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ar.go
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ar.go
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// Copyright 2018 The Bazel Authors. 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
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package main
import (
"encoding/binary"
"fmt"
"io"
"os"
"strconv"
"strings"
)
const (
// arHeader appears at the beginning of archives created by "ar" and
// "go tool pack" on all platforms.
arHeader = "!<arch>\n"
// entryLength is the size in bytes of the metadata preceding each file
// in an archive.
entryLength = 60
)
var zeroBytes = []byte("0 ")
type header struct {
NameRaw [16]byte
ModTimeRaw [12]byte
OwnerIdRaw [6]byte
GroupIdRaw [6]byte
FileModeRaw [8]byte
FileSizeRaw [10]byte
EndRaw [2]byte
}
func (h *header) name() string {
return strings.TrimRight(string(h.NameRaw[:]), " ")
}
func (h *header) size() int64 {
s, err := strconv.Atoi(strings.TrimRight(string(h.FileSizeRaw[:]), " "))
if err != nil {
panic(err)
}
return int64(s)
}
func (h *header) next() int64 {
size := h.size()
return size + size%2
}
func (h *header) deterministic() *header {
h2 := *h
copy(h2.ModTimeRaw[:], zeroBytes)
copy(h2.OwnerIdRaw[:], zeroBytes)
copy(h2.GroupIdRaw[:], zeroBytes)
copy(h2.FileModeRaw[:], zeroBytes) // GNU ar also clears this
return &h2
}
// stripArMetadata strips the archive metadata of non-deterministic data:
// - Timestamps
// - User IDs
// - Group IDs
// - File Modes
// The archive is modified in place.
func stripArMetadata(archivePath string) error {
archive, err := os.OpenFile(archivePath, os.O_RDWR, 0)
if err != nil {
return err
}
defer archive.Close()
magic := make([]byte, len(arHeader))
if _, err := io.ReadFull(archive, magic); err != nil {
return err
}
if string(magic) != arHeader {
return fmt.Errorf("%s is not an archive", archivePath)
}
for {
hdr := &header{}
if err := binary.Read(archive, binary.BigEndian, hdr); err == io.EOF {
return nil
} else if err != nil {
return err
}
// Seek back at the beginning of the header and overwrite it.
archive.Seek(-entryLength, os.SEEK_CUR)
if err := binary.Write(archive, binary.BigEndian, hdr.deterministic()); err != nil {
return err
}
if _, err := archive.Seek(hdr.next(), os.SEEK_CUR); err == io.EOF {
return nil
} else if err != nil {
return err
}
}
}