golang读写锁

golang读写锁，其特征在于

1. 读锁：可以同时进行多个协程读操作，不允许写操作
2. 写锁：只允许同时有一个协程进行写操作，不允许其他写操作和读操作

读写锁共有四个方法

1. RLock：获取读锁
2. RUnLock：释放读锁
3. Lock：获取写锁
4. UnLock：释放写锁

使用例子如下

package main

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

var gRWLock *sync.RWMutex

var gVar int

func init() {
    gRWLock = new(sync.RWMutex)
    gVar = 1
}

func main() {
    var wg sync.WaitGroup
    go Read(1, &wg)
    wg.Add(1)
    go Write(1, &wg)
    wg.Add(1)
    go Read(2, &wg)
    wg.Add(1)
    go Read(3, &wg)
    wg.Add(1)

    wg.Wait()
}

func Read(id int, wg *sync.WaitGroup) {
    fmt.Printf("Read Coroutine: %d start
", id)
    defer fmt.Printf("Read Coroutine: %d end
", id)
    gRWLock.RLock()
    fmt.Printf("gVar %d
", gVar)
    time.Sleep(time.Second)
    gRWLock.RUnlock()

    wg.Done()

}

func Write(id int, wg *sync.WaitGroup) {
    fmt.Printf("Write Coroutine: %d start
", id)
    defer fmt.Printf("Write Coroutine: %d end
", id)
    gRWLock.Lock()
    gVar = gVar + 100
    fmt.Printf("gVar %d
", gVar)
    time.Sleep(time.Second)
    gRWLock.Unlock()
    wg.Done()

}