How to Avoid Deadlock When Using sync.Cond in Go?

How to Effectively Utilize sync.Cond

Understanding the Problem

When working with sync.Cond, it's crucial to be aware of the potential race condition between locking and invoking the Wait method. In the example provided:

import (
    "sync"
    "time"
)

func main() {
    m := sync.Mutex{}
    c := sync.NewCond(&m)
    go func() {
        time.Sleep(1 * time.Second)
        c.Broadcast()
    }()
    m.Lock()
    time.Sleep(2 * time.Second)
    c.Wait()
}

The main goroutine intentionally introduces a delay between locking the mutex and calling c.Wait(). This simulates a race condition that triggers a deadlock panic upon execution.

Resolving the Race Condition

To avoid this issue, it's essential to acquire the lock explicitly before calling c.Wait(). By doing so, we ensure that the Cond waits only when the corresponding mutex is locked.

When to Use sync.Cond

Determining whether sync.Cond is the appropriate synchronization primitive for your scenario is equally important. While it's suitable for scenarios where multiple readers may wait for shared resources to become available, a simpler sync.Mutex might suffice for one-to-one write and read operations between goroutines.

Using Channels as an Alternative

In some cases, channels provide a more efficient method of data exchange compared to sync.Cond. For instance, in the above example, a channel can be employed to signal when the HTTP headers become available. This approach avoids the need for locking and waiting, resulting in improved performance.

Example: Using sync.Cond

If sync.Cond is the preferred approach, consider the following example:

var sharedRsc = make(map[string]interface{})
func main() {
    var wg sync.WaitGroup
    wg.Add(2)
    m := sync.Mutex{}
    c := sync.NewCond(&m)
    go func() {
        c.L.Lock()
        for len(sharedRsc) == 0 {
            c.Wait()
        }
        fmt.Println(sharedRsc["rsc1"])
        c.L.Unlock()
        wg.Done()
    }()

    go func() {
        c.L.Lock()
        for len(sharedRsc) == 0 {
            c.Wait()
        }
        fmt.Println(sharedRsc["rsc2"])
        c.L.Unlock()
        wg.Done()
    }()

    c.L.Lock()
    sharedRsc["rsc1"] = "foo"
    sharedRsc["rsc2"] = "bar"
    c.Broadcast()
    c.L.Unlock()
    wg.Wait()
}

Conclusion

While sync.Cond offers a solution for data sharing and synchronization, it's essential to carefully consider the suitability of this primitive for your specific requirements. Understanding the potential race condition and alternative synchronization mechanisms can help you utilize sync.Cond effectively in your Go applications.

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