在Go中返回锁定互斥锁的好方法

Question

Following problem: I have a function that only should allow one caller to execute. If someone tries to call the function and it is already busy the second caller should immediatly return with an error.

I tried the following:

1. Use a mutex

Would be pretty easy. But the problem is, you cannot check if a mutex is locked. You can only block on it. Therefore it does not work

2. Wait on a channel

var canExec = make(chan bool, 1)

func init() {
    canExec <- true
}

func onlyOne() error {
    select {
    case <-canExec:
    default:
        return errors.New("already busy")
    }

    defer func() {
        fmt.Println("done")
        canExec <- true
    }()

    // do stuff

}

What I don't like here:

• looks really messi
• if easy to mistakenly block on the channel / mistakenly write to the channel

3. Mixture of mutex and shared state

var open = true

var myMutex *sync.Mutex

func canExec() bool {
    myMutex.Lock()
    defer myMutex.Unlock()

    if open {
        open = false
        return true
    }

    return false
}

func endExec() {
    myMutex.Lock()
    defer myMutex.Unlock()

    open = true
}

func onlyOne() error {
    if !canExec() {
        return errors.New("busy")
    }
    defer endExec()

    // do stuff

    return nil
}

I don't like this either. Using a shard variable with mutex is not that nice.

Any other idea?

Answer 1

You can use a semaphore for this (go get golang.org/x/sync/semaphore)

package main

import (
    "errors"
    "fmt"
    "sync"
    "time"

    "golang.org/x/sync/semaphore"
)

var sem = semaphore.NewWeighted(1)

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            if err := onlyOne(); err != nil {
                fmt.Println(err)
            }
        }()
        time.Sleep(time.Second)
    }
    wg.Wait()
}

func onlyOne() error {
    if !sem.TryAcquire(1) {
        return errors.New("busy")
    }
    defer sem.Release(1)
    fmt.Println("working")
    time.Sleep(5 * time.Second)
    return nil
}