需要帮助理解为什么select {}不会永远阻止

Question

我正在努力使用通道来实现队列.具体来说,我试图使用通道的大小来限制同时goroutine的数量.即,我写了以下代码:

package main

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

func runTask (t string, ch *chan bool) {
        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))
        <- *ch
}

func main() {
        numWorkers := 3
        files := []string{"a", "b", "c", "d", "e", "f", "g", "h", "i", "j"}

        activeWorkers := make(chan bool, numWorkers)

        for _, f := range files {
                activeWorkers <- true
                fmt.Printf("activeWorkers is %d long.\n", len(activeWorkers))
                go runTask(f, &activeWorkers)
        }
        select{}
}

现在,代码崩溃了:

throw: all goroutines are asleep - deadlock!

我的期望是对select的调用会永远阻塞,让goroutines终止而不会出现死锁.

所以我有一个双重的问题:为什么不选择永久阻塞,而不是抛出一个time.Sleep()调用for循环后,我怎么能避免死锁？

干杯,

-mtw

Answer 1

Arlen Cuss已经写了一个很好的答案.我只是想为你的工作队列建议另一种设计.您可以只生成有限数量的工作人员goroutine,而不是限制您的频道可以缓冲的条目数量.像这样的东西:

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
    }
}

如果您只想等到所有任务都已执行,也可以使用sync.WaitGroup,但使用out通道的优势在于您可以稍后聚合结果.例如,如果每个任务返回该文件中的单词数,则可以使用最终循环来总结所有单个单词计数.