check payload size before hitting Apple (#67)

closes #65

it was documented and I tested it to be 4096 bytes

push/service.go

@@ -21,6 +21,8 @@ const (
 	Production2197  = "https://api.push.apple.com:2197"
 )
 
+const maxPayload = 4096 // 4KB at most
+
 // Service is the Apple Push Notification Service that you send notifications to.
 type Service struct {
 	Host   string
@@ -52,6 +54,14 @@ func NewClient(cert tls.Certificate) (*http.Client, error) {
 
 // Push sends a notification and waits for a response.
 func (s *Service) Push(deviceToken string, headers *Headers, payload []byte) (string, error) {
+	// check payload length before even hitting Apple.
+	if len(payload) > maxPayload {
+		return "", &Error{
+			Reason: ErrPayloadTooLarge,
+			Status: http.StatusRequestEntityTooLarge,
+		}
+	}
+
 	urlStr := fmt.Sprintf("%v/3/device/%v", s.Host, deviceToken)
 
 	req, err := http.NewRequest("POST", urlStr, bytes.NewReader(payload))

push/service_test.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -141,3 +142,24 @@ func TestTimestampError(t *testing.T) {
 		t.Errorf("Expected timestamp %v, got %v.", expected, e.Timestamp)
 	}
 }
+
+func TestPayloadTooLarge(t *testing.T) {
+	payload := []byte(strings.Repeat("0123456789abcdef", 256) + "x")
+
+	service := push.NewService(http.DefaultClient, "host")
+	_, err := service.Push("device-token", nil, payload)
+	if err == nil {
+		t.Fatal("Expected error, got none")
+	}
+	if _, ok := err.(*push.Error); !ok {
+		t.Fatalf("Expected push error, got %v.", err)
+	}
+
+	e := err.(*push.Error)
+	if e.Reason != push.ErrPayloadTooLarge {
+		t.Errorf("Expected PayloadTooLarge, got reason %q.", e.Reason)
+	}
+	if e.Status != http.StatusRequestEntityTooLarge {
+		t.Errorf("Expected status %v, got %v.", http.StatusRequestEntityTooLarge, e.Status)
+	}
+}