package main
import "fmt"
type T int
func IsClosed(ch <-chan T) bool {
select {
case <-ch:
return true
default:
}
return false
}
func main() {
c := make(chan T)
fmt.Println(IsClosed(c)) // false
close(c)
fmt.Println(IsClosed(c)) // true
}
closed function to check whether or not a channel has been closed, its usefulness would be very limited, just like the built-in len function for checking the current number of values stored in the value buffer of a channel. The reason is that the status of the checked channel may have changed just after a call to such functions returns, so that the returned value has already not been able to reflect the latest status of the just checked channel. Although it is okay to stop sending values to a channel ch if the call closed(ch) returns true, it is not safe to close the channel or continue sending values to the channel if the call closed(ch) returns false.
T).
func SafeClose(ch chan T) (justClosed bool) {
defer func() {
if recover() != nil {
// The return result can be altered
// in a defer function call.
justClosed = false
}
}()
// assume ch != nil here.
close(ch) // panic if ch is closed
return true // <=> justClosed = true; return
}
func SafeSend(ch chan T, value T) (closed bool) {
defer func() {
if recover() != nil {
closed = true
}
}()
ch <- value // panic if ch is closed
return false // <=> closed = false; return
}
sync.Once to close channels:
type MyChannel struct {
C chan T
once sync.Once
}
func NewMyChannel() *MyChannel {
return &MyChannel{C: make(chan T)}
}
func (mc *MyChannel) SafeClose() {
mc.once.Do(func() {
close(mc.C)
})
}
sync.Mutex to avoid closing a channel multiple times:
type MyChannel struct {
C chan T
closed bool
mutex sync.Mutex
}
func NewMyChannel() *MyChannel {
return &MyChannel{C: make(chan T)}
}
func (mc *MyChannel) SafeClose() {
mc.mutex.Lock()
defer mc.mutex.Unlock()
if !mc.closed {
close(mc.C)
mc.closed = true
}
}
func (mc *MyChannel) IsClosed() bool {
mc.mutex.Lock()
defer mc.mutex.Unlock()
return mc.closed
}
SafeClose function is called concurrently with a channel send operation to the same channel, data races might happen (though such data races generally don't do any harm).
SafeSend function is that its calls can't be used as send operations that follow the case keyword in select blocks. The other drawback of the above SafeSend and SafeClose functions is that many people, including me, would think the above solutions by using panic/recover and sync package are not graceful. Following, some pure-channel solutions without using panic/recover and sync package will be introduced, for all kinds of situations.
sync.WaitGroup is used to make the examples complete. It may be not always essential to use it in real practice.)
package main
import (
"time"
"math/rand"
"sync"
"log"
)
func main() {
rand.Seed(time.Now().UnixNano()) // needed before Go 1.20
log.SetFlags(0)
// ...
const Max = 100000
const NumReceivers = 100
wgReceivers := sync.WaitGroup{}
wgReceivers.Add(NumReceivers)
// ...
dataCh := make(chan int)
// the sender
go func() {
for {
if value := rand.Intn(Max); value == 0 {
// The only sender can close the
// channel at any time safely.
close(dataCh)
return
} else {
dataCh <- value
}
}
}()
// receivers
for i := 0; i < NumReceivers; i++ {
go func() {
defer wgReceivers.Done()
// Receive values until dataCh is
// closed and the value buffer queue
// of dataCh becomes empty.
for value := range dataCh {
log.Println(value)
}
}()
}
wgReceivers.Wait()
}
package main
import (
"time"
"math/rand"
"sync"
"log"
)
func main() {
rand.Seed(time.Now().UnixNano()) // needed before Go 1.20
log.SetFlags(0)
// ...
const Max = 100000
const NumSenders = 1000
wgReceivers := sync.WaitGroup{}
wgReceivers.Add(1)
// ...
dataCh := make(chan int)
stopCh := make(chan struct{})
// stopCh is an additional signal channel.
