Add an optional Lstater interface

The interface has one method returning the `FileInfo` and a flag telling if `Lstat` was called or not.

```go
type Lstater interface {
    LstatIfPossible(name string) (os.FileInfo, bool, error)
}
```

`Lstat` is currently only supported by the `OsFs`, but since that `Fs` can be used in others, they will also support it by proxy.

But not always, so hence this optional interface.

The interface is in this commit implemented for:

* BasePathFs
* OsFs
* CopyOnWriteFs
* ReadOnlyFs

Fixes #75

basepath.go

@@ -9,6 +9,8 @@ import (
 	"time"
 )
 
+var _ Lstater = (*BasePathFs)(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
 // the base path before calling the base Fs.
@@ -164,4 +166,16 @@ func (b *BasePathFs) Create(name string) (f File, err error) {
 	return &BasePathFile{File: sourcef, path: b.path}, nil
 }
 
+func (b *BasePathFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	name, err := b.RealPath(name)
+	if err != nil {
+		return nil, false, &os.PathError{Op: "lstat", Path: name, Err: err}
+	}
+	if lstater, ok := b.source.(Lstater); ok {
+		return lstater.LstatIfPossible(name)
+	}
+	fi, err := b.source.Stat(name)
+	return fi, false, err
+}
+
 // vim: ts=4 sw=4 noexpandtab nolist syn=go

copyOnWriteFs.go

@@ -8,6 +8,8 @@ import (
 	"time"
 )
 
+var _ Lstater = (*CopyOnWriteFs)(nil)
+
 // The CopyOnWriteFs is a union filesystem: a read only base file system with
 // a possibly writeable layer on top. Changes to the file system will only
 // be made in the overlay: Changing an existing file in the base layer which
@@ -76,18 +78,55 @@ func (u *CopyOnWriteFs) Chmod(name string, mode os.FileMode) error {
 func (u *CopyOnWriteFs) Stat(name string) (os.FileInfo, error) {
 	fi, err := u.layer.Stat(name)
 	if err != nil {
-		origErr := err
-		if e, ok := err.(*os.PathError); ok {
-			err = e.Err
-		}
-		if err == os.ErrNotExist || err == syscall.ENOENT || err == syscall.ENOTDIR {
+		isNotExist := u.isNotExist(err)
+		if isNotExist {
 			return u.base.Stat(name)
 		}
-		return nil, origErr
+		return nil, err
 	}
 	return fi, nil
 }
 
+func (u *CopyOnWriteFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	llayer, ok1 := u.layer.(Lstater)
+	lbase, ok2 := u.base.(Lstater)
+
+	if ok1 {
+		fi, b, err := llayer.LstatIfPossible(name)
+		if err == nil {
+			return fi, b, nil
+		}
+
+		if !u.isNotExist(err) {
+			return nil, b, err
+		}
+	}
+
+	if ok2 {
+		fi, b, err := lbase.LstatIfPossible(name)
+		if err == nil {
+			return fi, b, nil
+		}
+		if !u.isNotExist(err) {
+			return nil, b, err
+		}
+	}
+
+	fi, err := u.Stat(name)
+
+	return fi, false, err
+}
+
+func (u *CopyOnWriteFs) isNotExist(err error) bool {
+	if e, ok := err.(*os.PathError); ok {
+		err = e.Err
+	}
+	if err == os.ErrNotExist || err == syscall.ENOENT || err == syscall.ENOTDIR {
+		return true
+	}
+	return false
+}
+
 // Renaming files present only in the base layer is not permitted
 func (u *CopyOnWriteFs) Rename(oldname, newname string) error {
 	b, err := u.isBaseFile(oldname)

lstater.go

@@ -0,0 +1,27 @@
+// Copyright © 2018 Steve Francia <spf@spf13.com>.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package afero
+
+import (
+	"os"
+)
+
+// Lstater is an optional interface in Afero. It is only implemented by the
+// filesystems saying so.
+// It will call Lstat if the filesystem iself is, or it delegates to, the os filesystem.
+// Else it will call Stat.
+// In addtion to the FileInfo, it will return a boolean telling whether Lstat was called or not.
+type Lstater interface {
+	LstatIfPossible(name string) (os.FileInfo, bool, error)
+}

