Example explaining RWMutex composition on a struct

lockcomposition.go

package main

import (
	"fmt"
	"sync"
	"net/url"
	"math/rand"
	"strconv"
	"time"
)

type MutexHolder struct {
	sync.RWMutex
	Fields url.Values	
}

func main() {
	start := time.Now()
	var wg sync.WaitGroup
	wait := 3 * time.Second
	
	for i:=0; i<5; i++ {
	   m := MutexHolder {
		   Fields: map[string][]string{"key": {"value", "value2"}},
	   }
	
	   wg.Add(3)
	   go printByReference(&m, i, wait, &wg)
	   go printByReference(&m, i, wait, &wg)
           go printByReference(&m, i, wait, &wg)
	   // Goroutines will finish in `wait * count(printByReference calls)` seconds because RWMutex is shared
		
	   //wg.Add(3)	
	   //go printByValue(m, i, wait, &wg)
	   //go printByValue(m, i, wait, &wg)
	   //go printByValue(m, i, wait, &wg)	   
	   // Goroutines will finish in `wait` seconds because RWMutex is shared
	}
	
	wg.Wait()
	fmt.Println("Took: ", time.Since(start))
}

func printByReference(mh *MutexHolder, i int, wait time.Duration, wg *sync.WaitGroup) {
	defer wg.Done()
	
	mh.Lock()
        time.Sleep(wait)
	mh.Fields.Add("RandInt", strconv.Itoa(rand.Intn(100)))
	mh.Fields.Add("MyAddress", fmt.Sprintf("%p", &mh))
	mh.Fields.Set("LoopIndex", strconv.Itoa(i))

	fmt.Println(mh)
	mh.Unlock()
}

func printByValue(mh MutexHolder, i int, wait time.Duration, wg *sync.WaitGroup) {
	defer wg.Done()
	
	mh.Lock()
        time.Sleep(wait)
	mh.Fields.Add("RandInt", strconv.Itoa(rand.Intn(100)))
	mh.Fields.Add("MyAddress", fmt.Sprintf("%p", &mh))
	mh.Fields.Set("LoopIndex", strconv.Itoa(i))
	
	fmt.Println(mh)
	mh.Unlock()
}