entry_filter_windows.go

//go:build windows

package fastwalk

import (
	"io/fs"
	"os"
	"path/filepath"
	"sync"
	"syscall"
)

type fileKey struct {
	VolumeSerialNumber uint32
	FileIndexHigh      uint32
	FileIndexLow       uint32
}

type EntryFilter struct {
	mu   sync.Mutex
	seen map[fileKey]struct{}
}

func NewEntryFilter() *EntryFilter {
	return &EntryFilter{seen: make(map[fileKey]struct{}, 128)}
}

func (e *EntryFilter) Entry(path string, _ fs.DirEntry) bool {
	namep, err := syscall.UTF16PtrFromString(fixLongPath(path))
	if err != nil {
		return false
	}

	h, err := syscall.CreateFile(namep, 0, 0, nil, syscall.OPEN_EXISTING,
		syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
	if err != nil {
		return false
	}

	var d syscall.ByHandleFileInformation
	err = syscall.GetFileInformationByHandle(h, &d)
	syscall.CloseHandle(h)
	if err != nil {
		return false
	}

	key := fileKey{
		VolumeSerialNumber: d.VolumeSerialNumber,
		FileIndexHigh:      d.FileIndexHigh,
		FileIndexLow:       d.FileIndexLow,
	}

	e.mu.Lock()
	if e.seen == nil {
		e.seen = make(map[fileKey]struct{})
	}
	_, ok := e.seen[key]
	if !ok {
		e.seen[key] = struct{}{}
	}
	e.mu.Unlock()

	return ok
}

func isAbs(path string) (b bool) {
	v := filepath.VolumeName(path)
	if v == "" {
		return false
	}
	path = path[len(v):]
	if path == "" {
		return false
	}
	return os.IsPathSeparator(path[0])
}

// fixLongPath returns the extended-length (\\?\-prefixed) form of
// path when needed, in order to avoid the default 260 character file
// path limit imposed by Windows. If path is not easily converted to
// the extended-length form (for example, if path is a relative path
// or contains .. elements), or is short enough, fixLongPath returns
// path unmodified.
//
// See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
func fixLongPath(path string) string {
	// Do nothing (and don't allocate) if the path is "short".
	// Empirically (at least on the Windows Server 2013 builder),
	// the kernel is arbitrarily okay with < 248 bytes. That
	// matches what the docs above say:
	// "When using an API to create a directory, the specified
	// path cannot be so long that you cannot append an 8.3 file
	// name (that is, the directory name cannot exceed MAX_PATH
	// minus 12)." Since MAX_PATH is 260, 260 - 12 = 248.
	//
	// The MSDN docs appear to say that a normal path that is 248 bytes long
	// will work; empirically the path must be less then 248 bytes long.
	if len(path) < 248 {
		// Don't fix. (This is how Go 1.7 and earlier worked,
		// not automatically generating the \\?\ form)
		return path
	}

	// The extended form begins with \\?\, as in
	// \\?\c:\windows\foo.txt or \\?\UNC\server\share\foo.txt.
	// The extended form disables evaluation of . and .. path
	// elements and disables the interpretation of / as equivalent
	// to \. The conversion here rewrites / to \ and elides
	// . elements as well as trailing or duplicate separators. For
	// simplicity it avoids the conversion entirely for relative
	// paths or paths containing .. elements. For now,
	// \\server\share paths are not converted to
	// \\?\UNC\server\share paths because the rules for doing so
	// are less well-specified.
	if len(path) >= 2 && path[:2] == `\\` {
		// Don't canonicalize UNC paths.
		return path
	}
	if !isAbs(path) {
		// Relative path
		return path
	}

	const prefix = `\\?`

	pathbuf := make([]byte, len(prefix)+len(path)+len(`\`))
	copy(pathbuf, prefix)
	n := len(path)
	r, w := 0, len(prefix)
	for r < n {
		switch {
		case os.IsPathSeparator(path[r]):
			// empty block
			r++
		case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
			// /./
			r++
		case r+1 < n && path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
			// /../ is currently unhandled
			return path
		default:
			pathbuf[w] = '\\'
			w++
			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
				pathbuf[w] = path[r]
				w++
			}
		}
	}
	// A drive's root directory needs a trailing \
	if w == len(`\\?\c:`) {
		pathbuf[w] = '\\'
		w++
	}
	return string(pathbuf[:w])
}