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"
 }