dt614037527
dt614037527
2017-08-14 21:32

我靠运气解决的Golang僵局,需要说明

I'm trying to understand go channel and go routine. To do so, I'm doing online exercises. I found one here: http://whipperstacker.com/2015/10/05/3-trivial-concurrency-exercises-for-the-confused-newbie-gopher/

I resolved the 3rd one (named "Internet cafe"). But there's something I resolved by "luck", and it's bother me because I don't understand my issue and why my "hack" fixed it.

In my code below, I replace "enterChan <- next" by "go func() { enterChan <- next }()", and it solved my deadlock.

Can someone explain to me why it deadlock before, and why it works with this hack ? Is it a proper solution, or an ugly one ?

Don't hesitate to criticize my code, I'm searching to improve :)

Many thanks!

This is my code:

package main

import (
    "fmt"
    "math/rand"
    "strconv"
    "time"
)

const (
    maxNumberOfUser = 8
)

func useComputer(tourist string, leaverChan chan string) {
    seed := rand.NewSource(time.Now().UnixNano())
    random := rand.New(seed)
    fmt.Println(tourist, "is online")
    d := random.Intn(120-15) + 15
    time.Sleep(time.Duration(d) * time.Millisecond * 10)
    fmt.Println(tourist, "is done, having spent", d, "minutes online.")
    leaverChan <- tourist
}

func manageUsers(enterChan, leaverChan chan string, stopChan chan struct{}) {
    nbUsed := 0
    queue := make([]string, 0)
    for {
        select {
        case tourist := <-enterChan:
            if nbUsed < maxNumberOfUser {
                nbUsed++
                go useComputer(tourist, leaverChan)
            } else {
                fmt.Println(tourist, "waiting for turn.")
                queue = append(queue, tourist)
            }
        case tourist := <-leaverChan:
            nbUsed--
            fmt.Println(tourist, "is leaving, number of free place is now:", maxNumberOfUser-nbUsed)
            if len(queue) > 0 {
                next := queue[0]
                queue = queue[1:]
                go func() {
                    enterChan <- next
                }()
            } else if nbUsed == 0 {
                close(stopChan)
                return
            }

        }
    }
}

func main() {
    enterChan := make(chan string)
    leaverChan := make(chan string)
    stopChan := make(chan struct{})
    go manageUsers(enterChan, leaverChan, stopChan)

    for i := 1; i <= 25; i++ {
        enterChan <- "Tourist " + strconv.Itoa(i)
    }
    <-stopChan
    fmt.Println("The place is empty, let's close up and go to the beach!")
}
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