// Its sender is the receiver of channel
// dataCh, and its receivers are the
// senders of channel dataCh.
// senders
for i := 0; i < NumSenders; i++ {
go func() {
for {
// The try-receive operation is to try
// to exit the goroutine as early as
// possible. For this specified example,
// it is not essential.
select {
case <- stopCh:
return
default:
}
// Even if stopCh is closed, the first
// branch in the second select may be
// still not selected for some loops if
// the send to dataCh is also unblocked.
// But this is acceptable for this
// example, so the first select block
// above can be omitted.
select {
case <- stopCh:
return
case dataCh <- rand.Intn(Max):
}
}
}()
}
// the receiver
go func() {
defer wgReceivers.Done()
for value := range dataCh {
if value == Max-1 {
// The receiver of channel dataCh is
// also the sender of stopCh. It is
// safe to close the stop channel here.
close(stopCh)
return
}
log.Println(value)
}
}()
// ...
wgReceivers.Wait()
}
dataCh is never closed. Yes, channels don't have to be closed. A channel will be eventually garbage collected if no goroutines reference it any more, whether it is closed or not. So the gracefulness of closing a channel here is not to close the channel.
package main
import (
"time"
"math/rand"
"sync"
"log"
"strconv"
)
func main() {
rand.Seed(time.Now().UnixNano()) // needed before Go 1.20
log.SetFlags(0)
// ...
const Max = 100000
const NumReceivers = 10
const NumSenders = 1000
wgReceivers := sync.WaitGroup{}
wgReceivers.Add(NumReceivers)
// ...
dataCh := make(chan int)
stopCh := make(chan struct{})
// stopCh is an additional signal channel.
// Its sender is the moderator goroutine shown
// below, and its receivers are all senders
// and receivers of dataCh.
toStop := make(chan string, 1)
// The channel toStop is used to notify the
// moderator to close the additional signal
// channel (stopCh). Its senders are any senders
// and receivers of dataCh, and its receiver is
// the moderator goroutine shown below.
// It must be a buffered channel.
var stoppedBy string
// moderator
go func() {
stoppedBy = <-toStop
close(stopCh)
}()
// senders
for i := 0; i < NumSenders; i++ {
go func(id string) {
for {
value := rand.Intn(Max)
if value == 0 {
// Here, the try-send operation is
// to notify the moderator to close
// the additional signal channel.
select {
case toStop <- "sender#" + id:
default:
}
return
}
// The try-receive operation here is to
// try to exit the sender goroutine as
// early as possible. Try-receive and
// try-send select blocks are specially
// optimized by the standard Go
// compiler, so they are very efficient.
select {
case <- stopCh:
return
default:
}
// Even if stopCh is closed, the first
// branch in this select block might be
// still not selected for some loops
// (and for ever in theory) if the send
// to dataCh is also non-blocking. If
// this is unacceptable, then the above
// try-receive operation is essential.
select {
case <- stopCh:
return
case dataCh <- value:
}
}
}(strconv.Itoa(i))
}
// receivers
for i := 0; i < NumReceivers; i++ {
go func(id string) {
defer wgReceivers.Done()
for {
// Same as the sender goroutine, the
// try-receive operation here is to
// try to exit the receiver goroutine
// as early as possible.
select {
case <- stopCh:
return
default:
}
// Even if stopCh is closed, the first
// branch in this select block might be
// still not selected for some loops
// (and forever in theory) if the receive
// from dataCh is also non-blocking. If
// this is not acceptable, then the above
// try-receive operation is essential.
select {
case <- stopCh:
return
case value := <-dataCh:
if value == Max-1 {
// Here, the same trick is
// used to notify the moderator
// to close the additional
// signal channel.
select {
case toStop <- "receiver#" + id:
default:
}
return
}
log.Println(value)
}
}
}(strconv.Itoa(i))
}
// ...
wgReceivers.Wait()
log.Println("stopped by", stoppedBy)
}
toStop is one. This is to avoid the first notification is missed when it is sent before the moderator goroutine gets ready to receive notification from toStop.
toStop channel as the sum number of senders and receivers, then we don't need a try-send select block to notify the moderator.
