// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
// which must be held when changing the condition and
// when calling the Wait method.
//
// A Cond must not be copied after first use.
type Cond struct {
        noCopy noCopy

        // L is held while observing or changing the condition
        L Locker

        notify  notifyList
        checker copyChecker
}

func NewCond(l Locker) *Cond

cadence := sync.NewCond(sync.Mutex{})

// Broadcast wakes all goroutines waiting on c.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Broadcast()

go func() {
   for range time.Tick(1 * time.Millisecond) {
      cadence.Broadcast()
   }
}()

// Signal wakes one goroutine waiting on c, if there is any.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Signal()

// Wait atomically unlocks c.L and suspends execution
// of the calling goroutine. After later resuming execution,
// Wait locks c.L before returning. Unlike in other systems,
// Wait cannot return unless awoken by Broadcast or Signal.
//
// Because c.L is not locked when Wait first resumes, the caller
// typically cannot assume that the condition is true when
// Wait returns. Instead, the caller should Wait in a loop:
//
//    c.L.Lock()
//    for !condition() {
//        c.Wait()
//    }
//    ... make use of condition ...
//    c.L.Unlock()
//
func (c *Cond) Wait()

c.L.Lock()
for !condition() {
    c.Wait()
}
... make use of condition ...
c.L.Unlock()

package sync

import (
   "log"
   "sync"
   "testing"
   "time"
)

var done = false

func read(name string, c *sync.Cond) {
   c.L.Lock()
   for !done {
      c.Wait()
   }
   log.Println(name, "starts reading")
   c.L.Unlock()
}

func write(name string, c *sync.Cond) {
   log.Println(name, "starts writing")
   time.Sleep(time.Second)
   c.L.Lock()
   done = true
   c.L.Unlock()
   log.Println(name, "wakes all")
   c.Broadcast()
}

func TestSyncCond(t *testing.T) {
   cond := sync.NewCond(sync.Mutex{})

   go read("reader1", cond)
   go read("reader2", cond)
   go read("reader3", cond)
   write("writer", cond)

   time.Sleep(time.Second * 3)
}