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package sequential
import (
"os"
"path/filepath"
"strconv"
"sync"
"time"
"unsafe"
"golang.org/x/sys/windows"
)
// Create is a copy of [os.Create], modified to use sequential file access.
//
// It uses [windows.FILE_FLAG_SEQUENTIAL_SCAN] rather than [windows.FILE_ATTRIBUTE_NORMAL]
// as implemented in golang. Refer to the [Win32 API documentation] for details
// on sequential file access.
//
// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
func Create(name string) (*os.File, error) {
return openFileSequential(name, windows.O_RDWR|windows.O_CREAT|windows.O_TRUNC)
}
// Open is a copy of [os.Open], modified to use sequential file access.
//
// It uses [windows.FILE_FLAG_SEQUENTIAL_SCAN] rather than [windows.FILE_ATTRIBUTE_NORMAL]
// as implemented in golang. Refer to the [Win32 API documentation] for details
// on sequential file access.
//
// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
func Open(name string) (*os.File, error) {
return openFileSequential(name, windows.O_RDONLY)
}
// OpenFile is a copy of [os.OpenFile], modified to use sequential file access.
//
// It uses [windows.FILE_FLAG_SEQUENTIAL_SCAN] rather than [windows.FILE_ATTRIBUTE_NORMAL]
// as implemented in golang. Refer to the [Win32 API documentation] for details
// on sequential file access.
//
// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
func OpenFile(name string, flag int, _ os.FileMode) (*os.File, error) {
return openFileSequential(name, flag)
}
func openFileSequential(name string, flag int) (file *os.File, err error) {
if name == "" {
return nil, &os.PathError{Op: "open", Path: name, Err: windows.ERROR_FILE_NOT_FOUND}
}
r, e := openSequential(name, flag|windows.O_CLOEXEC)
if e != nil {
return nil, &os.PathError{Op: "open", Path: name, Err: e}
}
return os.NewFile(uintptr(r), name), nil
}
func makeInheritSa() *windows.SecurityAttributes {
var sa windows.SecurityAttributes
sa.Length = uint32(unsafe.Sizeof(sa))
sa.InheritHandle = 1
return &sa
}
func openSequential(path string, mode int) (fd windows.Handle, err error) {
if len(path) == 0 {
return windows.InvalidHandle, windows.ERROR_FILE_NOT_FOUND
}
pathp, err := windows.UTF16PtrFromString(path)
if err != nil {
return windows.InvalidHandle, err
}
var access uint32
switch mode & (windows.O_RDONLY | windows.O_WRONLY | windows.O_RDWR) {
case windows.O_RDONLY:
access = windows.GENERIC_READ
case windows.O_WRONLY:
access = windows.GENERIC_WRITE
case windows.O_RDWR:
access = windows.GENERIC_READ | windows.GENERIC_WRITE
}
if mode&windows.O_CREAT != 0 {
access |= windows.GENERIC_WRITE
}
if mode&windows.O_APPEND != 0 {
access &^= windows.GENERIC_WRITE
access |= windows.FILE_APPEND_DATA
}
sharemode := uint32(windows.FILE_SHARE_READ | windows.FILE_SHARE_WRITE)
var sa *windows.SecurityAttributes
if mode&windows.O_CLOEXEC == 0 {
sa = makeInheritSa()
}
var createmode uint32
switch {
case mode&(windows.O_CREAT|windows.O_EXCL) == (windows.O_CREAT | windows.O_EXCL):
createmode = windows.CREATE_NEW
case mode&(windows.O_CREAT|windows.O_TRUNC) == (windows.O_CREAT | windows.O_TRUNC):
createmode = windows.CREATE_ALWAYS
case mode&windows.O_CREAT == windows.O_CREAT:
createmode = windows.OPEN_ALWAYS
case mode&windows.O_TRUNC == windows.O_TRUNC:
createmode = windows.TRUNCATE_EXISTING
default:
createmode = windows.OPEN_EXISTING
}
// Use FILE_FLAG_SEQUENTIAL_SCAN rather than FILE_ATTRIBUTE_NORMAL as implemented in golang.
// https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
h, e := windows.CreateFile(pathp, access, sharemode, sa, createmode, windows.FILE_FLAG_SEQUENTIAL_SCAN, 0)
return h, e
}
// Helpers for CreateTemp
var (
rand uint32
randmu sync.Mutex
)
func reseed() uint32 {
return uint32(time.Now().UnixNano() + int64(os.Getpid()))
}
func nextSuffix() string {
randmu.Lock()
r := rand
if r == 0 {
r = reseed()
}
r = r*1664525 + 1013904223 // constants from Numerical Recipes
rand = r
randmu.Unlock()
return strconv.Itoa(int(1e9 + r%1e9))[1:]
}
// CreateTemp is a copy of [os.CreateTemp], modified to use sequential file access.
//
// It uses [windows.FILE_FLAG_SEQUENTIAL_SCAN] rather than [windows.FILE_ATTRIBUTE_NORMAL]
// as implemented in golang. Refer to the [Win32 API documentation] for details
// on sequential file access.
//
// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
func CreateTemp(dir, prefix string) (f *os.File, err error) {
if dir == "" {
dir = os.TempDir()
}
nconflict := 0
for i := 0; i < 10000; i++ {
name := filepath.Join(dir, prefix+nextSuffix())
f, err = openFileSequential(name, windows.O_RDWR|windows.O_CREAT|windows.O_EXCL)
if os.IsExist(err) {
if nconflict++; nconflict > 10 {
randmu.Lock()
rand = reseed()
randmu.Unlock()
}
continue
}
break
}
return
}
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