Source: cond.go in package sync


package sync

import (
	"sync/atomic"
	"unsafe"
)

Cond implements a condition variable, a rendezvous point for goroutines waiting for or announcing the occurrence of an event. Each Cond has an associated Locker L (often a *Mutex or *RWMutex), which must be held when changing the condition and when calling the Wait method. A Cond must not be copied after first use.

type Cond struct {
	noCopy noCopy

L is held while observing or changing the condition

	L Locker

	notify  notifyList
	checker copyChecker
}

NewCond returns a new Cond with Locker l.

func NewCond(l Locker) *Cond {
	return &Cond{L: l}
}

Wait atomically unlocks c.L and suspends execution of the calling goroutine. After later resuming execution, Wait locks c.L before returning. Unlike in other systems, Wait cannot return unless awoken by Broadcast or Signal. Because c.L is not locked when Wait first resumes, the caller typically cannot assume that the condition is true when Wait returns. Instead, the caller should Wait in a loop: c.L.Lock() for !condition() { c.Wait() } ... make use of condition ... c.L.Unlock()

func (c *Cond) Wait() {
	c.checker.check()
	t := runtime_notifyListAdd(&c.notify)
	c.L.Unlock()
	runtime_notifyListWait(&c.notify, t)
	c.L.Lock()
}

Signal wakes one goroutine waiting on c, if there is any. It is allowed but not required for the caller to hold c.L during the call.

func (c *Cond) Signal() {
	c.checker.check()
	runtime_notifyListNotifyOne(&c.notify)
}

Broadcast wakes all goroutines waiting on c. It is allowed but not required for the caller to hold c.L during the call.

func (c *Cond) Broadcast() {
	c.checker.check()
	runtime_notifyListNotifyAll(&c.notify)
}

copyChecker holds back pointer to itself to detect object copying.

type copyChecker uintptr

func (c *copyChecker) check() {
	if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
		!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
		uintptr(*c) != uintptr(unsafe.Pointer(c)) {
		panic("sync.Cond is copied")
	}
}

noCopy may be embedded into structs which must not be copied after the first use. See https://golang.org/issues/8005#issuecomment-190753527 for details.

type noCopy struct{}

Lock is a no-op used by -copylocks checker from `go vet`.

func (*noCopy) Lock()   {}