walk: avoid stat()'ing files unnecessarily

Before:

```
go test -bench=. .
goos: linux
goarch: amd64
pkg: github.com/tonistiigi/fsutil
cpu: Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz
BenchmarkWalker/[1]-target-8         	   27994	     41254 ns/op
BenchmarkWalker/[1]-**/target-8      	   29540	     40402 ns/op
BenchmarkWalker/[2]-*/target-8       	    1017	   1104834 ns/op
BenchmarkWalker/[2]-**/target-8      	     920	   1124731 ns/op
BenchmarkWalker/[3]-*/*/target-8     	      34	  40524094 ns/op
BenchmarkWalker/[3]-**/target-8      	      32	  40432908 ns/op
BenchmarkWalker/[4]-*/*/*/target-8   	      31	  44836714 ns/op
BenchmarkWalker/[4]-**/target-8      	      26	  44611379 ns/op
BenchmarkWalker/[5]-*/*/*/*/target-8 	      85	  14179975 ns/op
BenchmarkWalker/[5]-**/target-8      	      78	  13620031 ns/op
BenchmarkWalker/[6]-*/*/*/*/*/target-8         	      44	  23380013 ns/op
BenchmarkWalker/[6]-**/target-8                	      51	  22435264 ns/op
BenchmarkWalker/[6]-**-!*/*/**-8               	    2101	    580936 ns/op
```

After:

```
go test -bench=. .
goos: linux
goarch: amd64
pkg: github.com/tonistiigi/fsutil
cpu: Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz
BenchmarkWalker/[1]-target-8         	   40788	     26219 ns/op
BenchmarkWalker/[1]-**/target-8      	   41642	     26998 ns/op
BenchmarkWalker/[2]-*/target-8       	    1725	    660271 ns/op
BenchmarkWalker/[2]-**/target-8      	    1806	    645525 ns/op
BenchmarkWalker/[3]-*/*/target-8     	      64	  17609101 ns/op
BenchmarkWalker/[3]-**/target-8      	      66	  17563334 ns/op
BenchmarkWalker/[4]-*/*/*/target-8   	      40	  26241578 ns/op
BenchmarkWalker/[4]-**/target-8      	      43	  24575370 ns/op
BenchmarkWalker/[5]-*/*/*/*/target-8 	     121	   9564283 ns/op
BenchmarkWalker/[5]-**/target-8      	     126	   9412483 ns/op
BenchmarkWalker/[6]-*/*/*/*/*/target-8         	      63	  18703020 ns/op
BenchmarkWalker/[6]-**/target-8                	      73	  17287584 ns/op
BenchmarkWalker/[6]-**-!*/*/**-8               	    3792	    273148 ns/op
```

Signed-off-by: Nick Santos <[email protected]>

walker.go

@@ -2,6 +2,7 @@ package fsutil
 
 import (
  "context"
+ gofs "io/fs"
  "os"
  "path/filepath"
  "strings"
@@ -47,11 +48,11 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
  if err != nil {
  return errors.WithStack(&os.PathError{Op: "resolve", Path: root, Err: err})
  }
- fi, err := os.Stat(root)
+ rootFI, err := os.Stat(root)
  if err != nil {
  return errors.WithStack(err)
  }
- if !fi.IsDir() {
+ if !rootFI.IsDir() {
  return errors.WithStack(&os.PathError{Op: "walk", Path: root, Err: syscall.ENOTDIR})
  }
 
@@ -126,7 +127,7 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
  var parentDirs []visitedDir
 
  seenFiles := make(map[uint64]string)
- return filepath.Walk(root, func(path string, fi os.FileInfo, walkErr error) (retErr error) {
+ return filepath.WalkDir(root, func(path string, dirEntry gofs.DirEntry, walkErr error) (retErr error) {
  defer func() {
  if retErr != nil && isNotExist(retErr) {
  retErr = filepath.SkipDir
@@ -146,9 +147,10 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
  var (
  dir visitedDir
  isDir bool
+ fi gofs.FileInfo
  )
- if fi != nil {
- isDir = fi.IsDir()
+ if dirEntry != nil {
+ isDir = dirEntry.IsDir()
  }
 
  if includeMatcher != nil || excludeMatcher != nil {
@@ -161,6 +163,11 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
  }
 
  if isDir {
+ fi, err = dirEntry.Info()
+ if err != nil {
+ return err
+ }
+
  dir = visitedDir{
  fi: fi,
  path: path,
@@ -268,6 +275,14 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
 
  dir.calledFn = true
 
+ // The FileInfo might have already been read further up.
+ if fi == nil {
+ fi, err = dirEntry.Info()
+ if err != nil {
+ return err
+ }
+ }
+
  stat, err := mkstat(origpath, path, fi, seenFiles)
  if err != nil {
  return err

walker_test.go

@@ -411,6 +411,7 @@ func BenchmarkWalker(b *testing.B) {
  for _, scenario := range []struct {
  maxDepth int
  pattern string
+ exclude string
  expected int
  }{{
  maxDepth: 1,
@@ -460,9 +461,18 @@ func BenchmarkWalker(b *testing.B) {
  maxDepth: 6,
  pattern: "**/target",
  expected: 2388,
+ }, {
+ maxDepth: 6,
+ pattern: "**",
+ exclude: "*/*/**",
+ expected: 20,
  }} {
  scenario := scenario // copy loop var
- b.Run(fmt.Sprintf("[%d]-%s", scenario.maxDepth, scenario.pattern), func(b *testing.B) {
+ suffix := ""
+ if scenario.exclude != "" {
+ suffix = fmt.Sprintf("-!%s", scenario.exclude)
+ }
+ b.Run(fmt.Sprintf("[%d]-%s%s", scenario.maxDepth, scenario.pattern, suffix), func(b *testing.B) {
  tmpdir, err := os.MkdirTemp("", "walk")
  if err != nil {
  b.Error(err)
@@ -478,12 +488,17 @@ func BenchmarkWalker(b *testing.B) {
 
  for i := 0; i < b.N; i++ {
  count := 0
- err = Walk(context.Background(), tmpdir, &WalkOpt{
+ walkOpt := &WalkOpt{
  IncludePatterns: []string{scenario.pattern},
- }, func(path string, fi os.FileInfo, err error) error {
- count++
- return nil
- })
+ }
+ if scenario.exclude != "" {
+ walkOpt.ExcludePatterns = []string{scenario.exclude}
+ }
+ err = Walk(context.Background(), tmpdir, walkOpt,
+ func(path string, fi os.FileInfo, err error) error {
+ count++
+ return nil
+ })
  if err != nil {
  b.Error(err)
  }