Gocondition variable
type Cond struct {
        // L is held while observing or changing the condition
        L Locker
        // contains filtered or unexported fields
}
type Cond
    func NewCond(l Locker) *Cond
    func (c *Cond) Broadcast()
    func (c *Cond) Signal()
    func (c *Cond) Wait()
type Locker

type Locker interface {
        Lock()
        Unlock()
}
A Locker represents an object that can be locked and unlocked.

NewCond()LockerBroadcast()condition variablegoroutineSignal()goroutineWait()goroutinecondition variable
c.L.Lock()
for !condition() {
    c.Wait()
}
... make use of condition ...
c.L.Unlock()

Wait()Wait()wait()goroutinegoroutine
package main

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

func main() {
    c := sync.NewCond(&sync.Mutex{})
    var num int

    for i := 1; i <= 2; i++ {
        go func(id int) {
            fmt.Println("Enter Thread ID:", id)
            c.L.Lock()
            for num != 1 {
                fmt.Println("Enter loop: Thread ID:", id)
                c.Wait()
                fmt.Println("Exit loop: Thread ID:", id)
            }
            num++
            c.L.Unlock()
            fmt.Println("Exit Thread ID:", id)
        }(i)
    }

    time.Sleep(time.Second)
    fmt.Println("Sleep 1 second")

    num++
    c.Broadcast()
    time.Sleep(time.Second)
    fmt.Println("Program exit")
}

一次执行结果如下：

Enter Thread ID: 2
Enter loop: Thread ID: 2
Enter Thread ID: 1
Enter loop: Thread ID: 1
Sleep 1 second
Exit loop: Thread ID: 2
Exit Thread ID: 2
Exit loop: Thread ID: 1
Enter loop: Thread ID: 1
Program exit

goroutine 2goroutine 1goroutinecondition variable

参考资料：
Package sync；
Condition Variables。