fix: Double GZIP.

middlewares/compress.go

@@ -6,12 +6,26 @@ import (
 	"github.com/NYTimes/gziphandler"
 )
 
-// Compress is a middleware that allows redirections
-type Compress struct {
+const (
+	contentEncodingHeader = "Content-Encoding"
+)
+
+// Compress is a middleware that allows redirection
+type Compress struct{}
+
+// ServerHTTP is a function used by Negroni
+func (c *Compress) ServeHTTP(rw http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
+	if isEncoded(r.Header) {
+		next.ServeHTTP(rw, r)
+	} else {
+		newGzipHandler := gziphandler.GzipHandler(next)
+		newGzipHandler.ServeHTTP(rw, r)
+	}
 }
 
-// ServerHTTP is a function used by negroni
-func (c *Compress) ServeHTTP(rw http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
-	newGzipHandler := gziphandler.GzipHandler(next)
-	newGzipHandler.ServeHTTP(rw, r)
+func isEncoded(headers http.Header) bool {
+	header := headers.Get(contentEncodingHeader)
+	// According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding,
+	// content is not encoded if the header 'Content-Encoding' is empty or equals to 'identity'.
+	return header != "" && header != "identity"
 }

middlewares/compress_test.go

@@ -0,0 +1,69 @@
+package middlewares
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/NYTimes/gziphandler"
+	"github.com/containous/traefik/testhelpers"
+	"github.com/stretchr/testify/assert"
+)
+
+const (
+	acceptEncodingHeader = "Accept-Encoding"
+	varyHeader           = "Vary"
+	gzip                 = "gzip"
+)
+
+func TestShouldCompressWhenNoContentEncodingHeader(t *testing.T) {
+	handler := &Compress{}
+
+	req := testhelpers.MustNewRequest(http.MethodGet, "http://localhost", nil)
+	req.Header.Add(acceptEncodingHeader, gzip)
+
+	baseBody := generateBytes(gziphandler.DefaultMinSize)
+	next := func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write(baseBody)
+	}
+	rw := httptest.NewRecorder()
+
+	handler.ServeHTTP(rw, req, next)
+
+	assert.Equal(t, gzip, rw.Header().Get(contentEncodingHeader))
+	assert.Equal(t, acceptEncodingHeader, rw.Header().Get(varyHeader))
+
+	if assert.ObjectsAreEqualValues(rw.Body.Bytes(), baseBody) {
+		assert.Fail(t, "expected a compressed body", "got %v", rw.Body.Bytes())
+	}
+}
+
+func TestShouldNotCompressWhenContentEncodingHeader(t *testing.T) {
+	handler := &Compress{}
+
+	req := testhelpers.MustNewRequest(http.MethodGet, "http://localhost", nil)
+	req.Header.Add(acceptEncodingHeader, gzip)
+	req.Header.Add(contentEncodingHeader, gzip)
+
+	baseBody := generateBytes(gziphandler.DefaultMinSize)
+
+	next := func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write(baseBody)
+	}
+
+	rw := httptest.NewRecorder()
+	handler.ServeHTTP(rw, req, next)
+
+	assert.Equal(t, "", rw.Header().Get(contentEncodingHeader))
+	assert.Equal(t, "", rw.Header().Get(varyHeader))
+
+	assert.EqualValues(t, rw.Body.Bytes(), baseBody)
+}
+
+func generateBytes(len int) []byte {
+	var value []byte
+	for i := 0; i < len; i++ {
+		value = append(value, 0x61)
+	}
+	return value
+}