//
// Errors returned from this package may be tested against these errors
// with [errors.Is].
var (
	// ErrInvalid indicates an invalid argument.
	// Methods on File will return this error when the receiver is nil.
	ErrInvalid = fs.ErrInvalid // "invalid argument"

	ErrPermission = fs.ErrPermission // "permission denied"
	ErrExist      = fs.ErrExist      // "file already exists"

	}
	if pe.Path != "nonexist" {
		t.Fatalf("Open(nonexist): err.Path = %q, want %q", pe.Path, "nonexist")
	}

	_, err = sub.Open("./")
	if !errors.Is(err, ErrInvalid) {
		t.Fatalf("Open(./): error is %v, want %v", err, ErrInvalid)
	}

// and a non-nil error.
//
// Most clients are better served by the more efficient ReadDir method.
func (f *File) Readdir(n int) ([]FileInfo, error) {
	if f == nil {
		return nil, ErrInvalid
	}
	_, _, infos, err := f.readdir(n, readdirFileInfo)
	if infos == nil {
		// Readdir has historically always returned a non-nil empty slice, never nil,
		// even on error (except misuse with nil receiver above).

	is    bool
	isnot bool
}

var isExistTests = []isExistTest{
	{&fs.PathError{Err: fs.ErrInvalid}, false, false},
	{&fs.PathError{Err: fs.ErrPermission}, false, false},
	{&fs.PathError{Err: fs.ErrExist}, true, false},
	{&fs.PathError{Err: fs.ErrNotExist}, false, true},
	{&fs.PathError{Err: fs.ErrClosed}, false, false},
	{&os.LinkError{Err: fs.ErrInvalid}, false, false},
	{&os.LinkError{Err: fs.ErrPermission}, false, false},

// using [errors.Is].
var (
	ErrInvalid    = errInvalid()    // "invalid argument"
	ErrPermission = errPermission() // "permission denied"
	ErrExist      = errExist()      // "file already exists"
	ErrNotExist   = errNotExist()   // "file does not exist"
	ErrClosed     = errClosed()     // "file already closed"
)

func errInvalid() error    { return oserror.ErrInvalid }

	case io.SeekCurrent:
		newOffset = r.offset + offset
	case io.SeekEnd:
		newOffset = r.size + offset
	default:
		return 0, os.ErrInvalid
	}

	switch {
	case newOffset == r.offset:
		return newOffset, nil

	case newOffset < 0, newOffset > r.size:
		return 0, os.ErrInvalid

	case newOffset == 0:
		r.r = nil

	case newOffset == r.size:
		r.r = errorReader{io.EOF}

	default:

		return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
	}
	f.offset = offset
	return offset, nil
}

func (f *openMapFile) ReadAt(b []byte, offset int64) (int, error) {
	if offset < 0 || offset > int64(len(f.f.Data)) {
		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
	}
	n := copy(b, f.f.Data[offset:])
	if n < len(b) {
		return n, io.EOF
	}

import (
	"syscall"
)

// Stat returns the [FileInfo] structure describing file.
// If there is an error, it will be of type [*PathError].
func (f *File) Stat() (FileInfo, error) {
	if f == nil {
		return nil, ErrInvalid
	}
	var fs fileStat
	err := f.pfd.Fstat(&fs.sys)
	if err != nil {
		return nil, f.wrapErr("stat", err)
	}
	fillFileStatFromSys(&fs, f.name)
	return &fs, nil
}

// chroot-style security mechanism, and Sub does not change that fact.
func Sub(fsys FS, dir string) (FS, error) {
	if !ValidPath(dir) {
		return nil, &PathError{Op: "sub", Path: dir, Err: ErrInvalid}
	}
	if dir == "." {
		return fsys, nil
	}
	if fsys, ok := fsys.(SubFS); ok {
		return fsys.Sub(dir)
	}
	return &subFS{fsys, dir}, nil
}

type subFS struct {
	fsys FS
	dir  string
}

	if !isClosed {
		t.Fatal("File ref count should be zero")
	}

	// Closing a file again should result in invalid argument.
	if err = rlk.Close(); err != os.ErrInvalid {
		t.Fatal(err)
	}

	_, err = newRLockedFile(nil)
	if err != os.ErrInvalid {
		t.Fatal("Unexpected error", err)
	}
}

// Tests lock and unlock semantics.
func TestLockAndUnlock(t *testing.T) {
	f, err := os.CreateTemp("", "lock")