sequential_windows.go

Documentation: github.com/moby/sys/sequential

     1  package sequential
     2  
     3  import (
     4  	"os"
     5  	"path/filepath"
     6  	"strconv"
     7  	"sync"
     8  	"syscall"
     9  	"time"
    10  	"unsafe"
    11  
    12  	"golang.org/x/sys/windows"
    13  )
    14  
    15  // Create creates the named file with mode 0666 (before umask), truncating
    16  // it if it already exists. If successful, methods on the returned
    17  // File can be used for I/O; the associated file descriptor has mode
    18  // O_RDWR.
    19  // If there is an error, it will be of type *PathError.
    20  func Create(name string) (*os.File, error) {
    21  	return OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0)
    22  }
    23  
    24  // Open opens the named file for reading. If successful, methods on
    25  // the returned file can be used for reading; the associated file
    26  // descriptor has mode O_RDONLY.
    27  // If there is an error, it will be of type *PathError.
    28  func Open(name string) (*os.File, error) {
    29  	return OpenFile(name, os.O_RDONLY, 0)
    30  }
    31  
    32  // OpenFile is the generalized open call; most users will use Open
    33  // or Create instead.
    34  // If there is an error, it will be of type *PathError.
    35  func OpenFile(name string, flag int, _ os.FileMode) (*os.File, error) {
    36  	if name == "" {
    37  		return nil, &os.PathError{Op: "open", Path: name, Err: syscall.ENOENT}
    38  	}
    39  	r, err := openFileSequential(name, flag, 0)
    40  	if err == nil {
    41  		return r, nil
    42  	}
    43  	return nil, &os.PathError{Op: "open", Path: name, Err: err}
    44  }
    45  
    46  func openFileSequential(name string, flag int, _ os.FileMode) (file *os.File, err error) {
    47  	r, e := openSequential(name, flag|windows.O_CLOEXEC, 0)
    48  	if e != nil {
    49  		return nil, e
    50  	}
    51  	return os.NewFile(uintptr(r), name), nil
    52  }
    53  
    54  func makeInheritSa() *windows.SecurityAttributes {
    55  	var sa windows.SecurityAttributes
    56  	sa.Length = uint32(unsafe.Sizeof(sa))
    57  	sa.InheritHandle = 1
    58  	return &sa
    59  }
    60  
    61  func openSequential(path string, mode int, _ uint32) (fd windows.Handle, err error) {
    62  	if len(path) == 0 {
    63  		return windows.InvalidHandle, windows.ERROR_FILE_NOT_FOUND
    64  	}
    65  	pathp, err := windows.UTF16PtrFromString(path)
    66  	if err != nil {
    67  		return windows.InvalidHandle, err
    68  	}
    69  	var access uint32
    70  	switch mode & (windows.O_RDONLY | windows.O_WRONLY | windows.O_RDWR) {
    71  	case windows.O_RDONLY:
    72  		access = windows.GENERIC_READ
    73  	case windows.O_WRONLY:
    74  		access = windows.GENERIC_WRITE
    75  	case windows.O_RDWR:
    76  		access = windows.GENERIC_READ | windows.GENERIC_WRITE
    77  	}
    78  	if mode&windows.O_CREAT != 0 {
    79  		access |= windows.GENERIC_WRITE
    80  	}
    81  	if mode&windows.O_APPEND != 0 {
    82  		access &^= windows.GENERIC_WRITE
    83  		access |= windows.FILE_APPEND_DATA
    84  	}
    85  	sharemode := uint32(windows.FILE_SHARE_READ | windows.FILE_SHARE_WRITE)
    86  	var sa *windows.SecurityAttributes
    87  	if mode&windows.O_CLOEXEC == 0 {
    88  		sa = makeInheritSa()
    89  	}
    90  	var createmode uint32
    91  	switch {
    92  	case mode&(windows.O_CREAT|windows.O_EXCL) == (windows.O_CREAT | windows.O_EXCL):
    93  		createmode = windows.CREATE_NEW
    94  	case mode&(windows.O_CREAT|windows.O_TRUNC) == (windows.O_CREAT | windows.O_TRUNC):
    95  		createmode = windows.CREATE_ALWAYS
    96  	case mode&windows.O_CREAT == windows.O_CREAT:
    97  		createmode = windows.OPEN_ALWAYS
    98  	case mode&windows.O_TRUNC == windows.O_TRUNC:
    99  		createmode = windows.TRUNCATE_EXISTING
   100  	default:
   101  		createmode = windows.OPEN_EXISTING
   102  	}
   103  	// Use FILE_FLAG_SEQUENTIAL_SCAN rather than FILE_ATTRIBUTE_NORMAL as implemented in golang.
   104  	// https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx
   105  	const fileFlagSequentialScan = 0x08000000 // FILE_FLAG_SEQUENTIAL_SCAN
   106  	h, e := windows.CreateFile(pathp, access, sharemode, sa, createmode, fileFlagSequentialScan, 0)
   107  	return h, e
   108  }
   109  
   110  // Helpers for CreateTemp
   111  var rand uint32
   112  var randmu sync.Mutex
   113  
   114  func reseed() uint32 {
   115  	return uint32(time.Now().UnixNano() + int64(os.Getpid()))
   116  }
   117  
   118  func nextSuffix() string {
   119  	randmu.Lock()
   120  	r := rand
   121  	if r == 0 {
   122  		r = reseed()
   123  	}
   124  	r = r*1664525 + 1013904223 // constants from Numerical Recipes
   125  	rand = r
   126  	randmu.Unlock()
   127  	return strconv.Itoa(int(1e9 + r%1e9))[1:]
   128  }
   129  
   130  // CreateTemp is a copy of os.CreateTemp, modified to use sequential
   131  // file access. Below is the original comment from golang:
   132  // TempFile creates a new temporary file in the directory dir
   133  // with a name beginning with prefix, opens the file for reading
   134  // and writing, and returns the resulting *os.File.
   135  // If dir is the empty string, TempFile uses the default directory
   136  // for temporary files (see os.TempDir).
   137  // Multiple programs calling TempFile simultaneously
   138  // will not choose the same file. The caller can use f.Name()
   139  // to find the pathname of the file. It is the caller's responsibility
   140  // to remove the file when no longer needed.
   141  func CreateTemp(dir, prefix string) (f *os.File, err error) {
   142  	if dir == "" {
   143  		dir = os.TempDir()
   144  	}
   145  
   146  	nconflict := 0
   147  	for i := 0; i < 10000; i++ {
   148  		name := filepath.Join(dir, prefix+nextSuffix())
   149  		f, err = OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
   150  		if os.IsExist(err) {
   151  			if nconflict++; nconflict > 10 {
   152  				randmu.Lock()
   153  				rand = reseed()
   154  				randmu.Unlock()
   155  			}
   156  			continue
   157  		}
   158  		break
   159  	}
   160  	return
   161  }
   162
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