goruntine not running concurrently? - go

Question

I have the following program, I am new to gorountine, what I want to test is simple, I am calling a gorountine in a loop 100 times, it there is one time failure, the entire program fails, otherwise succeeds, and fail10Percent it delays 1 second, and check a random number if it is 4, let it fail.
package main
import (
"fmt"
"math/rand"
"time"
)
func fail10Percent(ch chan int) {
time.Sleep(1 * time.Second)
e := rand.Intn(10)
fmt.Println("Calculating rand.Intn(10) ", e)
if e == 4 {
ch <- 0
return
}
ch <- 1
}
func test() {
for i := 0; i < 100; i++ {
err := make(chan int)
go fail10Percent(err)
res := <-err
fmt.Println("=== result: ", res)
if res != 1 {
fmt.Println("failed")
return
}
}
fmt.Println("succeeded")
}
func main() {
test()
}
I expect the go fail10Percent(err) will run concurrently 100 times, which will only have 1 second delay, however, when I run it, I see the following result getting printed 1 second after 1 second, why is that, and how I can adjust my program to do what I want.
Calculating rand.Intn(10) 1
=== result: 1
Calculating rand.Intn(10) 7
=== result: 1
Calculating rand.Intn(10) 7
=== result: 1
Calculating rand.Intn(10) 9
=== result: 1
Calculating rand.Intn(10) 1
=== result: 1
Calculating rand.Intn(10) 8
=== result: 1
Calculating rand.Intn(10) 5
=== result: 1
Calculating rand.Intn(10) 0
=== result: 1
Calculating rand.Intn(10) 6
=== result: 1
Calculating rand.Intn(10) 0
=== result: 1
Calculating rand.Intn(10) 4
=== result: 0
failed

Answer 1

I've commented out the code for you so that you can understand.
package main
import (
"fmt"
"math/rand"
"sync"
)
func fail10Percent(ch chan int, w *sync.WaitGroup) {
defer w.Done()
num := rand.Intn(10)
fmt.Println("calculating rand.Intn(10) ", num)
if num == 4 {
ch <- 0 // Fail
return
}
ch <- 1 // Pass
}
func test() {
var ch = make(chan int, 1)
// Launch the receiver goroutine to listen if goroutine succeeded or failed based on the value sent to ch
go func() {
for recv := range ch {
switch recv {
// Fail
case 0:
fmt.Println("goroutine failed")
// Pass
case 1:
fmt.Println("goroutine succeed")
}
}
}()
// wg is a WaitGroup
var wg sync.WaitGroup
for i := 0; i < 100; i++ {
wg.Add(1)
go fail10Percent(ch, &wg)
}
// wg.Wait() to wait for all goroutines to complete
wg.Wait()
// Close the channel so that the receiver can stop
close(ch)
}
func main() {
test()
}
Update:
Simple solution without using sync.WaitGroup
package main
import (
"fmt"
"math/rand"
)
// Using a send only channel
func fail10Percent(ch chan<- int) {
num := rand.Intn(10)
fmt.Println("calculating rand.Intn(10) ", num)
if num == 4 {
ch <- 0 // Fail
return
}
ch <- 1 // Pass
}
func test() {
var ch = make(chan int, 1)
for i := 0; i < 100; i++ {
go fail10Percent(ch)
}
for i := 0; i < 100; i++ {
if recv := <-ch; recv == 0 {
fmt.Println("goroutine failed")
} else if recv == 1 {
fmt.Println("goroutine succeed")
}
}
close(ch)
}
func main() {
test()
}