Need help understanding why select{} isn't blocking forever

channel, go

Solution

Arlen Cuss has already written a good answer. I just want to suggest another design for your work queue. Instead of limiting the number of entries your channel can buffer, you can also just spawn a limited number of worker goroutines which feels more natural imho. Something like that:

package main

import "fmt"
import "time"
import "math/rand"

func runTask(t string) string {
    start := time.Now()
    fmt.Println("starting task", t)
    time.Sleep(time.Millisecond * time.Duration(rand.Int31n(1500))) // fake processing time
    fmt.Println("done running task", t, "in", time.Since(start))
    return t
}

func worker(in chan string, out chan string) {
    for t := range in {
        out <- runTask(t)
    }
}

func main() {
    numWorkers := 3

    // spawn workers
    in, out := make(chan string), make(chan string)
    for i := 0; i < numWorkers; i++ {
        go worker(in, out)
    }

    files := []string{"a", "b", "c", "d", "e", "f", "g", "h", "i", "j"}

    // schedule tasks
    go func() {
        for _, f := range files {
            in <- f
        }
    }()

    // get results
    for _ = range files {
        <-out
    }
}

You can also use a sync.WaitGroup if you just want to wait until all tasks have been executed, but using an `out` channel has the advantage that you can aggregate the results later. For example if each tasks returns the number of words in that file, the final loop might be used to sum up all individual word counts.

Problem

Original source