主要功能在退出前不等待通道读取？

Question

考虑以下使用 Go 例程和通道实现 Dining Philosophers 的尝试。

package main

import "fmt"

func philos(id int, left, right, plate chan bool) {
    fmt.Printf("Philosopher # %d wants to eat\n", id) 
    <-left
    <-right
    plate <- true
    left <- true
    right <- true
    fmt.Printf("Philosopher # %d finished eating\n", id) 
}

func main() {
    const numPhilos = 5 
    var forks [numPhilos]chan bool
    for i := 0; i < numPhilos; i++ {
        forks[i] = make(chan bool, 1)
        forks[i] <- true
    }   
    plates := make(chan bool)
    for i := 0; i < numPhilos; i++ {
        go philos(i, forks[(i-1+numPhilos)%numPhilos], forks[(i+numPhilos)%numPhilos], plates)
    }   
    for i := 0; i < numPhilos; i++ {
        <-plates
    }   
}

有时这会按预期工作，即所有哲学家都吃，例如：

Philosopher # 4 wants to eat
Philosopher # 3 wants to eat
Philosopher # 2 wants to eat
Philosopher # 1 wants to eat
Philosopher # 4 finished eating
Philosopher # 3 finished eating
Philosopher # 2 finished eating
Philosopher # 1 finished eating
Philosopher # 0 wants to eat
Philosopher # 0 finished eating

但是，有时会遗漏一位（或多位）哲学家（例如哲学家 #0，在以下情况下没有吃饭）：

Philosopher # 4 wants to eat
Philosopher # 1 wants to eat
Philosopher # 3 wants to eat
Philosopher # 2 wants to eat
Philosopher # 4 finished eating
Philosopher # 0 wants to eat
Philosopher # 2 finished eating
Philosopher # 1 finished eating
Philosopher # 3 finished eating

问题是：为什么会发生这种情况？

我已经知道的：

如果 main go 例程完成（即使其他一些例程仍在运行ning），程序将退出。
如果一个 go routine 试图从一个通道读取，并且该通道是空的（即之前没有人写入它），它将被阻塞。

现在，main 尝试从通道 plates 读取 5 次，因此在 philos 例程已运行五次之前它不会终止。但似乎它在这样做之前仍然设法以某种方式终止。我错过了什么吗？（似乎 plates 只读了 4 次。）

编辑：好吧，再考虑一下后，我得出结论，也许 philos 例程总是运行 5然而，在有时间打印哲学家吃东西之前，可能会被打断。事实上，如果我按如下方式更改顺序，它似乎总是有效：

func philos(id int, left, right, plate chan bool) {
    fmt.Printf("Philosopher # %d wants to eat\n", id) 
    <-left
    <-right
    left <- true
    right <- true
    fmt.Printf("Philosopher # %d finished eating\n", id) 
    plate <- true
}

不过，如果有人可以验证这个解释，那就太好了:)

Answer 1

您在标准输出中看到的与实际发生的不同。有时，main 从 plates 接收，然后在打印语句发生之前从 returns 接收。所以：

plate <- true
left <- true    // On this line or on
right <- true   // this line, main receives from plate and then returns before
fmt.Printf("Philosopher # %d finished eating\n", id) // this line executes

因为并发不是确定性的，所以这不会每次都发生。有时打印发生在 main returns 之前，有时则不会。这并不意味着通道读取没有发生。

Answer 2

实际上通道被读取了5次，但是由于main函数只是在等待通道被读取到第5次，所以它在philos函数到达这一行之前就退出了：

fmt.Printf("Philosopher # %d finished eating\n", id)`

为了使其正确打印，您需要在写入印版通道之前运行这一行。

主要功能在退出前不等待通道读取？

main function not waiting for channel read before exiting?

go

goroutine

channels