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77 lines
2 KiB
Go
77 lines
2 KiB
Go
// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build linux darwin dragonfly freebsd openbsd netbsd solaris
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package tar
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import (
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"io"
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"os"
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"runtime"
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"syscall"
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)
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func init() {
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sysSparseDetect = sparseDetectUnix
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}
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func sparseDetectUnix(f *os.File) (sph sparseHoles, err error) {
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// SEEK_DATA and SEEK_HOLE originated from Solaris and support for it
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// has been added to most of the other major Unix systems.
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var seekData, seekHole = 3, 4 // SEEK_DATA/SEEK_HOLE from unistd.h
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if runtime.GOOS == "darwin" {
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// Darwin has the constants swapped, compared to all other UNIX.
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seekData, seekHole = 4, 3
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}
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// Check for seekData/seekHole support.
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// Different OS and FS may differ in the exact errno that is returned when
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// there is no support. Rather than special-casing every possible errno
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// representing "not supported", just assume that a non-nil error means
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// that seekData/seekHole is not supported.
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if _, err := f.Seek(0, seekHole); err != nil {
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return nil, nil
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}
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// Populate the SparseHoles.
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var last, pos int64 = -1, 0
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for {
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// Get the location of the next hole section.
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if pos, err = fseek(f, pos, seekHole); pos == last || err != nil {
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return sph, err
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}
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offset := pos
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last = pos
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// Get the location of the next data section.
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if pos, err = fseek(f, pos, seekData); pos == last || err != nil {
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return sph, err
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}
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length := pos - offset
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last = pos
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if length > 0 {
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sph = append(sph, SparseEntry{offset, length})
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}
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}
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}
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func fseek(f *os.File, pos int64, whence int) (int64, error) {
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pos, err := f.Seek(pos, whence)
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if errno(err) == syscall.ENXIO {
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// SEEK_DATA returns ENXIO when past the last data fragment,
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// which makes determining the size of the last hole difficult.
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pos, err = f.Seek(0, io.SeekEnd)
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}
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return pos, err
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}
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func errno(err error) error {
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if perr, ok := err.(*os.PathError); ok {
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return perr.Err
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}
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return err
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}
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