fix: double compression.

vendor/github.com/NYTimes/gziphandler/gzip.go

@@ -3,6 +3,7 @@ package gziphandler
 import (
 	"bufio"
 	"compress/gzip"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -97,6 +98,7 @@ func (w *GzipResponseWriter) Write(b []byte) (int, error) {
 	}
 
 	// Save the write into a buffer for later use in GZIP responseWriter (if content is long enough) or at close with regular responseWriter.
+	// On the first write, w.buf changes from nil to a valid slice
 	w.buf = append(w.buf, b...)
 
 	// If the global writes are bigger than the minSize, compression is enable.
@@ -122,7 +124,9 @@ func (w *GzipResponseWriter) startGzip() error {
 	w.Header().Del(contentLength)
 
 	// Write the header to gzip response.
-	w.writeHeader()
+	if w.code != 0 {
+		w.ResponseWriter.WriteHeader(w.code)
+	}
 
 	// Initialize the GZIP response.
 	w.init()
@@ -146,14 +150,6 @@ func (w *GzipResponseWriter) WriteHeader(code int) {
 	w.code = code
 }
 
-// writeHeader uses the saved code to send it to the ResponseWriter.
-func (w *GzipResponseWriter) writeHeader() {
-	if w.code == 0 {
-		w.code = http.StatusOK
-	}
-	w.ResponseWriter.WriteHeader(w.code)
-}
-
 // init graps a new gzip writer from the gzipWriterPool and writes the correct
 // content encoding header.
 func (w *GzipResponseWriter) init() {
@@ -166,19 +162,18 @@ func (w *GzipResponseWriter) init() {
 
 // Close will close the gzip.Writer and will put it back in the gzipWriterPool.
 func (w *GzipResponseWriter) Close() error {
-	// Buffer not nil means the regular response must be returned.
-	if w.buf != nil {
-		w.writeHeader()
-		// Make the write into the regular response.
-		_, writeErr := w.ResponseWriter.Write(w.buf)
-		// Returns the error if any at write.
-		if writeErr != nil {
-			return fmt.Errorf("gziphandler: write to regular responseWriter at close gets error: %q", writeErr.Error())
-		}
-	}
-
-	// If the GZIP responseWriter is not set no needs to close it.
 	if w.gw == nil {
+		// Gzip not trigged yet, write out regular response.
+		if w.code != 0 {
+			w.ResponseWriter.WriteHeader(w.code)
+		}
+		if w.buf != nil {
+			_, writeErr := w.ResponseWriter.Write(w.buf)
+			// Returns the error if any at write.
+			if writeErr != nil {
+				return fmt.Errorf("gziphandler: write to regular responseWriter at close gets error: %q", writeErr.Error())
+			}
+		}
 		return nil
 	}
 
@@ -236,12 +231,22 @@ func NewGzipLevelHandler(level int) (func(http.Handler) http.Handler, error) {
 // NewGzipLevelAndMinSize behave as NewGzipLevelHandler except it let the caller
 // specify the minimum size before compression.
 func NewGzipLevelAndMinSize(level, minSize int) (func(http.Handler) http.Handler, error) {
+	return NewGzipHandler(level, minSize, &GzipResponseWriter{})
+}
+
+// NewGzipHandler behave as NewGzipLevelHandler except it let the caller
+// specify the minimum size before compression and a GzipWriter.
+func NewGzipHandler(level, minSize int, gw GzipWriter) (func(http.Handler) http.Handler, error) {
 	if level != gzip.DefaultCompression && (level < gzip.BestSpeed || level > gzip.BestCompression) {
 		return nil, fmt.Errorf("invalid compression level requested: %d", level)
 	}
 	if minSize < 0 {
-		return nil, fmt.Errorf("minimum size must be more than zero")
+		return nil, errors.New("minimum size must be more than zero")
 	}
+	if gw == nil {
+		return nil, errors.New("the GzipWriter must be defined")
+	}
+
 	return func(h http.Handler) http.Handler {
 		index := poolIndex(level)
 
@@ -249,13 +254,9 @@ func NewGzipLevelAndMinSize(level, minSize int) (func(http.Handler) http.Handler
 			w.Header().Add(vary, acceptEncoding)
 
 			if acceptsGzip(r) {
-				gw := &GzipResponseWriter{
-					ResponseWriter: w,
-					index:          index,
-					minSize:        minSize,
-
-					buf: []byte{},
-				}
+				gw.SetResponseWriter(w)
+				gw.setIndex(index)
+				gw.setMinSize(minSize)
 				defer gw.Close()
 
 				h.ServeHTTP(gw, r)

vendor/github.com/NYTimes/gziphandler/wrapper.go

@@ -0,0 +1,58 @@
+package gziphandler
+
+import (
+	"bufio"
+	"net"
+	"net/http"
+)
+
+const (
+	contentEncodingHeader = "Content-Encoding"
+)
+
+// ----------
+
+// http.ResponseWriter
+// http.Hijacker
+type GzipWriter interface {
+	Header() http.Header
+	Write([]byte) (int, error)
+	WriteHeader(int)
+	Hijack() (net.Conn, *bufio.ReadWriter, error)
+	Close() error
+	SetResponseWriter(http.ResponseWriter)
+	setIndex(int)
+	setMinSize(int)
+}
+
+func (w *GzipResponseWriter) SetResponseWriter(rw http.ResponseWriter) {
+	w.ResponseWriter = rw
+}
+
+func (w *GzipResponseWriter) setIndex(index int) {
+	w.index = index
+}
+
+func (w *GzipResponseWriter) setMinSize(minSize int) {
+	w.minSize = minSize
+}
+
+// --------
+
+type GzipResponseWriterWrapper struct {
+	GzipResponseWriter
+}
+
+func (g *GzipResponseWriterWrapper) Write(b []byte) (int, error) {
+	if g.gw == nil && isEncoded(g.Header()) {
+		return g.ResponseWriter.Write(b)
+	}
+	return g.GzipResponseWriter.Write(b)
+}
+
+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"
+}