Last value is missing when reading channel written in concurrent goroutines

Question

I'm fairly new in Go and I want to run several tasks asynchronously, wait for all of them to be finished and collect their results into a slice.

I was reading a lot of documentation and examples, specially this Nathan LeClaire's post, and came up with something close to what I want to do (see code below). The mechanic is simple:

• 10 goroutines are triggered and each of them writes a value in a channel.
• Another goroutine reads the channel and fills the slice.
• After all these stuff are done, the slice is printed.

However the outcome shows a 9-length slice (values from 0 to 8) and the 10th value (should be 9) seems missing. The program exits just fine and I don't know what is going on. Any hint is appreciated.

Here is a code example to play with http://play.golang.org/p/HUFOZLmCto:

package main

import (
    "fmt"
    "sync"
)

func main() {

    var wg sync.WaitGroup

    n := 10    
    c := make(chan int)

    wg.Add(n)

    for i := 0; i < n; i++ {
        go func(val int) {
            defer wg.Done()
            fmt.Println("Sending value to channel: ", val)
            c <- val
        }(i)
    }

    var array []int

    go func() {
        for val := range c {
            fmt.Println("Recieving from channel: ", val)
            array = append(array, val)
        }
    }()

    wg.Wait()
    fmt.Println("Array: ", array)
}

And this is the outcome:

Sending value to channel:  0
Recieving from channel:  0
Sending value to channel:  1
Recieving from channel:  1
Sending value to channel:  2
Recieving from channel:  2
Sending value to channel:  3
Recieving from channel:  3
Sending value to channel:  4
Recieving from channel:  4
Sending value to channel:  5
Recieving from channel:  5
Sending value to channel:  6
Recieving from channel:  6
Sending value to channel:  7
Recieving from channel:  7
Sending value to channel:  8
Recieving from channel:  8
Sending value to channel:  9
Array:  [0 1 2 3 4 5 6 7 8]     // Note 9 is missing here

Answer 1

You're exiting before the receiving goroutine has a chance to receive and handle the value. There is also a race condition on the array variable, where main may try to print the array during the appendoperation.

Note that even though an using unbuffered channel would create a synchronization point between the two loops, and guarantee that the receive loop has the value before wg.Done(), it wouldn't guarantee that the fmt.Println and append happen before main continues.

One way to arrange this is to put the receive loop in main, and wait on closing the c chan in it's own goroutine:

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

for val := range c {
    fmt.Println("Recieving from channel: ", val)
    array = append(array, val)
}

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

Answer 2

Pretty simple. At some point all the values are written, the waitGroup is released and a goroutine is filling the sice. Since the waitGroup is release its possible for the print to occur before the channel is drained into the slice.

To solve, move the wg.Done() into the reader to prevent the print happening before the draining.

package main

import (
    "fmt"
    "sync"
)

func main() {
    n := 10
    c := make(chan int)

    var wg sync.WaitGroup
    wg.Add(10)

    for i := 0; i < n; i++ {
        go func(val int) {
            fmt.Println("Sending value to channel: ", val)
            c <- val
        }(i)
    }

    var array []int

    go func() {

        for val := range c {
            fmt.Println("Recieving from channel: ", val)
            array = append(array, val)
            wg.Done()
        }
    }()

    wg.Wait()
    fmt.Println("Array: ", array)
}

Example in playground