Do not compress already compressed response in CompressHandler

server.go

@@ -330,6 +330,12 @@ func CompressHandler(h RequestHandler) RequestHandler {
 func CompressHandlerLevel(h RequestHandler, level int) RequestHandler {
 	return func(ctx *RequestCtx) {
 		h(ctx)
+		ce := ctx.Response.Header.PeekBytes(strContentEncoding)
+		if len(ce) > 0 {
+			// Do not compress responses with non-empty
+			// Content-Encoding.
+			return
+		}
 		if ctx.Request.Header.HasAcceptEncodingBytes(strGzip) {
 			ctx.Response.gzipBody(level)
 		} else if ctx.Request.Header.HasAcceptEncodingBytes(strDeflate) {

server_test.go

@@ -1145,6 +1145,29 @@ func TestCompressHandler(t *testing.T) {
 		t.Fatalf("unexpected body %q. Expecting %q", body, expectedBody)
 	}
 
+	// an attempt to compress already compressed response
+	ctx.Request.Reset()
+	ctx.Response.Reset()
+	ctx.Request.Header.Set("Accept-Encoding", "gzip, deflate, sdhc")
+	hh := CompressHandler(h)
+	hh(&ctx)
+	s = ctx.Response.String()
+	br = bufio.NewReader(bytes.NewBufferString(s))
+	if err := resp.Read(br); err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	ce = resp.Header.Peek("Content-Encoding")
+	if string(ce) != "gzip" {
+		t.Fatalf("unexpected Content-Encoding: %q. Expecting %q", ce, "gzip")
+	}
+	body, err = resp.BodyGunzip()
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	if string(body) != expectedBody {
+		t.Fatalf("unexpected body %q. Expecting %q", body, expectedBody)
+	}
+
 	// verify deflate-compressed response
 	ctx.Request.Reset()
 	ctx.Response.Reset()