Go WorkGroup doesn't synchronize functions

Question

This is just an abstract example. test will work as long as it receives messages from test2. Then when they are all done wg.Done() will be called, and application will close after wg.Wait().

package main

import (
    "fmt"
    "sync"
)

var a = make(chan bool)
var wg sync.WaitGroup

func main() {
    go test()
    wg.Add(1)
    for i := 0; i < 5; i++ {
        go test2()
        wg.Add(1)
    }
    wg.Wait()
}

func test() {
    defer wg.Done()
    for {
        val := <-a
        fmt.Println(val)
    }
}

func test2() {
    defer wg.Done()
    for i := 0; i < 3; i++ {
        a <- true
    }
}

At least this is theory, but it doesn't translate well to practice. It correctly displays true 15 times, but then application crashes with a deadlock, because wg.Done() is never called. I'm quite stumped, because this is how it's done in tutorials.

score 1 · Answer 1 · answered Mar 16 '23 at 15:06

1

The goroutine created in test will never terminate, because of that, the wait group count will never reach zero and it will wait indefinitely.

Terminate the endless for-loop after 3 writes and the program should work, because you are reading 3 times from the channel.

Also, you have to call wg.Add before creating the goroutine, not after. wg.Done may be called before you can add to it, causing a panic.

answered Mar 16 '23 at 15:06

Burak Serdar

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What should I do if we assume that I don't know how many messages I will receive? – Mar 16 '23 at 15:10
Usually, the sending end closes the channel when done, so the receiver end can range on the channel. range will receive from the channel until it closes. – Burak Serdar Mar 16 '23 at 15:17
I can't close the channel in test2 because I can't be sure that it was closed in the last step. – Mar 16 '23 at 15:24
You should close it in `test`, not `test2` – Burak Serdar Mar 16 '23 at 15:47
So the answer is that this type of application cannot be done with golang. – Mar 16 '23 at 15:54
1

No. The way you are doing is wrong. Of course it can be done with golang. – Burak Serdar Mar 16 '23 at 15:58
1

You seem to lack the basic understanding of how channels work. Work through the tour of go, and there you can see how it is done. – Burak Serdar Mar 16 '23 at 16:00

Go WorkGroup doesn't synchronize functions

1 Answers1