How to properly set/modify a value based on events/conditions that happen after process has started, while dealing with Goroutines without creating a race condition.
For example, the following "works (is buggy)" and output is:
ping, foo=true
ping, foo=false
ping, foo=true
ping, foo=true
ping, foo=true
https://play.golang.org/p/Y3FafF-nBc
package main
import "fmt"
type test struct {
ch chan string
foo bool
}
func (t *test) run() {
for {
select {
case v := <-t.ch:
fmt.Printf("%+v, foo=%+v
", v, t.foo)
t.foo = false
default:
}
}
}
func (t *test) Ping() {
t.ch <- "ping"
}
func New() *test {
t := &test{
ch: make(chan string),
}
go t.run()
return t
}
func main() {
t := New()
for i := 0; i <= 10; i++ {
if t.foo {
t.Ping()
}
if i%3 == 0 {
t.foo = true
}
}
}
But if compiled or run using the -race option, I get this output:
$ go run -race main.go
ping, foo=true
==================
WARNING: DATA RACE
Write at 0x00c4200761b8 by goroutine 6:
main.(*test).run()
/main.go:16 +0x1fb
Previous read at 0x00c4200761b8 by main goroutine:
main.main()
/main.go:37 +0x5e
Goroutine 6 (running) created at:
main.New()
/main.go:30 +0xd0
main.main()
/main.go:35 +0x33
==================
ping, foo=false
ping, foo=true
ping, foo=true
ping, foo=true
Found 1 data race(s)
exit status 66
Therefore, I would like to know what concurrency pattern could I use for being available to change the value of foo outside the gorutine and also inside the gorutine without creating a race condition.
You have some options:
atomic.Value : Sample (1)sync.RWMutex : Sample (3)sync/atomic : Sample (6)Also see: Use a sync.Mutex or a channel?
1- You may use atomic.Value:
A Value provides an atomic load and store of a consistently typed value. Values can be created as part of other data structures. The zero value for a Value returns nil from Load. Once Store has been called, a Value must not be copied.
A Value must not be copied after first use.
Like this working sample:
// to test the panic use go build -race
package main
import (
"fmt"
"sync/atomic"
)
type test struct {
ch chan string
atomic.Value
}
func (t *test) run() {
for {
select {
case v := <-t.ch:
fmt.Printf("%+v, foo=%+v
", v, t.Load())
t.Store(false)
default:
}
}
}
func (self *test) Ping() {
self.ch <- "ping"
}
func New() *test {
t := &test{
ch: make(chan string),
}
t.Store(false)
go t.run()
return t
}
func main() {
t := New()
for i := 0; i <= 10; i++ {
if x, _ := t.Load().(bool); x {
t.Ping()
}
// time.Sleep(time.Second)
if i%3 == 0 {
t.Store(true)
}
}
}
output with go build -race:
ping, foo=true
ping, foo=false
ping, foo=false
ping, foo=false
ping, foo=false
2- A little improvment to func (t *test) run():
func (t *test) run() {
for v := range t.ch {
fmt.Printf("%+v, foo=%+v
", v, t.Load())
t.Store(false)
}
}
3- You may use sync.RWMutex and sync.WaitGroup, like this working sample:
// to test the panic use go build -race
package main
import (
"fmt"
"sync"
)
type test struct {
ch chan string
foo bool
sync.RWMutex
sync.WaitGroup
}
func (t *test) run() {
for v := range t.ch {
t.Lock()
r := t.foo
t.foo = false
t.Unlock()
fmt.Printf("%+v, foo=%+v
", v, r)
}
t.Done()
}
func (self *test) Ping() {
self.ch <- "ping"
}
func New() *test {
t := &test{ch: make(chan string)}
t.Add(1)
go t.run()
return t
}
func main() {
t := New()
for i := 0; i <= 10; i++ {
t.RLock()
r := t.foo
t.RUnlock()
if r {
t.Ping()
}
// time.Sleep(time.Second)
if i%3 == 0 {
t.Lock()
t.foo = true
t.Unlock()
}
}
close(t.ch)
t.Wait()
}
output with go build -race:
ping, foo=true
ping, foo=true
ping, foo=false
