How to correctly use sync.Cond?

Question

I\'m having trouble figuring out how to correctly use sync.Cond. From what I can tell, a race condition exists between locking the Locker and invoking the condition\'s Wait meth

Answer 1

Here's a practical example with two go routines. They start one after another but the second one waits on a condition which is broadcast by the first one before proceeding:

package main

import (
    "sync"
    "fmt"
    "time"
)

func main() {
    lock := sync.Mutex{}
    lock.Lock()

    cond := sync.NewCond(&lock)

    waitGroup := sync.WaitGroup{}
    waitGroup.Add(2)

    go func() {
        defer waitGroup.Done()

        fmt.Println("First go routine has started and waits for 1 second before broadcasting condition")

        time.Sleep(1 * time.Second)

        fmt.Println("First go routine broadcasts condition")

        cond.Broadcast()
    }()

    go func() {
        defer waitGroup.Done()

        fmt.Println("Second go routine has started and is waiting on condition")

        cond.Wait()

        fmt.Println("Second go routine unlocked by condition broadcast")
    }()

    fmt.Println("Main go routine starts waiting")

    waitGroup.Wait()

    fmt.Println("Main go routine ends")
}

Output may vary slightly as the second go routine could start before the first one and viceversa:

Main go routine starts waiting
Second go routine has started and is waiting on condition
First go routine has started and waits for 1 second before broadcasting condition
First go routine broadcasts condition
Second go routine unlocked by condition broadcast
Main go routine ends

https://gist.github.com/fracasula/21565ea1cf0c15726ca38736031edc70

Answer 2

You need to make sure that c.Broadcast is called after your call to c.Wait. The correct version of your program would be:

package main

import (
    "fmt"
    "sync"
)

func main() {
    m := &sync.Mutex{}
    c := sync.NewCond(m)
    m.Lock()
    go func() {
        m.Lock() // Wait for c.Wait()
        c.Broadcast()
        m.Unlock()
    }()
    c.Wait() // Unlocks m, waits, then locks m again
    m.Unlock()
}

https://play.golang.org/p/O1r8v8yW6h