Honor Accept-Encoding header in gzip transcoder

This avoids sending compressed data to clients which do not support
it.

proxy/proxy.go

@@ -16,7 +16,7 @@ type Proxy struct {
 }
 
 type Transcoder interface {
-	Transcode(*ResponseWriter, *ResponseReader) error
+	Transcode(*ResponseWriter, *ResponseReader, http.Header) error
 }
 
 func New() *Proxy {
@@ -68,7 +68,7 @@ func (p *Proxy) handle(w http.ResponseWriter, r *http.Request) error {
 	defer resp.Body.Close()
 	rw := newResponseWriter(w)
 	rr := newResponseReader(resp)
-	err = p.proxyResponse(rw, rr)
+	err = p.proxyResponse(rw, rr, r.Header)
 	read := rr.counter.Count()
 	written := rw.rw.Count()
 	log.Printf("transcoded: %d -> %d (%3.1f%%)", read, written, float64(written)/float64(read)*100)
@@ -92,14 +92,14 @@ func forward(r *http.Request) (*http.Response, error) {
 	return http.DefaultTransport.RoundTrip(r)
 }
 
-func (p *Proxy) proxyResponse(w *ResponseWriter, r *ResponseReader) error {
+func (p *Proxy) proxyResponse(w *ResponseWriter, r *ResponseReader, headers http.Header) error {
 	w.takeHeaders(r)
 	transcoder, found := p.transcoders[r.ContentType()]
 	if !found {
 		return w.ReadFrom(r)
 	}
 	w.setChunked()
-	if err := transcoder.Transcode(w, r); err != nil {
+	if err := transcoder.Transcode(w, r, headers); err != nil {
 		return fmt.Errorf("transcoding error: %s", err)
 	}
 	return nil

transcoder/gif.go

@@ -3,11 +3,12 @@ package transcoder
 import (
 	"github.com/barnacs/compy/proxy"
 	"image/gif"
+	"net/http"
 )
 
 type Gif struct{}
 
-func (t *Gif) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Gif) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	img, err := gif.Decode(r)
 	if err != nil {
 		return err

transcoder/gzip.go

@@ -3,6 +3,8 @@ package transcoder
 import (
 	"compress/gzip"
 	"github.com/barnacs/compy/proxy"
+	"net/http"
+	"strings"
 )
 
 type Gzip struct {
@@ -10,7 +12,7 @@ type Gzip struct {
 	SkipGzipped bool
 }
 
-func (t *Gzip) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Gzip) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	if t.decompress(r) {
 		gzr, err := gzip.NewReader(r.Reader)
 		if err != nil {
@@ -21,13 +23,22 @@ func (t *Gzip) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error
 		r.Header().Del("Content-Encoding")
 		w.Header().Del("Content-Encoding")
 	}
-	if compress(r) {
+
+	shouldGzip := false
+	for _, v := range strings.Split(headers.Get("Accept-Encoding"), ", ") {
+		if strings.SplitN(v, ";", 2)[0] == "gzip" {
+			shouldGzip = true
+			break
+		}
+	}
+
+	if shouldGzip && compress(r) {
 		gzw := gzip.NewWriter(w.Writer)
 		defer gzw.Flush()
 		w.Writer = gzw
 		w.Header().Set("Content-Encoding", "gzip")
 	}
-	return t.Transcoder.Transcode(w, r)
+	return t.Transcoder.Transcode(w, r, headers)
 }
 
 func (t *Gzip) decompress(r *proxy.ResponseReader) bool {

transcoder/identity.go

@@ -2,10 +2,11 @@ package transcoder
 
 import (
 	"github.com/barnacs/compy/proxy"
+	"net/http"
 )
 
 type Identity struct{}
 
-func (i *Identity) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (i *Identity) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	return w.ReadFrom(r)
 }

transcoder/jpeg.go

@@ -3,6 +3,7 @@ package transcoder
 import (
 	"github.com/barnacs/compy/proxy"
 	"github.com/pixiv/go-libjpeg/jpeg"
+	"net/http"
 )
 
 type Jpeg struct {
@@ -20,7 +21,7 @@ func NewJpeg(quality int) *Jpeg {
 	}
 }
 
-func (t *Jpeg) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Jpeg) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	img, err := jpeg.Decode(r, t.decOptions)
 	if err != nil {
 		return err

transcoder/minify.go

@@ -6,6 +6,7 @@ import (
 	"github.com/tdewolff/minify/css"
 	"github.com/tdewolff/minify/html"
 	"github.com/tdewolff/minify/js"
+	"net/http"
 )
 
 type Minifier struct {
@@ -24,6 +25,6 @@ func NewMinifier() *Minifier {
 	}
 }
 
-func (t *Minifier) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Minifier) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	return t.m.Minify(r.ContentType(), w, r)
 }

transcoder/png.go

@@ -3,11 +3,12 @@ package transcoder
 import (
 	"github.com/barnacs/compy/proxy"
 	"image/png"
+	"net/http"
 )
 
 type Png struct{}
 
-func (t *Png) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Png) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	img, err := png.Decode(r)
 	if err != nil {
 		return err