Reading from a goroutine channel without blocking

You can easily verify

func helper(c chan<- error) {
    time.Sleep(5 * time.Second)
    c <- errors.New("") // send errors/nil on c
}

func worker() error {
    fmt.Println("do one")

    c := make(chan error, 1)
    go helper(c)

    err := <-c
    fmt.Println("do two")

    return err
}

func main() {
    worker()
}

Q: Is the statement err := <- c blocking worker? I don't think so, since the channel is buffered.

A: err := <- c will block worker.

Q: If it is blocking, how do I make it non-blocking? My requirement is to have worker and its caller continue with rest of the work, without waiting for the value to appear on the channel.

A: If you don't want blocking, just remove err := <-c. If you need err at the end, just move err := <-c to the end.

You can not read channel without blocking, if you go through without blocking, can can no more exec this code, unless your code is in a loop.

Loop:
    for {
        select {
        case <-c:
            break Loop
        default:
            //default will go through without blocking
        }
        // do something
    }

And have you ever seen errgroup or waitgroup？

It use atomic, cancel context and sync.Once to implement this.

https://github.com/golang/sync/blob/master/errgroup/errgroup.go

https://github.com/golang/go/blob/master/src/sync/waitgroup.go

Or you can just use it, go you func and then wait for error in any place you want.

In your code, the rest of the work is independent of whether the helper encountered an error. You can simply receive from the channel after the rest of the work is completed.

func worker() error {
    //do some work
    c := make(chan error, 1)
    go helper(c)
    //do rest of the work
    return <-c
}

I think you need this code..

run this code

package main

import (
    "log"
    "sync"
)

func helper(c chan<- error) {

    for {
        var err error = nil
        // do job

        if err != nil {
            c <- err // send errors/nil on c
            break
        }
    }

}

func worker(c chan error) error {
    log.Println("first log")

    go func() {
        helper(c)
    }()

    count := 1
    Loop:
        for {
            select {
            case err := <- c :
                return err
            default:
                log.Println(count, " log")
                count++
                isFinished := false
                // do your job
                if isFinished {
                    break Loop // remove this when you test

                }
            }
        }
    return nil
}

func main() {
    wg := sync.WaitGroup{}
    wg.Add(1)
    go func() {
        c := make(chan error, 1)
        worker(c)
        wg.Done()
    }()
    wg.Wait()
}