go concurrency all goroutines are asleep - deadlock

Question

Sorry about the noob question but I'm having a hard time wrapping my head around the concurrency part of go. Basically this program below is a simplified version of a larger one I'm writing, thus I want to keep the structure similar to below.

Basically instead of waiting 4 seconds I want to run addCount(..) concurrent using the unbuffered channel and when all elements in the int_slice has been processed I want to do another operation on them. However this program ends with a "panic: close of closed channel" and if I remove the closing of the channel I'm getting the output I'm expecting but it panics with: "fatal error: all goroutines are asleep - deadlock"

How can I implement the concurrency part correctly in this scenario?

Thanks in advance!

package main

import (
    "fmt"
    "time"
)

func addCount(num int, counter chan<- int) {
    time.Sleep(time.Second * 2)
    counter <- num * 2
}

func main() {
    counter := make(chan int)
    int_slice := []int{2, 4}

    for _, item := range int_slice {
        go addCount(item, counter)
        close(counter)
    }

    for item := range counter {
        fmt.Println(item)
    }
}

Answer 1

For the sake of having examples here's a slightly modified version of what @eugenioy submitted. It allows for the use of an unbuffered channel and the reading of values as they come in instead of at the end like a regular for loop.

package main

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

func addCount(num int, counter chan<- int, wg *sync.WaitGroup) {
    // clear one from the sync group
    defer wg.Done()
    // not needed, unless you wanted to slow down the output
    time.Sleep(time.Second * 2)
    counter <- num * 2
}

func main() {
    // variable names don't have underscores in Go
    intSlice := []int{2, 4}

    counter := make(chan int)

    var wg sync.WaitGroup

    for _, item := range intSlice {
        // add one to the sync group, to mark we should wait for one more
        wg.Add(1)
        go addCount(item, counter, &wg)
    }

    // by wrapping wait and close in a go routine I can start reading the channel before its done, I also don't need to know the size of the
    // slice
    go func() {
        wg.Wait()
        close(counter)
    }()

    for item := range counter {
        fmt.Println(item)
    }
}

Answer 2

Here are the issues I spotted in the code, and below a working version based on your implementation.

• If a goroutine tries to write to an "unbuffered" channel, it will block until someone reads from it. Since you are not reading until they finish writing to the channel, you have a deadlock there.

• Closing the channel while they are blocked breaks the deadlock, but gives an error since they now can't write to a closed channel.

Solution involves:

• Creating a buffered channel so that they can write without blocking.

• Using a sync.WaitGroup so that you wait for the goroutines to finish before closing the channel.

• Reading from the channel at the end, when all is done.

See here, with comments:

    package main

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

    func addCount(num int, counter chan<- int, wg *sync.WaitGroup) {
        // clear one from the sync group
        defer wg.Done()
        time.Sleep(time.Second * 2)
        counter <- num * 2
    }

    func main() {
        int_slice := []int{2, 4}
        // make the slice buffered using the slice size, so that they can write without blocking
        counter := make(chan int, len(int_slice))

        var wg sync.WaitGroup

        for _, item := range int_slice {
            // add one to the sync group, to mark we should wait for one more
            wg.Add(1)
            go addCount(item, counter, &wg)
        }

        // wait for all goroutines to end
        wg.Wait()

        // close the channel so that we not longer expect writes to it
        close(counter)

        // read remaining values in the channel
        for item := range counter {
            fmt.Println(item)
        }

    }

Answer 3

package main

import (
    "fmt"
    "time"
)

func addCount(num int, counter chan <- int) {
    time.Sleep(time.Second * 2)
    counter <- num * 2
}

func main() {
    counter := make(chan int)
    int_slice := []int{2, 4}

    for _, item := range int_slice {
        go addCount(item, counter)

        fmt.Println(<-counter)
    }
}