golang concurrency sync issue

Issue

I am facing a synchronization issue when using goroutines. My program outputs unpredictable results. I checked the docu and for unbuffered channels there is no way to check if all msgs have been processed. I simplified the issue to this little demo code that still demonstrates the problem. Clearly this is not an issue with Golang but with my code. Obviously I am not using the right concurrency pattern.

Question is how to resolve this. If possible I would neither want to close the channel nor stop the hive goroutine. It think it would be great if I could assume that once all bee goroutines are finished that hive is done for now, too (that is what I tried by using wg.Wait()).

package main

import(
    "fmt"
    "sync"
    "time"
)



func main() {
    count := int64(0)
    c := make(chan int64)
    var wg sync.WaitGroup

    // bees
    for i:=0; i<5000;i++{
        wg.Add(1)
        go func(in chan int64) {
            defer wg.Done()
            time.Sleep(100)
            in <- 2
        }(c)
    }

    // hive
    go func() {
        for out := range c {
            count += out
        }
    }()

    wg.Wait()
    // bang! but why?
    fmt.Println(count)  
}

// every now and again the program prints out before it is finished
// $ go run pattern1.go
// 10000
// $ go run pattern1.go
// 9998
// $ go run pattern1.go
// 9998
// $ go run pattern1.go
// 10000
// $ go run pattern1.go
// 10000
// $ go run pattern1.go
// 9998

Solution

You’re never waiting for the “hive” loop to finish, so sometimes you print the count value before it’s complete.

It’s easiest to use the WaitGroup to signal when to close the channel, and block main on the for range loop:

go func() {
    wg.Wait()
    close(c)
}()

for out := range c {
    count += out
}

http://play.golang.org/p/jK24dtG2je

Answered By – JimB

Answer Checked By – Terry (GoLangFix Volunteer)

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