Go1.16引入目录遍历优化解析

type DirEntry interface {
        Name() string
        IsDir() bool
        Type() FileMode
        Info() (FileInfo, error)
}

func IterateDir(path string) int {
    // go1.16 的 os.ReadDir 就是这么实现的，为了测试我们把它展开成对(*os.File).ReadDir的调用
	f, err := os.Open(path)
	if err != nil {
		panic(err)
	}
	defer f.Close()
 
	files, err := f.ReadDir(-1)
	if err != nil {
		panic(err)
	}
	length := 0
	for _, finfo := range files {
		length = max(length, len(finfo.Name()))
	}
	return length
}
 
func IterateDir2(path string) int {
    // 1.16之前遍历目录的常用方法之一
	f, err := os.Open(path)
	if err != nil {
		panic(err)
	}
	defer f.Close()
 
	files, err := f.Readdir(-1)
	if err != nil {
		panic(err)
	}
	length := 0
	for _, finfo := range files {
		length = max(length, len(finfo.Name()))
	}
	return length
}
 
func BenchmarkIter1(b *testing.B) {
	for range b.N {
		IterateDir("../test")
	}
}
 
func BenchmarkIter2(b *testing.B) {
	for range b.N {
		IterateDir2("../test")
	}
}

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).
		// Keep it that way to avoid breaking overly sensitive callers.
		infos = []FileInfo{}
	}
	return infos, err
}

func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
	...
 
	for n != 0 {
		// 使用系统调用获得目录项的数据
        // 目录项的元信息一般是存储在目录本身的数据里的，所以读这些信息和读普通文件很类似
		if d.bufp >= d.nbuf {
			d.bufp = 0
			var errno error
			d.nbuf, errno = f.pfd.ReadDirent(*d.buf)
			runtime.KeepAlive(f)
			if errno != nil {
				return names, dirents, infos, &PathError{Op: "readdirent", Path: f.name, Err: errno}
			}
			if d.nbuf <= 0 {
				break // EOF
			}
		}
 
		buf := (*d.buf)[d.bufp:d.nbuf]
		reclen, ok := direntReclen(buf)
		if !ok || reclen > uint64(len(buf)) {
			break
		}
        // 注意这行
		rec := buf[:reclen]
 
		if mode == readdirName {
			names = append(names, string(name))
		} else if mode == readdirDirEntry {
			// 这里的代码后面再看
		} else {
			info, err := lstat(f.name + "/" + string(name))
			if IsNotExist(err) {
				// File disappeared between readdir + stat.
				// Treat as if it didn't exist.
				continue
			}
			if err != nil {
				return nil, nil, infos, err
			}
			infos = append(infos, info)
		}
	}
 
	if n > 0 && len(names)+len(dirents)+len(infos) == 0 {
		return nil, nil, nil, io.EOF
	}
	return names, dirents, infos, nil
}

// rec := buf[:reclen] 防止你忘了rec是哪来的
de, err := newUnixDirent(f.name, string(name), direntType(rec))
if IsNotExist(err) {
	// File disappeared between readdir and stat.
	// Treat as if it didn't exist.
	continue
}
if err != nil {
	return nil, dirents, nil, err
}
dirents = append(dirents, de)

type unixDirent struct {
	parent string
	name   string
	typ    FileMode
	info   FileInfo
}
 
func newUnixDirent(parent, name string, typ FileMode) (DirEntry, error) {
	ude := &unixDirent{
		parent: parent,
		name:   name,
		typ:    typ,
	}
    // 检测文件类型信息是否有效
	if typ != ^FileMode(0) && !testingForceReadDirLstat {
		return ude, nil
	}
 
	info, err := lstat(parent + "/" + name)
	if err != nil {
		return nil, err
	}
 
	ude.typ = info.Mode().Type()
	ude.info = info
	return ude, nil
}

func (d *unixDirent) Info() (FileInfo, error) {
	if d.info != nil {
		return d.info, nil
	}
    // 只会调用一次
	return lstat(d.parent + "/" + d.name)
}

// https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/src/filesystem/dir-common.h#L270
inline file_type
get_file_type(const std::filesystem::__gnu_posix::dirent& d [[gnu::unused]])
{
#ifdef _GLIBCXX_HAVE_STRUCT_DIRENT_D_TYPE
  switch (d.d_type)
  {
  case DT_BLK:
    return file_type::block;
  case DT_CHR:
    return file_type::character;
  case DT_DIR:
    return file_type::directory;
  case DT_FIFO:
    return file_type::fifo;
  case DT_LNK:
    return file_type::symlink;
  case DT_REG:
    return file_type::regular;
  case DT_SOCK:
    return file_type::socket;
  case DT_UNKNOWN:
    return file_type::unknown;
  default:
    return file_type::none;
  }
#else
  return file_type::none;
#endif
}
 
// 如果操作系统以及文件系统不支持，则回退到lstat
// https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/include/bits/fs_dir.h#L342
file_type
_M_file_type() const
{
    if (_M_type != file_type::none && _M_type != file_type::symlink)
	    return _M_type;
    return status().type();
}