Expected output as well as deadlock in a worker pool-CodePudding

I'm learning Go concurrency and wrote the obligatory worker pool example, where there are N pieces of work and M workers (N > M). I'm running into a deadlock (all goroutines are asleep), which I can't figure out; however, I'm also getting the expected output before the deadlock occurs, which has me even more confused. Can someone please point out the things I'm doing wrong?

My code is this:

package main

import (
    "fmt"
    "sync"
)

// A simple worker pool implementation using channels and WaitGroups.
// Our workers simply read from a channel of integers from an input
// channel and write their squares to an output channel.

func addJobs(jobsCh chan<- int, wg *sync.WaitGroup) {
    // 100 numbers to crunch (jobs)
    for i := 1; i < 101; i   {
        jobsCh <- i
    }
    wg.Done()
}

func worker(jobsCh <-chan int, resultsCh chan<- int, wg2 *sync.WaitGroup) {
    for num := range jobsCh {
        resultsCh <- num * num
    }
    wg2.Done()
}

func addWorkers(jobsCh <-chan int, resultsCh chan<- int, wg *sync.WaitGroup) {
    var wg2 sync.WaitGroup
    // 10 workers
    for i := 0; i < 10; i   {
        wg2.Add(1)
        go worker(jobsCh, resultsCh, &wg2)
    }
    wg.Done()
}

func readResults(resultsCh <-chan int, wg *sync.WaitGroup) {
    for sq := range resultsCh {
        fmt.Printf("%v ", sq)
    }
    wg.Done()
}

func main() {
    var wg sync.WaitGroup
    jobsCh := make(chan int)
    resultsCh := make(chan int)

    wg.Add(1)
    go addJobs(jobsCh, &wg)

    wg.Add(1)
    go addWorkers(jobsCh, resultsCh, &wg)

    wg.Add(1)
    go readResults(resultsCh, &wg)

    wg.Wait()
}

This prints the squares of the numbers (in random order), as expected, but also runs into a deadlock. Please see this playground link. :(

CodePudding user response：

Close jobsCh to cause workers to exit:

func addJobs(jobsCh chan<- int, wg *sync.WaitGroup) {
    // 100 numbers to crunch (jobs)
    for i := 1; i < 101; i   {
        jobsCh <- i
    }
    close(jobsCh)  // <-- add this line
    wg.Done()
}

After workers are done, close resultsCh to cause results loop to exit:

func addWorkers(jobsCh <-chan int, resultsCh chan<- int, wg *sync.WaitGroup) {
    var wg2 sync.WaitGroup
    // 10 workers
    for i := 0; i < 10; i   {
        wg2.Add(1)
        go worker(jobsCh, resultsCh, &wg2)
    }
    wg2.Wait()       // <-- add this line
    close(resultsCh) // and this line
    wg.Done()
}