无论是否关闭频道，缩小地图都无法正常工作

Question

This is homework and beginner question. I edited the question since I made a mistake that I found.

I am trying to do parallel frequency maps from a text and I get an error in the last operation (reducing maps)

The code seem to work up to this point.

If I close the channel I get error: "panic: send on closed channel"

If I do not close the channel I get: "fatal error: all goroutines are asleep - deadlock!"

func WordCount(text string) {

    text = strings.ToLower(text)
    re := regexp.MustCompile("\\w+")
    sentence := re.FindAllString(text, -1)

    numberOfGroups := 4
    piece := len(sentence) / numberOfGroups

    wordChannel := make(chan map[string]int)

    wg := new(sync.WaitGroup)
    wg.Add(numberOfGroups)

    for i := 0; i < numberOfGroups; i ++ {
        go processToCounting(sentence[i*piece:(i+1)*piece], wordChannel, wg)
    }

    wg.Wait()
    fmt.Print(<-wordChannel)
    fmt.Print("
")

    finalMap := make(map[string]int)
    close(wordChannel)

    for i := 0; i < numberOfGroups; i++ {
        for k, v := range <- wordChannel {
            finalMap[k] += v
        }
    }
}

func processToCounting(textSlice []string, wordChannel chan map[string]int, wg *sync.WaitGroup) {
    freq := make(map[string]int)
    for _, v := range textSlice {
        freq[v]++
    }
    wg.Done()
    wordChannel <- freq
}

Answer 1

1. First question: panic

If I close the channel I get error: "panic: send on closed channel"

Why? One of you goroutines is trying to write to the channel which you already closed in the calling (main) goroutine. In your case in WordCount function.

In the current version of your code the panic is not reproducible with my test sentence, but you can easily cause this e.g. if you would call close(wordChannel) before wg.Wait().

Lets look at the Bug in processToCounting which may cause the panic:

wg.Done() // tells to the WaitGroup that the gouroutine is Done (decrements the counter of goroutines)
wordChannel <- freq // trying to write to the channel

Here wg.Done() signals to the WaitGroup that the goroutine is Done before actual writing to the channel has happened. The calling goroutine (WordCount function) at some point thinks that all gouroutines are done (wg.Wait() line) and closes the channel. But one of your goroutine which have not finished writing, will try to write to the closed channel. Then you will get the panic.

How to fix:

Use defer in processToCounting function

defer wg.Done() // triggers wg.Done right before the function returns (but after writing to the channel in your case) 

What to read:

See for beginners in A Tour of Go / Concurrency

Sending on a closed channel will cause a panic.

and documentation for: close

Sending to or closing a closed channel causes a run-time panic.

2. Second question: deadlock

If I do not close the channel I get: "fatal error: all goroutines are asleep - deadlock!"

You have a for-loop which reads from the channel. This for-loop is locked forever. Waiting for new values from the channel, but nobody will write there anymore.

See in A Tour of Go / Concurrency

The loop for i := range c receives values from the channel repeatedly until it is closed.

and see documentation for Channels

Receivers always block until there is data to receive