...
toStop := make(chan string, NumReceivers + NumSenders)
...
value := rand.Intn(Max)
if value == 0 {
toStop <- "sender#" + id
return
}
...
if value == Max-1 {
toStop <- "receiver#" + id
return
}
...
package main
import (
"time"
"math/rand"
"sync"
"log"
)
func main() {
rand.Seed(time.Now().UnixNano()) // needed before Go 1.20
log.SetFlags(0)
// ...
const Max = 100000
const NumReceivers = 100
const NumThirdParties = 15
wgReceivers := sync.WaitGroup{}
wgReceivers.Add(NumReceivers)
// ...
dataCh := make(chan int)
closing := make(chan struct{}) // signal channel
closed := make(chan struct{})
// The stop function can be called
// multiple times safely.
stop := func() {
select {
case closing<-struct{}{}:
<-closed
case <-closed:
}
}
// some third-party goroutines
for i := 0; i < NumThirdParties; i++ {
go func() {
r := 1 + rand.Intn(3)
time.Sleep(time.Duration(r) * time.Second)
stop()
}()
}
// the sender
go func() {
defer func() {
close(closed)
close(dataCh)
}()
for {
select{
case <-closing: return
default:
}
select{
case <-closing: return
case dataCh <- rand.Intn(Max):
}
}
}()
// receivers
for i := 0; i < NumReceivers; i++ {
go func() {
defer wgReceivers.Done()
for value := range dataCh {
log.Println(value)
}
}()
}
wgReceivers.Wait()
}
stop function is learned from a comment made by Roger Peppe.
package main
import (
"time"
"math/rand"
"sync"
"log"
"strconv"
)
func main() {
rand.Seed(time.Now().UnixNano()) // needed before Go 1.20
log.SetFlags(0)
// ...
const Max = 1000000
const NumReceivers = 10
const NumSenders = 1000
const NumThirdParties = 15
wgReceivers := sync.WaitGroup{}
wgReceivers.Add(NumReceivers)
// ...
dataCh := make(chan int) // will be closed
middleCh := make(chan int) // will never be closed
closing := make(chan string) // signal channel
closed := make(chan struct{})
var stoppedBy string
// The stop function can be called
// multiple times safely.
stop := func(by string) {
select {
case closing <- by:
<-closed
case <-closed:
}
}
// the middle layer
go func() {
exit := func(v int, needSend bool) {
close(closed)
if needSend {
dataCh <- v
}
close(dataCh)
}
for {
select {
case stoppedBy = <-closing:
exit(0, false)
return
case v := <- middleCh:
select {
case stoppedBy = <-closing:
exit(v, true)
return
case dataCh <- v:
}
}
}
}()
// some third-party goroutines
for i := 0; i < NumThirdParties; i++ {
go func(id string) {
r := 1 + rand.Intn(3)
time.Sleep(time.Duration(r) * time.Second)
stop("3rd-party#" + id)
}(strconv.Itoa(i))
}
// senders
for i := 0; i < NumSenders; i++ {
go func(id string) {
for {
value := rand.Intn(Max)
if value == 0 {
stop("sender#" + id)
return
}
select {
case <- closed:
return
default:
}
select {
case <- closed:
return
case middleCh <- value:
}
}
}(strconv.Itoa(i))
}
// receivers
for range [NumReceivers]struct{}{} {
go func() {
defer wgReceivers.Done()
for value := range dataCh {
log.Println(value)
}
}()
}
// ...
wgReceivers.Wait()
log.Println("stopped by", stoppedBy)
}
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reflect standard package.sync standard package.sync/atomic standard package.