Some notes on closing channels in golang

Golang is a development-efficient programming language that uses efficient coroutines to support concurrent programming. However, because a coroutine is a lightweight execution thread, it is closed differently from other threads. In this article, we will cover some considerations about shutdown in Golang.

1. Close the channel

In Golang, closing the channel is an important concept. Closing a channel means closing it without further sending of values. Closing the channel will cause any receive operations on the channel to succeed immediately, and receive operations will return the values ​​already present in the channel, and a zero value after all values ​​have been received.

For example, the following code demonstrates how to close a channel:

package main

import "fmt"

func main() {
    ch := make(chan int)

    go func() {
        for i := 0; i < 10; i++ {
            ch <- i
        }
        close(ch)  // 关闭信道
    }()

    for {
        v, ok := <-ch
        if !ok {
            break  // 信道已关闭
        }
        fmt.Println(v)
    }
}

It should be noted that the channel can only be closed by the sender, not the receiver. If you try to close the channel from the receiver, this may cause a runtime panic.

2. Avoid falling into deadlock

In Golang, the shutdown operation may cause the coroutine to fall into a deadlock state. This is because when a coroutine is waiting for a closed channel, it will wait forever. Therefore, we should follow some rules to avoid this happening.

First of all, we should always use a for loop to receive the value from the channel. This way, when the channel is closed, the loop will automatically exit, and we can determine if the channel is closed by checking the second return value.

Secondly, we should try to avoid using unbuffered channels in coroutines. This is because when using an unbuffered channel, the sender will be blocked until the value is received by the receiver. So if we close an unbuffered channel, the receiver will never receive the value and there won't be any way to exit the coroutine.

3. Close is not necessary

In some cases, closing a channel is not necessary. For example, if we just need to send some values ​​in the channel and then exit the program immediately, there is no need to close the channel. When the program exits, all unclosed channels will be automatically closed.

In addition, if our program only contains a single coroutine, and the life cycle of the channel is equal to or less than the life cycle of the coroutine, there is no need to close the channel. Because as the coroutine ends, all unclosed channels will be automatically closed.

4. Use sync.WaitGroup to wait for the coroutine to complete

In Golang, concurrent programming is very common. In order to ensure that the coroutine has completed all operations, we can use sync.WaitGroup to wait for the completion of the coroutine. When the coroutine is completed, we can release the coroutine by calling the sync.WaitGroup.Done() method.

Here is an example that demonstrates how to use sync.WaitGroup to wait for a coroutine to complete:

package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup

    ch := make(chan int)

    for i := 0; i < 2; i++ {
        wg.Add(1)

        go func() {
            defer wg.Done()

            for v := range ch {
                fmt.Println(v)
            }
        }()
    }

    for i := 0; i < 10; i++ {
        ch <- i
    }

    close(ch)

    wg.Wait()
}

Summary

Close is very important in Golang as it involves one of concurrent programming key aspects. We should pay attention to how to close a channel correctly and avoid deadlock. We should always wait for all coroutines to complete and use tools such as sync.WaitGroup to ensure safe exit of coroutines.

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