In which function should I pass the WaitGroup? - go

Question

I have made a simple code example to understand the usage of pipeline, here it is.
package main
import (
"fmt"
"sync"
"time"
)
func main() {
ch1 := make(chan int, 10) // Use buffered channel so as to avoid clogging
ch2 := make(chan string, 10)
var wg sync.WaitGroup
for i := 0; i < 3; i++ {
wg.Add(1)
go func1(i, ch1, &wg)
go func2(ch1, ch2)
}
wg.Wait()
close(ch1)
for val := range ch2 {
fmt.Println(val)
}
}
func func1(seconds int, ch chan<- int, wg *sync.WaitGroup) {
defer wg.Done()
time.Sleep(time.Duration(seconds) * time.Second)
fmt.Println(seconds)
ch <- seconds
}
func func2(ch1 chan int, ch2 chan string) {
for range ch1 {
ch2 <- "hello"
}
close(ch2)
}
Now, the problem is I don't get consistent output ( I understand it's some concurrency issue, which I haven't fully understood ).
Output
> go run pipeline-loop.go
0
1
2
hello
hello
> go run pipeline-loop.go
0
1
2
hello
hello
hello
> go run pipeline-loop.go
0
1
2
hello
hello
> go run pipeline-loop.go
0
1
2
hello
hello
> go run pipeline-loop.go
0
1
2
hello
hello
panic: close of closed channel
goroutine 6 [running]:
main.func2(0xc00006c000, 0xc000056180)
/home/projects/go-tuts/pipeline-loop.go:36 +0x72
created by main.main
/home/projects/go-tuts/pipeline-loop.go:16 +0x10f
exit status 2
Another guy changed the code ( and it was working ) and put func2 outside the loop but I want func2 for each iteration of the func1.
Problem
So, I want to understand as to where should the WaitGroup and close(ch) be used ?
Thanks.
Temporarya
( A golang noobie )
Update
Based on a user's answer, I changed the code, now I get output as expected ( but not the solution to this question ) but still there's a deadlock.
https://play.golang.org/p/O_rp_FLvNh8
package main
import (
"fmt"
"sync"
"time"
)
func main() {
ch1 := make(chan int, 10) // Use buffered channel so as to avoid clogging
ch2 := make(chan string, 10)
var wg1 sync.WaitGroup
// var wg2 sync.WaitGroup
for i := 0; i < 3; i++ {
wg1.Add(1)
go func1(i, ch1)
go func2(ch1, ch2, &wg1)
}
for val := range ch2 {
fmt.Println(val)
}
wg1.Wait()
close(ch1)
close(ch2)
}
// func func1(seconds int, ch chan<- int, wg *sync.WaitGroup) {
func func1(seconds int, ch chan<- int) {
// defer wg.Done()
time.Sleep(time.Duration(seconds) * time.Second)
fmt.Println(seconds)
ch <- seconds
}
func func2(ch1 chan int, ch2 chan string, wg *sync.WaitGroup) {
defer wg.Done()
for range ch1 {
ch2 <- "hello"
}
}

Answer 1

There are multiple problems in your code.
In the loop, you are spawning multiple (3) goroutines that runs func2, and in func2, you send data to ch2 and call close(ch2). This is a problem. It can happen that when one goroutine is seding data to ch2, the other has closed that channel, which causes:
panic: close of closed channel
goroutine 6 [running]:
main.func2(0xc00006c000, 0xc000056180)
/home/projects/go-tuts/pipeline-loop.go:36 +0x72
created by main.main
/home/projects/go-tuts/pipeline-loop.go:16 +0x10f
exit status 2
In general, you don't need close a chanel multiple times - you only need to shut them once they are all finished. You need another WaitGroup for this; you need to pass both functions a WaitGroup.
Further reading: https://blog.golang.org/pipelines
UPDATE:
Personally I use a pattern for "works" that produce data into a same channel and the channel needs to be closed after all works are done:
for something {
wg.Add(1)
go func(i int) {
work(ch)
wg.Done()
}
}
go func() {
wg.Wait()
close()
}()
I think it is a good idea that keeps the API clean from WorkGroup as WorkGroup is about how you sync the work instead of how the work is done.
I have changed your code into this pattern: https://play.golang.org/p/vdCNsxWhgyQ

Answer 2

I suspect you only want one channel to read from ch1 and write to ch2. There is not much point in creating 3 go-routines to do the same thing (and you also end up closing the same channel mutliple time which causes a panic as leaf bebop pointed out)
func main() {
ch1 := make(chan int, 10) // Use buffered channel so as to avoid clogging
ch2 := make(chan string, 10)
var wg sync.WaitGroup
for i := 0; i < 3; i++ {
wg.Add(1)
go func1(i, ch1, &wg)
}
go func2(ch1, ch2)
wg.Wait()
close(ch1)
for val := range ch2 {
fmt.Println(val)
}
}