如何正确使用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 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:

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

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.

Answer 1

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.