lstater_test.go

@@ -0,0 +1,102 @@
+// Copyright ©2018 Steve Francia <spf@spf13.com>
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package afero
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLstatIfPossible(t *testing.T) {
+	wd, _ := os.Getwd()
+	defer func() {
+		os.Chdir(wd)
+	}()
+
+	osFs := &OsFs{}
+
+	workDir, err := TempDir(osFs, "", "afero-lstate")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() {
+		osFs.RemoveAll(workDir)
+	}()
+
+	memWorkDir := "/lstate"
+
+	memFs := NewMemMapFs()
+	overlayFs1 := &CopyOnWriteFs{base: osFs, layer: memFs}
+	overlayFs2 := &CopyOnWriteFs{base: memFs, layer: osFs}
+	overlayFsMemOnly := &CopyOnWriteFs{base: memFs, layer: NewMemMapFs()}
+	basePathFs := &BasePathFs{source: osFs, path: workDir}
+	basePathFsMem := &BasePathFs{source: memFs, path: memWorkDir}
+	roFs := &ReadOnlyFs{source: osFs}
+	roFsMem := &ReadOnlyFs{source: memFs}
+
+	pathFileMem := filepath.Join(memWorkDir, "aferom.txt")
+
+	WriteFile(osFs, filepath.Join(workDir, "afero.txt"), []byte("Hi, Afero!"), 0777)
+	WriteFile(memFs, filepath.Join(pathFileMem), []byte("Hi, Afero!"), 0777)
+
+	os.Chdir(workDir)
+	if err := os.Symlink("afero.txt", "symafero.txt"); err != nil {
+		t.Fatal(err)
+	}
+
+	pathFile := filepath.Join(workDir, "afero.txt")
+	pathSymlink := filepath.Join(workDir, "symafero.txt")
+
+	checkLstat := func(l Lstater, name string, shouldLstat bool) os.FileInfo {
+		statFile, isLstat, err := l.LstatIfPossible(name)
+		if err != nil {
+			t.Fatalf("Lstat check failed: %s", err)
+		}
+		if isLstat != shouldLstat {
+			t.Fatalf("Lstat status was %t for %s", isLstat, name)
+		}
+		return statFile
+	}
+
+	testLstat := func(l Lstater, pathFile, pathSymlink string) {
+		shouldLstat := pathSymlink != ""
+		statRegular := checkLstat(l, pathFile, shouldLstat)
+		statSymlink := checkLstat(l, pathSymlink, shouldLstat)
+		if statRegular == nil || statSymlink == nil {
+			t.Fatal("got nil FileInfo")
+		}
+
+		symSym := statSymlink.Mode()&os.ModeSymlink == os.ModeSymlink
+		if symSym == (pathSymlink == "") {
+			t.Fatal("expected the FileInfo to describe the symlink")
+		}
+
+		_, _, err := l.LstatIfPossible("this-should-not-exist.txt")
+		if err == nil || !os.IsNotExist(err) {
+			t.Fatalf("expected file to not exist, got %s", err)
+		}
+	}
+
+	testLstat(osFs, pathFile, pathSymlink)
+	testLstat(overlayFs1, pathFile, pathSymlink)
+	testLstat(overlayFs2, pathFile, pathSymlink)
+	testLstat(basePathFs, "afero.txt", "symafero.txt")
+	testLstat(overlayFsMemOnly, pathFileMem, "")
+	testLstat(basePathFsMem, "aferom.txt", "")
+	testLstat(roFs, pathFile, pathSymlink)
+	testLstat(roFsMem, pathFileMem, "")
+}

match.go

@@ -33,8 +33,8 @@ import (
 // built-ins from that package.
 func Glob(fs Fs, pattern string) (matches []string, err error) {
 	if !hasMeta(pattern) {
-		// afero does not support Lstat directly.
-		if _, err = lstatIfOs(fs, pattern); err != nil {
+		// Lstat not supported by a ll filesystems.
+		if _, err = lstatIfPossible(fs, pattern); err != nil {
 			return nil, nil
 		}
 		return []string{pattern}, nil

os.go

@@ -19,6 +19,8 @@ import (
 	"time"
 )
 