ping, foo=true
ping, foo=false
ping, foo=true
4- So let's follow this approach https://talks.golang.org/2013/bestpractices.slide#29:
Original Code:
package main
import (
"fmt"
"time"
)
type Server struct{ quit chan bool }
func NewServer() *Server {
s := &Server{make(chan bool)}
go s.run()
return s
}
func (s *Server) run() {
for {
select {
case <-s.quit:
fmt.Println("finishing task")
time.Sleep(time.Second)
fmt.Println("task done")
s.quit <- true
return
case <-time.After(time.Second):
fmt.Println("running task")
}
}
}
func (s *Server) Stop() {
fmt.Println("server stopping")
s.quit <- true
<-s.quit
fmt.Println("server stopped")
}
func main() {
s := NewServer()
time.Sleep(2 * time.Second)
s.Stop()
}
5- Let's simplify it:
package main
import (
"fmt"
"time"
)
var quit = make(chan bool)
func main() {
go run()
time.Sleep(2 * time.Second)
fmt.Println("server stopping")
quit <- true // signal to quit
<-quit // wait for quit signal
fmt.Println("server stopped")
}
func run() {
for {
select {
case <-quit:
fmt.Println("finishing task")
time.Sleep(time.Second)
fmt.Println("task done")
quit <- true
return
case <-time.After(time.Second):
fmt.Println("running task")
}
}
}
output:
running task
running task
server stopping
finishing task
task done
server stopped
6- Simplified version of your sample:
// to test the panic use go build -race
package main
import "fmt"
import "sync/atomic"
var ch = make(chan string)
var state int32
func main() {
go run()
for i := 0; i <= 10; i++ {
if atomic.LoadInt32(&state) == 1 {
ch <- "ping"
}
if i%3 == 0 {
atomic.StoreInt32(&state, 1)
}
}
}
func run() {
for v := range ch {
fmt.Printf("%+v, state=%+v
", v, atomic.LoadInt32(&state))
atomic.StoreInt32(&state, 0)
}
}
output:
ping, state=1
ping, state=0
ping, state=1
ping, state=0
ping, state=1
ping, state=0
7- working sample with channels and without using Lock() (The Go Playground):
// to test the panic use go build -race
package main
import "fmt"
func main() {
go run()
for i := 0; i <= 10; i++ {
signal <- struct{}{}
if <-read {
ping <- "ping"
}
if i%3 == 0 {
write <- true
}
}
}
func run() {
foo := false
for {
select {
case <-signal:
fmt.Println("signal", foo)
read <- foo
case foo = <-write:
fmt.Println("write", foo)
case v := <-ping:
fmt.Println(v, foo)
foo = false
}
}
}
var (
ping = make(chan string)
signal = make(chan struct{})
read = make(chan bool)
write = make(chan bool)
)
output:
signal false
write true
signal true
ping true
signal false
signal false
write true
signal true
ping true
signal false
signal false
write true
signal true
ping true
signal false
signal false
write true
signal true
ping true
package main
import (
"sync"
"time"
"fmt"
)
var myvar int
var mut sync.Mutex
func main() {
for {
go other()
go printer()
time.Sleep(time.Duration(1) * time.Second)
}
}
func other() {
mut.Lock()
myvar = myvar +1
mut.Unlock()
}
func printer() {
mut.Lock()
fmt.Println(myvar)
mut.Unlock()
}
$ go build -race t1.go
$ ./t1
1
2
3
4
5
6
7
7
9
10
$ go build t2.go
$ go build -race t2.go
$ ./t2
==================
WARNING: DATA RACE
Read at 0x000000580ce8 by goroutine 7:
runtime.convT2E()
/usr/local/go/src/runtime/iface.go:155 +0x0
main.printer()
/.../.../.../GOPATH/t2.go:23 +0x65
Previous write at 0x000000580ce8 by goroutine 6:
main.other()
/.../.../.../GOPATH/t2.go:19 +0x3d
Goroutine 7 (running) created at:
main.main()
/.../.../.../GOPATH/t2.go:13 +0x5a
Goroutine 6 (finished) created at:
main.main()
/.../.../.../GOPATH/t2.go:12 +0x42
==================
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