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condchan.go
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condchan.go
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// CondChan is a sync.Cond with the ability to wait in select statement.
package condchan
import (
"sync"
"sync/atomic"
"unsafe"
)
// CondChan 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 CondChan struct {
L sync.Locker
ch chan struct{}
chL sync.RWMutex
noCopy noCopy
checker copyChecker
}
type selectFn func(<-chan struct{})
// New returns a new CondChan with Locker l.
func New(l sync.Locker) *CondChan {
return &CondChan{
L: l,
ch: make(chan struct{}),
chL: sync.RWMutex{},
}
}
// Select atomically unlocks cc.L and executes fn.
// After later resuming execution, Wait locks cc.L before returning.
//
// fn is executed passing channel in to it.
// Passed channel will signal by emitting struct{} or by closing.
// Inside fn should be select statement using passed channel together with the other channels that signals execution continuation.
func (cc *CondChan) Select(fn selectFn) {
cc.checker.check()
cc.chL.RLock()
ch := cc.ch
cc.chL.RUnlock()
cc.L.Unlock()
fn(ch)
cc.L.Lock()
}
// Wait atomically unlocks cc.L and suspends execution of the calling goroutine.
// After later resuming execution, Wait locks cc.L before returning.
// Unlike in other systems, Wait cannot return unless awoken by Broadcast or Signal.
func (cc *CondChan) Wait() {
cc.checker.check()
cc.chL.RLock()
ch := cc.ch
cc.chL.RUnlock()
cc.L.Unlock()
<-ch
cc.L.Lock()
}
// Signal wakes one goroutine waiting on cc, if there is any.
// It is allowed but not required for the caller to hold cc.L during the call.
func (cc *CondChan) Signal() {
cc.checker.check()
cc.chL.RLock()
select {
case cc.ch <- struct{}{}:
default:
}
cc.chL.RUnlock()
}
// Broadcast wakes all goroutines waiting on cc.
// It is allowed but not required for the caller to hold cc.L during the call.
func (cc *CondChan) Broadcast() {
cc.checker.check()
cc.chL.Lock()
close(cc.ch)
cc.ch = make(chan struct{})
cc.chL.Unlock()
}
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Below code is borrowed from sync.cond ///////////////////////////////////////////////////////////////////////////////
// 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() {}
func (*noCopy) Unlock() {}