feat: use atomic pointer for global feature flag client (#332)

Author: cbosss

featureflag/global.go

@@ -1,27 +1,27 @@
 package featureflag
 
 import (
-	"sync"
-
 	"github.com/sirupsen/logrus"
+	"go.uber.org/atomic"
+	"unsafe"
 )
 
-var globalLock sync.Mutex
-var globalClient Client = MockClient{}
+// See https://blog.dubbelboer.com/2015/08/23/rwmutex-vs-atomicvalue-vs-unsafepointer.html
+var (
+	defaultClient Client = MockClient{}
+	globalClient         = atomic.NewUnsafePointer(unsafe.Pointer(&defaultClient))
+)
 
 func SetGlobalClient(client Client) {
 	if client == nil {
 		return
 	}
-	globalLock.Lock()
-	globalClient = client
-	globalLock.Unlock()
+	globalClient.Store(unsafe.Pointer(&client))
 }
 
 func GetGlobalClient() Client {
-	globalLock.Lock()
-	defer globalLock.Unlock()
-	return globalClient
+	c := (*Client)(globalClient.Load())
+	return *c
 }
 
 // Init will initialize global client with a launch darkly client

featureflag/global_test.go

@@ -0,0 +1,16 @@
+package featureflag
+
+import (
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestGlobalAccess(t *testing.T) {
+	// initial value should be default
+	require.Equal(t, defaultClient, GetGlobalClient())
+
+	// setting new global should be reflected
+	n := &ldClient{}
+	SetGlobalClient(n)
+	require.Equal(t, n, GetGlobalClient())
+}

go.mod

@@ -36,6 +36,7 @@ require (
 	github.com/tidwall/pretty v1.0.1 // indirect
 	github.com/xtgo/uuid v0.0.0-20140804021211-a0b114877d4c // indirect
 	go.mongodb.org/mongo-driver v1.9.0
+	go.uber.org/atomic v1.9.0
 	golang.org/x/net v0.0.0-20200813134508-3edf25e44fcc
 	gopkg.in/DataDog/dd-trace-go.v1 v1.34.0
 	gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f // indirect

go.sum

@@ -389,6 +389,8 @@ go.mongodb.org/mongo-driver v1.9.0/go.mod h1:0sQWfOeY63QTntERDJJ/0SuKK0T1uVSgKCu
 go.opencensus.io v0.21.0/go.mod h1:mSImk1erAIZhrmZN+AvHh14ztQfjbGwt4TtuofqLduU=
 go.opencensus.io v0.22.0/go.mod h1:+kGneAE2xo2IficOXnaByMWTGM9T73dGwxeWcUqIpI8=
 go.uber.org/atomic v1.4.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
+go.uber.org/atomic v1.9.0 h1:ECmE8Bn/WFTYwEW/bpKD3M8VtR/zQVbavAoalC1PYyE=
+go.uber.org/atomic v1.9.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
 go.uber.org/multierr v1.1.0/go.mod h1:wR5kodmAFQ0UK8QlbwjlSNy0Z68gJhDJUG5sjR94q/0=
 go.uber.org/zap v1.10.0/go.mod h1:vwi/ZaCAaUcBkycHslxD9B2zi4UTXhF60s6SWpuDF0Q=
 golang.org/x/crypto v0.0.0-20180904163835-0709b304e793/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=