Go map Data Race

cache.go

package cache

import (
	"sync"
)

// UserCache stores a concurrent-safe cache of authenticated users
type UserCache struct {
	mu    sync.RWMutex // guards Cache
	Cache map[string]string
}

// Get gets a value from the cache, concurrent-safe
func (cache *UserCache) Get(key string) (string, bool) {
	cache.mu.RLock()
	defer cache.mu.RUnlock()
	value, ok := cache.Cache[key]
	return value, ok
}

// Set a key/value in the cache, concurrent-safe
func (cache *UserCache) Set(key, value string) {
	cache.mu.Lock()
	cache.Cache[key] = value
	cache.mu.Unlock()
}

cache_test.go

package cache

import (
	"testing"
	"time"
)

func TestUserCacheSet(t *testing.T) {
	cache := UserCache{Cache: make(map[string]string)}

	var tests = []struct {
		key, value string
	}{
		{"a", "A"},
		{"b", "B"},
		{"c", "C"},
	}

	// set values
	for _, test := range tests {
		// incorrect!
		/*
		go func() {
			cache.Set(test.key, test.value)
		}()
		*/
		
		// corrected
		go func(k, v string) {
			cache.Set(k, v)
		}(test.key, test.value)
	}

	time.Sleep(time.Second)
	for _, test := range tests {
		got, ok := cache.Get(test.key)
		if !ok {
			t.Errorf(
				"cache.Get(\"%s\") returned not ok, should have been ok",
				test.key,
			)
		}

		if got != test.value && ok {
			t.Errorf(
				"cache.Get(\"%s\") returned \"%s\", wanted \"%s\"",
				test.key, got, test.value,
			)
		}
	}
}

test.out

$ go test -race .
==================
WARNING: DATA RACE
Read by goroutine 7:
  github.com/micahhausler/cache.TestUserCacheSet.func1()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:22 +0x2e

Previous write by goroutine 6:
  github.com/micahhausler/cache.TestUserCacheSet()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:20 +0x2a6
  testing.tRunner()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:456 +0xdc

Goroutine 7 (running) created at:
  github.com/micahhausler/cache.TestUserCacheSet()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:23 +0x2f9
  testing.tRunner()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:456 +0xdc

Goroutine 6 (running) created at:
  testing.RunTests()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:561 +0xaa3
  testing.(*M).Run()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:494 +0xe4
  main.main()
      github.com/micahhausler/cache/_test/_testmain.go:54 +0x20f
==================
==================
WARNING: DATA RACE
Read by goroutine 7:
  github.com/micahhausler/cache.TestUserCacheSet.func1()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:22 +0x41

Previous write by goroutine 6:
  github.com/micahhausler/cache.TestUserCacheSet()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:20 +0x2a6
  testing.tRunner()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:456 +0xdc

Goroutine 7 (running) created at:
  github.com/micahhausler/cache.TestUserCacheSet()
      /Users/micahhausler/code/go/src/github.com/micahhausler/cache/cache_test.go:23 +0x2f9
  testing.tRunner()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:456 +0xdc

Goroutine 6 (running) created at:
  testing.RunTests()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:561 +0xaa3
  testing.(*M).Run()
      /usr/local/Cellar/go/1.5.2/libexec/src/testing/testing.go:494 +0xe4
  main.main()
      github.com/micahhausler/cache/_test/_testmain.go:54 +0x20f
==================
--- FAIL: TestUserCacheSet (1.00s)
	cache_test.go:33: cache.Get("a") returned not ok, should have been ok
	cache_test.go:33: cache.Get("b") returned not ok, should have been ok
FAIL
FAIL	github.com/micahhausler/cache	1.016s