没有等待组，将数据从不同的go例程写入相同的chanel可以正常工作

Question

When writing data into same channel using multiple go routines with waitgroup after waiting wg.Wait() getting exception saying all go routines are asleep or deedlock.

package main

import (
    "fmt"
    "runtime"
    "sync"
)

var wg sync.WaitGroup

func CreateMultipleRoutines() {
    ch := make(chan int)

    for i := 0; i < 10; i++ { // creates 10 go routines and adds to waitgroup
        wg.Add(1)
        go func() {
            for j := 0; j < 10; j++ {
                ch <- j
            }
            wg.Done() // indication of go routine is done to main routine
        }()
    }

    fmt.Println(runtime.NumGoroutine())
    wg.Wait()           //wait for all go routines to complete
    close(ch)           // closing channel after completion of wait fo go routines
    for v := range ch { // range can be used since channel is closed
        fmt.Println(v)
    }
    fmt.Println("About to exit program ...")
}

When tried to implement this without waitgroup I am able to read data from channel by looping exact number of times data pushed to channel but i cant range since there will be panic when we close channel. here is the example code

package main

import (
    "fmt"
    "runtime"
)

func main() {
    ch := make(chan int)


    for i := 0; i < 10; i++ { // creates 10 go routines and adds to waitgroup
        go func(i int) {
            for j := 0; j < 10; j++ {
                ch <- j * i
            }
        }(i)
    }

    fmt.Println(runtime.NumGoroutine())

    for v := 0; v < 100; v++ {
        fmt.Println(<-ch)
    }
    fmt.Println("About to exit program ...")
}

I want to understand why waitgroup in wait state is still waiting even though all go routines are signalled Done() which inturn makes number of go routines to zero

Answer 1

By default a chan holds no items, so all go routines are blocked on sending, until something reads from it. They never actually reach the wg.Done() statement.

A solution would be to close the channel in it's own go routine. Wrap your wg.Wait() and close(ch) lines like this:

go func() {
    wg.Wait() //wait for all go routines to complete
    close(ch) // closing channel after completion of wait fo go routines
}()

Then you can range over the channel, which will only close after all of the sending go routines have finished (and implicitly all values have been received).