Race Condition

race.go

package main

import (
	"log"
	"time"
)

type Inner struct {
	Count int
}

var cachedInner = map[string]*Inner{}

const KEY = "key"

func Get() *Inner {
	var d *Inner
	var ok bool

	d, ok = cachedInner[KEY]
	if !ok {
		d = getInner()
	}

	return d
}

func getInner() *Inner {
	log.Println("Generating Inner for Thread")
	data := &Inner{}
	cachedInner[KEY] = data
	return data
}

func process() {
	inner := Get()
	inner.Count++
	log.Println(inner.Count)
}

func main() {
	log.SetFlags(log.Ldate | log.Ltime | log.Llongfile)

	go process()
	go process()

	time.Sleep(2 * time.Second)
}

Race Condition output:

piyush:~ [] $ go run -race /tmp/sample.go 
2015/04/04 08:38:21 /tmp/sample.go:29: Generating Inner for Thread
2015/04/04 08:38:21 /tmp/sample.go:38: 1
==================
WARNING: DATA RACE
Read by goroutine 6:
  runtime.mapaccess2_faststr()
      /usr/local/Cellar/go/1.4.2/libexec/src/runtime/hashmap_fast.go:281 +0x0
  main.Get()
      /tmp/sample.go:20 +0x7f
  main.process()
      /tmp/sample.go:36 +0x2c

Previous write by goroutine 5:
  runtime.mapassign1()
      /usr/local/Cellar/go/1.4.2/libexec/src/runtime/hashmap.go:383 +0x0
  main.getInner()
      /tmp/sample.go:31 +0x19f
  main.Get()
      /tmp/sample.go:22 +0xaf
  main.process()
      /tmp/sample.go:36 +0x2c

Goroutine 6 (running) created at:
  main.main()
      /tmp/sample.go:45 +0x55

Goroutine 5 (finished) created at:
  main.main()
      /tmp/sample.go:44 +0x44
==================
==================
WARNING: DATA RACE
Read by goroutine 6:
  main.process()
      /tmp/sample.go:37 +0x3f

Previous write by goroutine 5:
  main.getInner()
      /tmp/sample.go:30 +0x132
  main.Get()
      /tmp/sample.go:22 +0xaf
  main.process()
      /tmp/sample.go:36 +0x2c

Goroutine 6 (running) created at:
  main.main()
      /tmp/sample.go:45 +0x55

Goroutine 5 (finished) created at:
  main.main()
      /tmp/sample.go:44 +0x44
==================
2015/04/04 08:38:21 /tmp/sample.go:38: 2
^Cexit status 2