Go run -race main.go hasn't returned yet

Getting Help Code Review
#1

If I run this code, with “go -race main.go” it never returns. When I run go run main.go, sometimes I get

"fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
/home/peter/go/src/github.com/stackskill0/concur/channels/n21/main.go:37 +0x1da

goroutine 6 [chan receive]:
main.main.func2(0xc42001e0c0, 0xc4200120f0)
/home/peter/go/src/github.com/stackskill0/concur/channels/n21/main.go:27 +0x7c
created by main.main
/home/peter/go/src/github.com/stackskill0/concur/channels/n21/main.go:22 +0xd6

goroutine 7 [semacquire]:
sync.runtime_Semacquire(0xc4200120fc)
/usr/local/go/src/runtime/sema.go:56 +0x39
sync.(*WaitGroup).Wait(0xc4200120f0)
/usr/local/go/src/sync/waitgroup.go:131 +0x72
main.main.func3(0xc4200120f0, 0xc42001e0c0)
/home/peter/go/src/github.com/stackskill0/concur/channels/n21/main.go:33 +0x2b
created by main.main
/home/peter/go/src/github.com/stackskill0/concur/channels/n21/main.go:32 +0x102
exit status 2"

I’m getting a deadlock and don’t understand why. Thanks for looking!


import (
	"fmt"
	"sync"
)

func main() {

	myCh := make(chan string)

	var wq sync.WaitGroup

	wq.Add(2)
	go func() {
		for i := 0; i < 10; i++ {
			myCh <- "anom 1"
		}
		wq.Done()
	}()

	go func() {
		for i := 0; i < 10; i++ {
			myCh <- "anom 2"
		}

		fmt.Println(<-myCh)
		fmt.Println("Clear!")
		wq.Done()
	}()

	go func() {
		wq.Wait()
		close(myCh)
	}()

	for ch := range myCh {
		fmt.Println("ch:", ch)
	}

}
#2

You have a logic race between the

fmt.Println(<-myCh)
fmt.Println("Clear!")
wq.Done()

in one of your goroutines and the

for ch := range myCh {
	fmt.Println("ch:", ch)
}

in the main routine. If the last loop manages to read all the channel values the first <-myCh will block, the waitgroup will never get done, the channel will never close and the program doesn’t terminate, potentially crashing with the deadlock output.

1 Like
#3

Use defer wq.Done( ) inside of your go routine and put your wq.Wait() outside your go routine in the bottom.

#4

Thanks for you help. I’m not sure learning more about the waitgroup code. It seems most people don’ use it. Anyway, I feel I kind of cheated, but I don’t get a race condition anymore.
This is what I ended up with:

package main

import (
	"fmt"
	"sync"
)

var wq sync.WaitGroup

func main() {

	myCh := make(chan string)

	wq.Add(2)
	go func() {
		defer wq.Done()
		for i := 0; i < 10; i++ {
			myCh <- "anom 1"
		}
	}()

	go func() {
		defer wq.Done()
		for i := 0; i < 10; i++ {
			myCh <- "anom 2"
		}

		/* 		fmt.Println(<-myCh)
		   		fmt.Println("Clear!") */

	}()

	go func() {
		wq.Wait()
		close(myCh)
		fmt.Println("Closed myCH")

	}()

	for ch := range myCh {
		fmt.Println("ch:", ch)
	}

}
#5

Yep, that looks fine. I don’t think you should feel cheated, nor avoid waitgroups. They are quite common, at least I use them all the time.

2 Likes
#6

Thanks!