+var _ Lstater = (*OsFs)(nil)
+
 // OsFs is a Fs implementation that uses functions provided by the os package.
 //
 // For details in any method, check the documentation of the os package
@@ -92,3 +94,8 @@ func (OsFs) Chmod(name string, mode os.FileMode) error {
 func (OsFs) Chtimes(name string, atime time.Time, mtime time.Time) error {
 	return os.Chtimes(name, atime, mtime)
 }
+
+func (OsFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	fi, err := os.Lstat(name)
+	return fi, true, err
+}

path.go

@@ -60,7 +60,7 @@ func walk(fs Fs, path string, info os.FileInfo, walkFn filepath.WalkFunc) error
 
 	for _, name := range names {
 		filename := filepath.Join(path, name)
-		fileInfo, err := lstatIfOs(fs, filename)
+		fileInfo, err := lstatIfPossible(fs, filename)
 		if err != nil {
 			if err := walkFn(filename, fileInfo, err); err != nil && err != filepath.SkipDir {
 				return err
@@ -77,15 +77,13 @@ func walk(fs Fs, path string, info os.FileInfo, walkFn filepath.WalkFunc) error
 	return nil
 }
 
-// if the filesystem is OsFs use Lstat, else use fs.Stat
-func lstatIfOs(fs Fs, path string) (info os.FileInfo, err error) {
-	_, ok := fs.(*OsFs)
-	if ok {
-		info, err = os.Lstat(path)
-	} else {
-		info, err = fs.Stat(path)
+// if the filesystem supports it, use Lstat, else use fs.Stat
+func lstatIfPossible(fs Fs, path string) (os.FileInfo, error) {
+	if lfs, ok := fs.(Lstater); ok {
+		fi, _, err := lfs.LstatIfPossible(path)
+		return fi, err
 	}
-	return
+	return fs.Stat(path)
 }
 
 // Walk walks the file tree rooted at root, calling walkFn for each file or
@@ -100,7 +98,7 @@ func (a Afero) Walk(root string, walkFn filepath.WalkFunc) error {
 }
 
 func Walk(fs Fs, root string, walkFn filepath.WalkFunc) error {
-	info, err := lstatIfOs(fs, root)
+	info, err := lstatIfPossible(fs, root)
 	if err != nil {
 		return walkFn(root, nil, err)
 	}

readonlyfs.go

@@ -6,6 +6,8 @@ import (
 	"time"
 )
 
+var _ Lstater = (*ReadOnlyFs)(nil)
+
 type ReadOnlyFs struct {
 	source Fs
 }
@@ -34,6 +36,14 @@ func (r *ReadOnlyFs) Stat(name string) (os.FileInfo, error) {
 	return r.source.Stat(name)
 }
 
+func (r *ReadOnlyFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	if lsf, ok := r.source.(Lstater); ok {
+		return lsf.LstatIfPossible(name)
+	}
+	fi, err := r.Stat(name)
+	return fi, false, err
+}
+
 func (r *ReadOnlyFs) Rename(o, n string) error {
 	return syscall.EPERM
 }

unionFile.go

@@ -123,6 +123,24 @@ func (f *UnionFile) Name() string {
 	return f.base.Name()
 }
 
+// UnionKey can be implemented to use an alternate key than FileInfo.Name when
+// weaving the two directories together. The use case(s) are uncommon and special,
+// but this will allow a union file system with multiple files with the same filename.
+// This needs to be implemented by your own implementation of os.FileInfo as
+// returned in Readdir.
+type UnionKey interface {
+	AferoUnionKey() string
+}
+
+func (f *UnionFile) key(ofi os.FileInfo) string {
+	switch tp := ofi.(type) {
+	case UnionKey:
+		return tp.AferoUnionKey()
+	default:
+		return tp.Name()
+	}
+}
+
 // Readdir will weave the two directories together and
 // return a single view of the overlayed directories
 func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
@@ -135,7 +153,7 @@ func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 				return nil, err
 			}
 			for _, fi := range rfi {
-				files[fi.Name()] = fi
+				files[f.key(fi)] = fi
 			}
 		}
 
@@ -146,7 +164,7 @@ func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 			}
 			for _, fi := range rfi {
 				if _, exists := files[fi.Name()]; !exists {
-					files[fi.Name()] = fi
+					files[f.key(fi)] = fi
 				}
 			}
 		}