fix data race with multiple map writers

When the client is initialized with params, the normalizeUser function
will concurrently write the params map. This causes the go runtime to
panic saying "fatal error: concurrent map writes".

This change fixes the data race by always copying the map when
normalizing the user.

client.go

@@ -211,13 +211,11 @@ func fetchConfig(user User, configName string, t *transport) configResponse {
 }
 
 func normalizeUser(user User, options Options) User {
-	var env map[string]string
-	if len(options.Environment.Params) > 0 {
-		env = options.Environment.Params
-	} else {
-		env = make(map[string]string)
+	env := make(map[string]string)
+	// Copy to avoid data race. We modify the map below.
+	for k, v := range options.Environment.Params {
+		env[k] = v
 	}
-
 	if options.Environment.Tier != "" {
 		env["tier"] = options.Environment.Tier
 	}

client_test.go

@@ -0,0 +1,34 @@
+package statsig
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestNormalizeUserDataRace(t *testing.T) {
+	const (
+		goroutines = 10
+		duration   = time.Second
+	)
+	options := Options{
+		Environment: Environment{
+			Params: map[string]string{
+				"foo": "bar",
+			},
+			Tier: "awesome",
+		},
+	}
+	start := time.Now()
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+	for g := 0; g < goroutines; g++ {
+		go func() {
+			defer wg.Done()
+			for time.Since(start) < duration {
+				normalizeUser(User{UserID: "cruise-llc"}, options)
+			}
+		}()
+	}
+	wg.Wait()
+}