fix: defer to os.Open for windows

See: golang/go#44279

Author: adamdecaf

merge.go

@@ -187,10 +187,38 @@ func MergeDir(dir string, conditions Conditions, opts *MergeDirOptions) ([]*File
 	if opts.AcceptFile == nil {
 		opts.AcceptFile = DefaultFileAcceptor
 	}
-	if opts.FS == nil {
-		opts.FS = os.DirFS(dir)
-		dir = "."
-	}
+	// fmt.Printf("BEFORE: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// if runtime.GOOS == "windows" {
+	// 	// Go running on windows does not support os.DirFS properly
+	// 	// See: https://github.com/golang/go/issues/44279
+	// 	fmt.Println("  windows") //nolint:forbidigo
+	// 	opts.FS = &osFilesystem{}
+	// 	dir = filepath.Join(filepath.VolumeName(dir), dir)
+	// 	fmt.Printf("MIDDLE2: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// }
+	// if opts.FS == nil {
+	// 	fmt.Println("opts.FS == nil") //nolint:forbidigo
+	// 	opts.FS = os.DirFS(dir)
+	// 	fmt.Printf("MIDDLE1: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// } else {
+	// 	var err error
+	// 	fmt.Printf("MIDDLE2A: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// 	opts.FS, err = fs.Sub(opts.FS, dir)
+	// 	if err != nil {
+	// 		return nil, fmt.Errorf("fs.Sub of %v and %v failed: %w", opts.FS, dir, err)
+	// 	}
+	// 	// if runtime.GOOS == "windows" {
+	// 	// 	dir = ""
+	// 	// } else {
+	// 	// 	dir = "."
+	// 	// }
+	// 	fmt.Printf("MIDDLE2B: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// }
+	// fmt.Printf("AFTER: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+
+	// dir + sub: "C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001\\a\\b\\c"
+	// BEFORE: dir="a\\b\\c"  opts.FS="C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001"
+	// AFTER: dir="a\\b\\c"  opts.FS="C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001"
 
 	sorted := &outFile{}
 	var setup sync.Once
@@ -278,15 +306,39 @@ func MergeDir(dir string, conditions Conditions, opts *MergeDirOptions) ([]*File
 	return convertToFiles(sorted, conditions)
 }
 
+// osFilesystem is an io/fs.FS which defers to the os package for opening files
+// See: https://github.com/golang/go/issues/44279
+type osFilesystem struct{}
+
+func (*osFilesystem) Open(name string) (fs.File, error) {
+	fmt.Printf("osFilesystem.Open(%v)\n", name) //nolint:forbidigo
+	return os.Open(name)
+}
+
+var _ fs.FS = new(osFilesystem)
+
 func walkDir(fsys fs.FS, dir string, discoveredPaths chan string) error {
-	reader, ok := fsys.(fs.ReadDirFS)
-	if !ok {
-		return fmt.Errorf("unexpected %T wanted fs.ReadDirFS", fsys)
-	}
+	fmt.Printf("walkDir: dir=%v  fsys=%#v\n", dir, fsys) //nolint:forbidigo
 
-	items, err := reader.ReadDir(dir)
+	var items []fs.DirEntry
+	var err error
+
+	if fsys != nil {
+		fmt.Printf("walkDir: A\n") //nolint:forbidigo
+		if rr, ok := fsys.(fs.ReadDirFS); ok {
+			fmt.Printf("walkDir: B\n") //nolint:forbidigo
+			items, err = rr.ReadDir(dir)
+		}
+	}
+	if err != nil {
+		return fmt.Errorf("fs.readdir %s failed: %w", dir, err)
+	}
+	if len(items) == 0 {
+		fmt.Printf("walkDir: C\n") //nolint:forbidigo
+		items, err = os.ReadDir(dir)
+	}
 	if err != nil {
-		return fmt.Errorf("listing %s failed: %w", dir, err)
+		return fmt.Errorf("os.readdir %s failed: %w", dir, err)
 	}
 
 	for i := range items {
@@ -356,7 +408,14 @@ func readValidateOptsFromFile(path string, opts *MergeDirOptions) *ValidateOpts
 	if opts.ValidateOptsExtension != "" {
 		where := strings.TrimSuffix(path, filepath.Ext(path)) + opts.ValidateOptsExtension
 
-		fd, err := opts.FS.Open(where)
+		var fd fs.File
+		var err error
+
+		if opts.FS != nil {
+			fd, err = opts.FS.Open(where)
+		} else {
+			fd, err = os.Open(where)
+		}
 		if err != nil {
 			return nil
 		}
@@ -369,12 +428,18 @@ func readValidateOptsFromFile(path string, opts *MergeDirOptions) *ValidateOpts
 	return nil
 }
 
-func readFile(fs fs.FS, path string, as FileAcceptance, validateOpts *ValidateOpts) (*File, error) {
+func readFile(fsys fs.FS, path string, as FileAcceptance, validateOpts *ValidateOpts) (*File, error) {
 	if as == SkipFile {
 		return nil, nil
 	}
 
-	fd, err := fs.Open(path)
+	var fd fs.File
+	var err error
+	if fsys != nil {
+		fd, err = fsys.Open(path)
+	} else {
+		fd, err = os.Open(path)
+	}
 	if err != nil {
 		return nil, fmt.Errorf("opening %s failed: %w", path, err)
 	}

merge_test.go

@@ -585,9 +585,30 @@ func TestMergeDir__DefaultFileAcceptor(t *testing.T) {
 	require.Equal(t, SkipFile, output)
 }
 
-func TestMergeDir_SubFS(t *testing.T) {
+func TestMergeDir(t *testing.T) {
+	dir := t.TempDir()
+
+	src, err := os.Open(filepath.Join("test", "testdata", "ppd-debit.ach"))
+	require.NoError(t, err)
+	t.Cleanup(func() { src.Close() })
+
+	dst, err := os.Create(filepath.Join(dir, "input.ach"))
+	require.NoError(t, err)
+
+	_, err = io.Copy(dst, src)
+	require.NoError(t, err)
+	require.NoError(t, dst.Close())
+
+	var conditions Conditions
+	merged, err := MergeDir(dir, conditions, nil)
+	require.NoError(t, err)
+	require.Len(t, merged, 1)
+}
+
+func TestMergeDir_WithFS(t *testing.T) {
 	dir := t.TempDir()
 	sub := filepath.Join("a", "b", "c")
+	fmt.Printf("dir + sub: %v\n", filepath.Join(dir, sub)) //nolint:forbidigo
 	require.NoError(t, os.MkdirAll(filepath.Join(dir, sub), 0777))
 
 	src, err := os.Open(filepath.Join("test", "testdata", "ppd-debit.ach"))
@@ -602,11 +623,20 @@ func TestMergeDir_SubFS(t *testing.T) {
 	require.NoError(t, dst.Close())
 
 	var conditions Conditions
+
+	// partial dir + sub
 	merged, err := MergeDir(sub, conditions, &MergeDirOptions{
 		FS: os.DirFS(dir),
 	})
 	require.NoError(t, err)
 	require.Len(t, merged, 1)
+
+	// full dir
+	merged, err = MergeDir(".", conditions, &MergeDirOptions{
+		FS: os.DirFS(filepath.Join(dir, sub)),
+	})
+	require.NoError(t, err)
+	require.Len(t, merged, 1)
 }
 
 func TestMergeDirHelpers(t *testing.T) {