chanel 使用与原理 二

有了上篇的基本了解，可以翻阅源码了

涉及的数据结构

// Go/src/runtime/chan.go

type hchan struct {
    qcount   uint           // total data in the queue
    dataqsiz uint           // size of the circular queue
    buf      unsafe.Pointer // points to an array of dataqsiz elements
    elemsize uint16
    closed   uint32
    elemtype *_type // element type
    sendx    uint   // send index
    recvx    uint   // receive index
    recvq    waitq  // list of recv waiters
    sendq    waitq  // list of send waiters

    // lock protects all fields in hchan, as well as several
    // fields in sudogs blocked on this channel.
    //
    // Do not change another G's status while holding this lock
    // (in particular, do not ready a G), as this can deadlock
    // with stack shrinking.
    lock mutex
}

type waitq struct {
    first *sudog
    last  *sudog
}


// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
type sudog struct {
    // The following fields are protected by the hchan.lock of the
    // channel this sudog is blocking on. shrinkstack depends on
    // this for sudogs involved in channel ops.

    g *g

    // isSelect indicates g is participating in a select, so
    // g.selectDone must be CAS'd to win the wake-up race.
    isSelect bool
    next     *sudog
    prev     *sudog
    elem     unsafe.Pointer // data element (may point to stack)

    // The following fields are never accessed concurrently.
    // For channels, waitlink is only accessed by g.
    // For semaphores, all fields (including the ones above)
    // are only accessed when holding a semaRoot lock.

    acquiretime int64
    releasetime int64
    ticket      uint32
    parent      *sudog // semaRoot binary tree
    waitlink    *sudog // g.waiting list or semaRoot
    waittail    *sudog // semaRoot
    c           *hchan // channel
}

makechan()

• 当 make(chan int,3) 带有设置缓存大小的参数，则会分配一段连续空间，buf 指向这段内存空间
• c.buf = add(unsafe.Pointer(c), hchanSize) 分配 hchanSize 大小的空间，其中常量 hchanSize hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))

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

    // compiler checks this but be safe.
    ···

    var c *hchan // 返回的是指针
    switch {
    case size == 0 || elem.size == 0:
        // Queue or element size is zero.
        c = (*hchan)(mallocgc(hchanSize, nil, true))
        // Race detector uses this location for synchronization.
        c.buf = c.raceaddr()  
    case elem.kind&kindNoPointers != 0:
        // Elements do not contain pointers.
        // Allocate hchan and buf in one call.
        c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil, true))
        c.buf = add(unsafe.Pointer(c), hchanSize)   // <============
    default:
        // Elements contain pointers.
        c = new(hchan)
        c.buf = mallocgc(uintptr(size)*elem.size, elem, true)   // <==============
    }
    ···
    return c
}

chansend()

我们截取不同情况的代码段。

如果 chanel 为 nil

如果 chanel 为空，即没有用 make 分配内存，那么会调用 gopark 方法

if c == nil {
        if !block {
            return false
        }
        gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
        throw("unreachable")
    }

而 gopark 方法会将当前的 goroutine 休眠，然后回调通过参数传来的 unlockf 方法。注意看回上面的代码，调用 gopark 方法时传递的 unlockf 参数为 nil，所以会一直休眠。

// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
// Reason explains why the goroutine has been parked.
// It is displayed in stack traces and heap dumps.
// Reasons should be unique and descriptive.
// Do not re-use reasons, add new ones.
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, traceskip int) {
    if reason != waitReasonSleep {
        checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
    }
    mp := acquirem()
    gp := mp.curg
    status := readgstatus(gp)
    if status != _Grunning && status != _Gscanrunning {
        throw("gopark: bad g status")
    }
    mp.waitlock = lock
    mp.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))
    gp.waitreason = reason
    mp.waittraceev = traceEv
    mp.waittraceskip = traceskip
    releasem(mp)
    // can't do anything that might move the G between Ms here.
    mcall(park_m)
}

这时， Go 语言启动的时候会有一个 goroutine sysmon 一直检测系统的运行情况，其中有一个方法 checkdead()，当检测到所有 goroutine 都处于休眠，即死锁，便抛出错误。

//  /Go/src/runtime/proc.go

func checkdead() {
    ...
    throw("all goroutines are asleep - deadlock!")  
}

如果 chanel 已经被关闭了

直接引发 panic：

lock(&c.lock)

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

如果能发送数据

发送数据还分三种情况：

1 当前 hchan 的 recvq 接收队列上已经有 goroutine 阻塞

lock(&c.lock)
    ···
    if sg := c.recvq.dequeue(); sg != nil {
        // 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
    }

send 方法判断到接收方 sudog 的 elem 字段存有对应的内存空间地址值的话，调用 sendDirect 方法。这里啰嗦一下，sudog 的 elem 是在 func chanrecv 方法中赋值的，将接收方 goroutine 用来接收数据的栈空间地址赋值给 elem。

// 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 sg.elem != nil {
        sendDirect(c.elemtype, sg, ep)
        sg.elem = nil
    }
    gp := sg.g
    unlockf()
    gp.param = unsafe.Pointer(sg)
    if sg.releasetime != 0 {
        sg.releasetime = cputicks()
    }
    goready(gp, skip+1)
}

sendDirect 方法中 memmove 方法直接拷贝 t.size 个字节到目的内存空间。

func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
    // src is on our stack, dst is a slot on another stack.

    // 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)
    // No need for cgo write barrier checks because dst is always
    // Go memory.
    memmove(dst, src, t.size)
}

2 当前 hchan.buf 还有可用空间：将数据放到 buffer 里面。

if c.qcount < c.dataqsiz {
        // Space is available in the channel buffer. Enqueue the element to send.
        qp := chanbuf(c, c.sendx)
        if raceenabled {
            raceacquire(qp)
            racerelease(qp)
        }
        typedmemmove(c.elemtype, qp, ep)
        c.sendx++
        if c.sendx == c.dataqsiz {
            c.sendx = 0
        }
        c.qcount++
        unlock(&c.lock)
        return true
    }

3 当前 hchan.buf 已满：阻塞当前 goroutine

// Block on the channel. Some receiver will complete our operation for us.
    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
    mysg.g = gp
    mysg.isSelect = false
    mysg.c = c
    gp.waiting = mysg
    gp.param = nil
    c.sendq.enqueue(mysg)
    goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)

goparkunlock 方法将当前 goroutine 休眠，并且释放锁资源

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling goready(gp).
func goparkunlock(lock *mutex, reason waitReason, traceEv byte, traceskip int) {
    gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
}