How to correctly use sync.Cond?

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 method. This example adds an artificial delay between the two lines in the main goroutine to simulate the race condition:

package main

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()
}

^{[Run on the Go Playground]}

This causes an immediate panic:

<pre>fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Syncsemacquire(0x10330208, 0x1)
/usr/local/go/src/runtime/sema.go:241 +0x2e0
sync.(*Cond).Wait(0x10330200, 0x0)
/usr/local/go/src/sync/cond.go:63 +0xe0
main.main()
/tmp/sandbox301865429/main.go:17 +0x1a0</pre>

What am I doing wrong? How do I avoid this apparent race condition? Is there a better synchronization construct I should be using?

Edit: I realize I should have better explained the problem I'm trying to solve here. I have a long-running goroutine that downloads a large file and a number of other goroutines that need access to the HTTP headers when they are available. This problem is harder than it sounds.

I can't use channels since only one goroutine would then receive the value. And some of the other goroutines would be trying to retrieve the headers long after they are already available.

The downloader goroutine could simply store the HTTP headers in a variable and use a mutex to safeguard access to them. However, this doesn't provide a way for the other goroutines to "wait" for them to become available.

I had thought that both a sync.Mutex and sync.Cond together could accomplish this goal but it appears that this is not possible.

OP answered his own, but did not directly answer the original question, I am going to post how to correctly use sync.Cond.

You do not really need sync.Cond if you have one goroutine for each write and read - a single sync.Mutex would suffice to communicate between them. sync.Cond could useful in situations where multiple readers wait for the shared resources to be available.

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

    go func() {
        // this go routine wait for changes to the sharedRsc
        c.L.Lock()
        for len(sharedRsc) == 0 {
            c.Wait()
        }
        fmt.Println(sharedRsc["rsc2"])
        c.L.Unlock()
        wg.Done()
    }()

    // this one writes changes to sharedRsc
    c.L.Lock()
    sharedRsc["rsc1"] = "foo"
    sharedRsc["rsc2"] = "bar"
    c.Broadcast()
    c.L.Unlock()
    wg.Wait()
}

Playground

Having said that, using channels is still the recommended way to pass data around if the situation permitting.

Note: sync.WaitGroup here is only used to wait for the goroutines to complete their executions.

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

package main

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

func main() {
	m := sync.Mutex{}
	m.Lock() // main gouroutine is owner of lock
	c := sync.NewCond(&m)
	go func() {
		m.Lock() // obtain a lock
		defer m.Unlock()
		fmt.Println("3. goroutine is owner of lock")
		time.Sleep(2 * time.Second) // long computing - because you are the owner, you can change state variable(s)
		c.Broadcast()               // State has been changed, publish it to waiting goroutines
		fmt.Println("4. goroutine will release lock soon (deffered Unlock")
	}()
	fmt.Println("1. main goroutine is owner of lock")
	time.Sleep(1 * time.Second) // initialization
	fmt.Println("2. main goroutine is still lockek")
	c.Wait() // Wait temporarily release a mutex during wating and give opportunity to other goroutines to change the state.
	// Because you don't know, whether this is state, that you are waiting for, is usually called in loop.
	m.Unlock()
	fmt.Println("Done")
}

http://play.golang.org/p/fBBwoL7_pm

Looks like you c.Wait for Broadcast which would never happens with your time intervals.
With

time.Sleep(3 * time.Second) //Broadcast after any Wait for it
c.Broadcast()

your snippet seems to work http://play.golang.org/p/OE8aP4i6gY .Or am I missing something that you try to achive?

I finally discovered a way to do this and it doesn't involve sync.Cond at all - just the mutex.

type Task struct {
    m       sync.Mutex
    headers http.Header
}

func NewTask() *Task {
    t := &Task{}
    t.m.Lock()
    go func() {
        defer t.m.Unlock()
        // ...do stuff...
    }()
    return t
}

func (t *Task) WaitFor() http.Header {
    t.m.Lock()
    defer t.m.Unlock()
    return t.headers
}

How does this work?

The mutex is locked at the beginning of the task, ensuring that anything calling WaitFor() will block. Once the headers are available and the mutex unlocked by the goroutine, each call to WaitFor() will execute one at a time. All future calls (even after the goroutine ends) will have no problem locking the mutex, since it will always be left unlocked.

Yes you can use one channel to pass Header to multiple Go routines.

headerChan := make(chan http.Header)

go func() { // This routine can be started many times
	header := <-headerChan	// Wait for header
	// Do things with the header
}()

// Feed the header to all waiting go routines
for more := true; more; {
	select {
	case headerChan <- r.Header:
	default: more = false
	}
}

This can be done with channels pretty easily and the code will be clean. Below is the example. Hope this helps!

package main

import (
	"fmt"
	"net/http"
	"sync"
)

func main() {
	done := make(chan struct{})
	var wg sync.WaitGroup
	// fork required number of goroutines
	for i := 0; i < 5; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			<-done
			fmt.Println("read the http headers from here")
		}()
	}
	time.Sleep(1) //download your large file here
	fmt.Println("Unblocking goroutines...")
	close(done) // this will unblock all the goroutines
	wg.Wait()
}

The problem in your code was that your signal was emitted once and a receiving go routine was not ready for it so the signal was missed. You should do broadcasting in a loop.

   package main
    
    import (
    	"sync"
    	"time"
    )
    
    func main() {
    	m := sync.Mutex{}
    	c := sync.NewCond(&m)
    	go func() {
    		time.Sleep(1 * time.Second)
    		for range time.Tick(time.Millisecond) {
    			c.Broadcast()
    		}
    	}()
    	m.Lock()
    	time.Sleep(2 * time.Second)
    	c.Wait()
    	m.Unlock()
    
    	//do stuff
    }

In the excellent book "Concurrency in Go" they provide the following easy solution while leveraging the fact that a channel that is closed will release all waiting clients.

package main
import (
    "fmt"
	"time"
)
func main() {
	httpHeaders := []string{}
	headerChan := make(chan interface{})
	var consumerFunc= func(id int, stream <-chan interface{}, funcHeaders *[]string)         
    {
		<-stream
		fmt.Println("Consumer ",id," got headers:", funcHeaders )	
	}
	for i:=0;i<3;i++ {
		go consumerFunc(i, headerChan, &httpHeaders)
	}
	fmt.Println("Getting headers...")
	time.Sleep(2*time.Second)
	httpHeaders=append(httpHeaders, "test1");
	fmt.Println("Publishing headers...")
	close(headerChan )
	time.Sleep(5*time.Second)
}

https://play.golang.org/p/cE3SiKWNRIt