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// Copyright 2016 Google Inc. All Rights Reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package storage
import (
var crc32cTable = crc32.MakeTable(crc32.Castagnoli)
// Reader reads a Cloud Storage object.
// It implements io.Reader.
type Reader struct {
body io.ReadCloser
remain, size int64
contentType string
cacheControl string
checkCRC bool // should we check the CRC?
wantCRC uint32 // the CRC32c value the server sent in the header
gotCRC uint32 // running crc
// Close closes the Reader. It must be called when done reading.
func (r *Reader) Close() error {
return r.body.Close()
func (r *Reader) Read(p []byte) (int, error) {
n, err := r.body.Read(p)
if r.remain != -1 {
r.remain -= int64(n)
if r.checkCRC {
r.gotCRC = crc32.Update(r.gotCRC, crc32cTable, p[:n])
// Check CRC here. It would be natural to check it in Close, but
// everybody defers Close on the assumption that it doesn't return
// anything worth looking at.
if r.remain == 0 && r.gotCRC != r.wantCRC {
return n, fmt.Errorf("storage: bad CRC on read: got %d, want %d",
r.gotCRC, r.wantCRC)
return n, err
// Size returns the size of the object in bytes.
// The returned value is always the same and is not affected by
// calls to Read or Close.
func (r *Reader) Size() int64 {
return r.size
// Remain returns the number of bytes left to read, or -1 if unknown.
func (r *Reader) Remain() int64 {
return r.remain
// ContentType returns the content type of the object.
func (r *Reader) ContentType() string {
return r.contentType
// CacheControl returns the cache control of the object.
func (r *Reader) CacheControl() string {
return r.cacheControl