add audit/admin trace support for browser requests (#10947)

To support this functionality we had to fork
the gorilla/rpc package with relevant changes

pkg/rpc/compression_selector.go

@@ -0,0 +1,84 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.ORIG file.
+//
+// Copyright 2020 MinIO, Inc. All rights reserved.
+// forked from https://github.com/gorilla/rpc/v2
+// modified to be used with MinIO under Apache
+// 2.0 license that can be found in the LICENSE file.
+package rpc
+
+import (
+	"compress/flate"
+	"compress/gzip"
+	"io"
+	"net/http"
+	"strings"
+	"unicode"
+)
+
+// gzipWriter writes and closes the gzip writer.
+type gzipWriter struct {
+	w *gzip.Writer
+}
+
+func (gw *gzipWriter) Write(p []byte) (n int, err error) {
+	defer gw.w.Close()
+	return gw.w.Write(p)
+}
+
+// gzipEncoder implements the gzip compressed http encoder.
+type gzipEncoder struct {
+}
+
+func (enc *gzipEncoder) Encode(w http.ResponseWriter) io.Writer {
+	w.Header().Set("Content-Encoding", "gzip")
+	return &gzipWriter{gzip.NewWriter(w)}
+}
+
+// flateWriter writes and closes the flate writer.
+type flateWriter struct {
+	w *flate.Writer
+}
+
+func (fw *flateWriter) Write(p []byte) (n int, err error) {
+	defer fw.w.Close()
+	return fw.w.Write(p)
+}
+
+// flateEncoder implements the flate compressed http encoder.
+type flateEncoder struct {
+}
+
+func (enc *flateEncoder) Encode(w http.ResponseWriter) io.Writer {
+	fw, err := flate.NewWriter(w, flate.DefaultCompression)
+	if err != nil {
+		return w
+	}
+	w.Header().Set("Content-Encoding", "deflate")
+	return &flateWriter{fw}
+}
+
+// CompressionSelector generates the compressed http encoder.
+type CompressionSelector struct {
+}
+
+// Select method selects the correct compression encoder based on http HEADER.
+func (*CompressionSelector) Select(r *http.Request) Encoder {
+	encHeader := r.Header.Get("Accept-Encoding")
+	encTypes := strings.FieldsFunc(encHeader, func(r rune) bool {
+		return unicode.IsSpace(r) || r == ','
+	})
+
+	for _, enc := range encTypes {
+		switch enc {
+		case "gzip":
+			return &gzipEncoder{}
+		case "deflate":
+			return &flateEncoder{}
+		}
+	}
+
+	return DefaultEncoder
+}