Make IOFS.ReadDir check for fs.ReadDirFile

And implement `fs.ReadDirFile` for `BasePathFile`.

There are other `File` implementations that could also benefit from the above, but
this is a start.

The primary motivation behind this is to allow `fs.WalkDir` use the native
implementation whenever possible, as that new function was added in Go 1.16 with
speed as its main selling point.

This commit also adds a benchmark for `fs.WalkDir` that, when compared to the main branch:

```bash
name        old time/op    new time/op    delta
WalkDir-10     369µs ± 1%     196µs ± 3%  -46.89%  (p=0.029 n=4+4)

name        old alloc/op   new alloc/op   delta
WalkDir-10    85.3kB ± 0%    40.2kB ± 0%  -52.87%  (p=0.029 n=4+4)

name        old allocs/op  new allocs/op  delta
WalkDir-10       826 ± 0%       584 ± 0%  -29.30%  (p=0.029 n=4+4)
```

Fixes #365

basepath.go

@@ -1,14 +1,18 @@
 package afero
 
 import (
+	"io/fs"
 	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
 	"time"
 )
 
-var _ Lstater = (*BasePathFs)(nil)
+var (
+	_ Lstater        = (*BasePathFs)(nil)
+	_ fs.ReadDirFile = (*BasePathFile)(nil)
+)
 
 // The BasePathFs restricts all operations to a given path within an Fs.
 // The given file name to the operations on this Fs will be prepended with
@@ -33,6 +37,14 @@ func (f *BasePathFile) Name() string {
 	return strings.TrimPrefix(sourcename, filepath.Clean(f.path))
 }
 
+func (f *BasePathFile) ReadDir(n int) ([]fs.DirEntry, error) {
+	if rdf, ok := f.File.(fs.ReadDirFile); ok {
+		return rdf.ReadDir(n)
+
+	}
+	return readDirFile{f.File}.ReadDir(n)
+}
+
 func NewBasePathFs(source Fs, path string) Fs {
 	return &BasePathFs{source: source, path: path}
 }

iofs.go

@@ -8,6 +8,7 @@ import (
 	"io/fs"
 	"os"
 	"path"
+	"sort"
 	"time"
 )
 
@@ -67,10 +68,20 @@ func (iofs IOFS) Glob(pattern string) ([]string, error) {
 }
 
 func (iofs IOFS) ReadDir(name string) ([]fs.DirEntry, error) {
-	items, err := ReadDir(iofs.Fs, name)
+	f, err := iofs.Fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	if rdf, ok := f.(fs.ReadDirFile); ok {
+		return rdf.ReadDir(-1)
+	}
+	items, err := f.Readdir(-1)
+
 	if err != nil {
 		return nil, iofs.wrapError("readdir", name, err)
 	}
+	sort.Sort(byName(items))
 
 	ret := make([]fs.DirEntry, len(items))
 	for i := range items {

iofs_test.go

@@ -6,9 +6,11 @@ package afero
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"io/fs"
 	"os"
+	"path/filepath"
 	"runtime"
 	"testing"
 	"testing/fstest"
@@ -61,6 +63,77 @@ func TestIOFS(t *testing.T) {
 			t.Error(err)
 		}
 	})
+
+}
+
+func TestIOFSNativeDirEntryWhenPossible(t *testing.T) {
+	t.Parallel()
+
+	osfs := NewBasePathFs(NewOsFs(), t.TempDir())
+
+	err := osfs.MkdirAll("dir1/dir2", os.ModePerm)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 1; i <= 2; i++ {
+		f, err := osfs.Create(fmt.Sprintf("dir1/dir2/test%d.txt", i))
+		if err != nil {
+			t.Fatal(err)
+		}
+		f.Close()
+	}
+
+	dir2, err := osfs.Open("dir1/dir2")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assertDirEntries := func(entries []fs.DirEntry) {
+		if len(entries) != 2 {
+			t.Fatalf("expected 2, got %d", len(entries))
+		}
+		for _, entry := range entries {
+			if _, ok := entry.(dirEntry); ok {
+				t.Fatal("DirEntry not native")
+			}
+		}
+	}
+
+	dirEntries, err := dir2.(fs.ReadDirFile).ReadDir(-1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertDirEntries(dirEntries)
+
+	iofs := NewIOFS(osfs)
+
+	fileCount := 0
+	err = fs.WalkDir(iofs, "", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if !d.IsDir() {
+			fileCount++
+		}
+
+		if _, ok := d.(dirEntry); ok {
+			t.Fatal("DirEntry not native")
+		}
+
+		return nil
+
+	})
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if fileCount != 2 {
+		t.Fatalf("expected 2, got %d", fileCount)
+	}
+
 }
 
 func TestFromIOFS(t *testing.T) {
@@ -416,3 +489,48 @@ func assertPermissionError(t *testing.T, err error) {
 		t.Errorf("Expected (*fs.PathError).Err == fs.ErrPermisson, got %[1]T (%[1]v)", err)
 	}
 }
+
+func BenchmarkWalkDir(b *testing.B) {
+	osfs := NewBasePathFs(NewOsFs(), b.TempDir())
+
+	createSomeFiles := func(dirname string) {
+		for i := 0; i < 10; i++ {
+			f, err := osfs.Create(filepath.Join(dirname, fmt.Sprintf("test%d.txt", i)))
+			if err != nil {
+				b.Fatal(err)
+			}
+			f.Close()
+		}
+	}
+
+	depth := 10
+	for level := depth; level > 0; level-- {
+		dirname := ""
+		for i := 0; i < level; i++ {
+			dirname = filepath.Join(dirname, fmt.Sprintf("dir%d", i))
+			err := osfs.MkdirAll(dirname, 0755)
+			if err != nil && !os.IsExist(err) {
+				b.Fatal(err)
+			}
+		}
+		createSomeFiles(dirname)
+	}
+
+	iofs := NewIOFS(osfs)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		err := fs.WalkDir(iofs, "", func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			return nil
+
+		})
+
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+
+}