os: add File.ReadDir method and DirEntry type

ReadDir provides a portable, efficient way to read a directory
and discover the type of directory entries.
This enables a more efficient file system walk, yet to be added.

See #41467 for the proposal review for the API.

Fixes #41467.

Change-Id: I461a526793ae46df48821aa448b04f1705546739
Reviewed-on: https://go-review.googlesource.com/c/go/+/261540
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 25 insertions, 26 deletions

diff --git a/src/os/dir_windows.go b/src/os/dir_windows.go
index 9e5d6bd505..1c3f2f0d57 100644
--- a/src/os/dir_windows.go
+++ b/src/os/dir_windows.go
@@ -10,20 +10,14 @@ import (
 	"syscall"
 )
 
-func (file *File) readdir(n int) (fi []FileInfo, err error) {
-	if file == nil {
-		return nil, syscall.EINVAL
-	}
+func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
 	if !file.isdir() {
-		return nil, &PathError{"Readdir", file.name, syscall.ENOTDIR}
+		return nil, nil, nil, &PathError{"readdir", file.name, syscall.ENOTDIR}
 	}
 	wantAll := n <= 0
-	size := n
 	if wantAll {
 		n = -1
-		size = 100
 	}
-	fi = make([]FileInfo, 0, size) // Empty with room to grow.
 	d := &file.dirinfo.data
 	for n != 0 && !file.dirinfo.isempty {
 		if file.dirinfo.needdata {
@@ -34,9 +28,6 @@ func (file *File) readdir(n int) (fi []FileInfo, err error) {
 					break
 				} else {
 					err = &PathError{"FindNextFile", file.name, e}
-					if !wantAll {
-						fi = nil
-					}
 					return
 				}
 			}
@@ -46,24 +37,32 @@ func (file *File) readdir(n int) (fi []FileInfo, err error) {
 		if name == "." || name == ".." { // Useless names
 			continue
 		}
-		f := newFileStatFromWin32finddata(d)
-		f.name = name
-		f.path = file.dirinfo.path
-		f.appendNameToPath = true
+		if mode == readdirName {
+			names = append(names, name)
+		} else {
+			f := newFileStatFromWin32finddata(d)
+			f.name = name
+			f.path = file.dirinfo.path
+			f.appendNameToPath = true
+			if mode == readdirDirEntry {
+				dirents = append(dirents, dirEntry{f})
+			} else {
+				infos = append(infos, f)
+			}
+		}
 		n--
-		fi = append(fi, f)
 	}
-	if !wantAll && len(fi) == 0 {
-		return fi, io.EOF
+	if !wantAll && len(names)+len(dirents)+len(infos) == 0 {
+		return nil, nil, nil, io.EOF
 	}
-	return fi, nil
+	return names, dirents, infos, nil
 }
 
-func (file *File) readdirnames(n int) (names []string, err error) {
-	fis, err := file.Readdir(n)
-	names = make([]string, len(fis))
-	for i, fi := range fis {
-		names[i] = fi.Name()
-	}
-	return names, err
+type dirEntry struct {
+	fs *fileStat
 }
+
+func (de dirEntry) Name() string            { return de.fs.Name() }
+func (de dirEntry) IsDir() bool             { return de.fs.IsDir() }
+func (de dirEntry) Type() FileMode          { return de.fs.Mode().Type() }
+func (de dirEntry) Info() (FileInfo, error) { return de.fs, nil }