Go: using one channels to receive results from multiple goroutines

Issue

I have a CSV file with thousands of records.
I process each record in a goroutine and I want to gather all the results of processed records in a slice to write them down in another file.
Simplified version of my code is:

var wg sync.WaitGroup

func main() {
    c := make(chan string, 1)
    csvfile, err := os.Open("data.csv")
    reader := csv.NewReader(bufio.NewReader(csvfile))
    //...
    for {
        line, err := reader.Read()
        // ...
        wg.Add(1)
        go processRecord(line, c)
    }
    //...
    results := make([]string, 0)
    for r := range c { // ERROR HERE: fatal error: all goroutines are asleep - deadlock!
        results = append(results , r)
    }
    // write results to file
    wg.Wait()
    close(c)
}

func processRecord(data []string, c chan string) {
    defer wg.Done()
    // ...
    c <- *result
}

In for loop I get all the results but loop continues after getting last result and I get this error:

fatal error: all goroutines are asleep - deadlock! 

What is wrong with this code? I tried different QAs on SO without any success.

Solution

The for-loop will only terminate once the channel is closed. You are closing the channel after for-loop terminates, hence the deadlock.

You can fix by putting the for-loop into a goroutine:

results := make([]string, 0)
done:=make(chan struct{})
go func() {
    for r := range c {
        results = append(results , r)
    }
    close(done)
}()
wg.Wait()
close(c)
<-done
// read results

Answered By – Burak Serdar

Answer Checked By – Senaida (GoLangFix Volunteer)

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