Updated with PR comments

Makefile

@@ -1,7 +1,7 @@
 all: test bench vet check-gofmt
 
 bench:
-	go test --race -bench . -run "Benchmark" ./form
+	go test -race -bench . -run "Benchmark" ./form
 
 build:
 	go build ./...
@@ -10,7 +10,7 @@ check-gofmt:
 	scripts/check_gofmt.sh
 
 test:
-	go test --race ./...
+	go test -race ./...
 
 vet:
 	go vet ./...

error_test.go

@@ -23,7 +23,9 @@ func TestErrorResponse(t *testing.T) {
 	defer ts.Close()
 
 	SetBackend("api", &BackendConfiguration{
-		Type: APIBackend, URL: ts.URL, HTTPClient: &http.Client{},
+		APIBackend,
+		ts.URL,
+		&http.Client{},
 	})
 
 	err := GetBackend(APIBackend).Call("GET", "/v1/account", "sk_test_badKey", nil, nil, nil)

stripe.go

@@ -76,7 +76,6 @@ type Backend interface {
 
 // BackendConfiguration is the internal implementation for making HTTP calls to Stripe.
 type BackendConfiguration struct {
-	mu         sync.Mutex
 	Type       SupportedBackend
 	URL        string
 	HTTPClient *http.Client
@@ -103,6 +102,7 @@ const (
 // Backends are the currently supported endpoints.
 type Backends struct {
 	API, Uploads Backend
+	mu           sync.RWMutex
 }
 
 // stripeClientUserAgent contains information about the current runtime which
@@ -160,32 +160,40 @@ func SetHTTPClient(client *http.Client) {
 func NewBackends(httpClient *http.Client) *Backends {
 	return &Backends{
 		API: &BackendConfiguration{
-			Type: APIBackend, URL: APIURL, HTTPClient: httpClient},
+			APIBackend, APIURL, httpClient},
 		Uploads: &BackendConfiguration{
-			Type: UploadsBackend, URL: UploadsURL, HTTPClient: httpClient},
+			UploadsBackend, UploadsURL, httpClient},
 	}
 }
 
 // GetBackend returns the currently used backend in the binding.
 func GetBackend(backend SupportedBackend) Backend {
-	var ret Backend
 	switch backend {
 	case APIBackend:
-		if backends.API == nil {
-			backends.API = &BackendConfiguration{
-				Type: backend, URL: apiURL, HTTPClient: httpClient}
+		backends.mu.RLock()
+		ret := backends.API
+		backends.mu.RUnlock()
+		if ret != nil {
+			return ret
 		}
-
-		ret = backends.API
+		backends.mu.Lock()
+		defer backends.mu.Unlock()
+		backends.API = &BackendConfiguration{backend, apiURL, httpClient}
+		return backends.API
 	case UploadsBackend:
-		if backends.Uploads == nil {
-			backends.Uploads = &BackendConfiguration{
-				Type: backend, URL: uploadsURL, HTTPClient: httpClient}
+		backends.mu.RLock()
+		ret := backends.Uploads
+		backends.mu.RUnlock()
+		if ret != nil {
+			return ret
 		}
-		ret = backends.Uploads
+		backends.mu.Lock()
+		defer backends.mu.Unlock()
+		backends.Uploads = &BackendConfiguration{backend, uploadsURL, httpClient}
+		return backends.Uploads
 	}
 
-	return ret
+	return nil
 }
 
 // SetBackend sets the backend used in the binding.
@@ -241,8 +249,6 @@ func (s *BackendConfiguration) CallMultipart(method, path, key, boundary string,
 // NewRequest is used by Call to generate an http.Request. It handles encoding
 // parameters and attaching the appropriate headers.
 func (s *BackendConfiguration) NewRequest(method, path, key, contentType string, body io.Reader, params *Params) (*http.Request, error) {
-	s.mu.Lock()
-	defer s.mu.Unlock()
 	if !strings.HasPrefix(path, "/") {
 		path = "/" + path
 	}
@@ -298,8 +304,6 @@ func (s *BackendConfiguration) NewRequest(method, path, key, contentType string,
 // the backend's HTTP client to execute the request and unmarshals the response
 // into v. It also handles unmarshaling errors returned by the API.
 func (s *BackendConfiguration) Do(req *http.Request, v interface{}) error {
-	s.mu.Lock()
-	defer s.mu.Unlock()
 	if LogLevel > 1 {
 		Logger.Printf("Requesting %v %v%v\n", req.Method, req.URL.Host, req.URL.Path)
 	}