c is closed, zeros *ep and returns (true, false).
// Otherwise, fills in *ep with an element and returns (true, true).
// A non-nil ep must point to the heap or the caller's stack.
func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
// raceenabled: don't need to check ep, as it is always on the stack
// or is new memory allocated by reflect.
if debugChan {
print("chanrecv: chan=", c, "\n")
}
if c == nil {/
if !block {// 对于select chan，向nil的chan接收消息会直接返回false
return
}
...
}
// Fast path: check for failed non-blocking operation without acquiring the lock.
if !block && empty(c) {// 对于select chan，chan缓存为空时
// After observing that the channel is not ready for receiving, we observe whether the
// channel is closed.
//
// Reordering of these checks could lead to incorrect behavior when racing with a close.
// For example, if the channel was open and not empty, was closed, and then drained,
// reordered reads could incorrectly indicate "open and empty". To prevent reordering,
// we use atomic loads for both checks, and rely on emptying and closing to happen in
// separate critical sections under the same lock. This assumption fails when closing
// an unbuffered channel with a blocked send, but that is an error condition anyway.
if atomic.Load(&c.closed) == 0 { // 如果chan已经close，直接返回
// Because a channel cannot be reopened, the later observation of the channel
// being not closed implies that it was also not closed at the moment of the
// first observation. We behave as if we observed the channel at that moment
// and report that the receive cannot proceed.
return
}
// The channel is irreversibly closed. Re-check whether the channel has any pending data
// to receive, which could have arrived between the empty and closed checks above.
// Sequential consistency is also required here, when racing with such a send.
if empty(c) {// 再次检查缓存是否为空
// The channel is irreversibly closed and empty.
if raceenabled {
raceacquire(c.raceaddr())
}
if ep != nil {
typedmemclr(c.elemtype, ep) //清空ep中的数据
}
return true, false
}
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
}
lock(&c.lock)
if c.closed != 0 && c.qcount == 0 {// 再次检查
if raceenabled {
raceacquire(c.raceaddr())
}
unlock(&c.lock)
if ep != nil {
typedmemclr(c.elemtype, ep)
}
return true, false
}
if sg := c.sendq.dequeue(); sg != nil {// 如果发送队列中有等待的发送者，直接从发送者接收数据
// Found a waiting sender. If buffer is size 0, receive value
// directly from sender. Otherwise, receive from head of queue
// and add sender's value to the tail of the queue (both map to
// the same buffer slot because the queue is full).
recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
return true, true
}
if c.qcount > 0 {// 如果有缓存
// Receive directly from queue
qp := chanbuf(c, c.recvx)// 获取缓存的位置
if raceenabled {
racenotify(c, c.recvx, nil)
}
if ep != nil {// 将数据拷贝至ep中
typedmemmove(c.elemtype, ep, qp)
}
typedmemclr(c.elemtype, qp)
c.recvx++ // 指向下一个位置
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount-- // 减少缓存数量
unlock(&c.lock)
return true, true
}
if !block {// 对于select channel操作，无缓存时直接返回false
unlock(&c.lock)
return false, false
}
return true, success
...
}
func recvDirect(t *_type, sg *sudog, dst unsafe.Pointer) {
// dst is on our stack or the heap, src is on another stack.
// The channel is locked, so src will not move during this
// operation.
src := sg.elem
typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
memmove(dst, src, t.size) // 拷贝数据
}
此场景下使用recv时，需要注意：
block为false- 向
nil的chanrecv会直接返回selected=false,received=false - 向
closed且缓存为空的chanrecv会获取到chan类型的零值 - 若已经有等待的发送者，
- 若是无缓存
chan，会直接从发送者接收数据（从发送者直接拷贝数据到接收者） - 否则，取待接收位置的缓存，将发送数据存储至待发送位置的缓存
- 若是无缓存
- 若有缓存，则取待接收位置的缓存
- 若没有缓存，直接返回selected=false,received=false
send/close操作均可以唤醒send goroutine，区别在于:close后，若有缓存，recv会返回缓存，若没有、缓存，recv会返回零值send后返回true,true
总结
最后以一张图总结2个case(1个send/recv case、1个default case)场景下使用send/recv的处理逻辑:
与单独使用(包含select单个send/recv)的最大区别在于不会造成goroutine阻塞，不满足条件时，直接返回结果。
标签：缓存,false,chanrecv,chansend,send,closed,sg,channel 来源： https://blog.csdn.net/xz_studying/article/details/117194406
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