如何运行goroutine的单个实例

Question

I have a goroutine that will be run multiple times. But it can only run one at a time (single instance). What is the correct/idiomatic way to make sure a certain goroutine can run only one at a time?

Here is my contrived example code to illustrate the point:

func main() {
    // Contrived example!!!!!!
    // theCaller() may be run at multiple, unpredictable times
    // theJob() must only be run one at a time
    go theCaller()
    go theCaller()
    go theCaller()
}

func theCaller() {
    if !jobIsRunning { // race condition here!
        jobIsRunning = true
        go theJob()
    }
}

var jobIsRunning bool

// Can run multiple times, but only one at a time
func theJob() {
    defer jobDone()
    do_something()
}

func jobDone() {
    jobIsRunning = false
}

Answer 1

Based on question and other comments from the OP, it looks like the goal is to start a new job if and only if a job is not already running.

Use a boolean variable protected by a sync.Mutex to record the running state of of the job. Set the variable to true when starting a job and to false when the job completes. Test this variable to determine if a job should be started.

var (
    jobIsRunning   bool
    JobIsrunningMu sync.Mutex
)

func maybeStartJob() {
    JobIsrunningMu.Lock()
    start := !jobIsRunning
    jobIsRunning = true
    JobIsrunningMu.Unlock()
    if start {
        go func() {
            theJob()
            JobIsrunningMu.Lock()
            jobIsRunning = false
            JobIsrunningMu.Unlock()
        }()
    }
}

func main() {
    maybeStartJob()
    maybeStartJob()
    maybeStartJob()
}

The lower-level sync/atomic package can also be used and may have better performance than using a mutex.

var jobIsRunning uint32

func maybeStartJob() {
    if atomic.CompareAndSwapUint32(&jobIsRunning, 0, 1) {
        go func() {
            theJob()
            atomic.StoreUint32(&jobIsRunning, 0)
        }()
    }
}

The sync/atomic package documentation warns that the functions in the package require great care to use correctly and that most applications should use the sync package.