sync.RWMutex

gistfile1.go

package main

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

var (
	writeBeforeRead bool
)

func init() {
	flag.BoolVar(&writeBeforeRead, "write-before-read", false, "")
}

func main() {
	flag.Parse()

	var m sync.RWMutex
	readlocked := make(chan int)
	var wg sync.WaitGroup

	// hold read lock first
	wg.Add(1)
	go func() {
		defer wg.Done()
		m.RLock()
		fmt.Printf("goroutine 1: read locked\n")
		close(readlocked)
		time.Sleep(3 * time.Second)
		m.RUnlock()
		fmt.Printf("goroutine 1: read unlocked\n")
	}()

	// this want to write
	wg.Add(1)
	go func() {
		defer wg.Done()
		<-readlocked
		if !writeBeforeRead {
			// wait a while, let reader to get lock first
			time.Sleep(1 * time.Second)
		}
		fmt.Printf("goroutine 2: trying to lock write\n")
		m.Lock()
		fmt.Printf("goroutine 2: write locked\n")
		time.Sleep(3 * time.Second)
		m.Unlock()
		fmt.Printf("goroutine 2: write unlocked\n")
	}()

	// this want to read
	wg.Add(1)
	go func() {
		defer wg.Done()
		<-readlocked
		if writeBeforeRead {
			// wait a while, let writer to get lock first
			time.Sleep(1 * time.Second)
		}
		fmt.Printf("goroutine 3: trying to lock read\n")
		m.RLock()
		fmt.Printf("goroutine 3: read locked\n")
		time.Sleep(3 * time.Second)
		m.RUnlock()
		fmt.Printf("goroutine 3: read unlocked\n")
	}()

	wg.Wait()
}