fix: defer to os.Open for windows

See: golang/go#44279

Author: adamdecaf

merge.go

@@ -25,6 +25,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 
@@ -187,10 +188,24 @@ func MergeDir(dir string, conditions Conditions, opts *MergeDirOptions) ([]*File
 	if opts.AcceptFile == nil {
 		opts.AcceptFile = DefaultFileAcceptor
 	}
+	fmt.Printf("BEFORE: dir=%q  opts.FS=%#v\n", dir, opts.FS)
 	if opts.FS == nil {
-		opts.FS = os.DirFS(dir)
-		dir = "."
+		// Go running on windows does not support os.DirFS properly
+		// See: https://github.com/golang/go/issues/44279
+		if runtime.GOOS == "windows" {
+			fmt.Println("  windows")
+			opts.FS = &osFilesystem{}
+		} else {
+			fmt.Println("  other")
+			opts.FS = os.DirFS(dir)
+			dir = "."
+		}
+		fmt.Printf("MIDDLE1: dir=%q  opts.FS=%#v\n", dir, opts.FS)
+	}
+	if runtime.GOOS == "windows" {
+		dir = filepath.Join(filepath.VolumeName(dir), dir)
 	}
+	fmt.Printf("AFTER: dir=%q  opts.FS=%#v\n", dir, opts.FS)
 
 	sorted := &outFile{}
 	var setup sync.Once
@@ -278,6 +293,16 @@ 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) {
+	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 {

merge_test.go

@@ -588,6 +588,7 @@ func TestMergeDir__DefaultFileAcceptor(t *testing.T) {
 func TestMergeDir_SubFS(t *testing.T) {
 	dir := t.TempDir()
 	sub := filepath.Join("a", "b", "c")
+	fmt.Printf("dir + sub: %q\n", filepath.Join(dir, sub))
 	require.NoError(t, os.MkdirAll(filepath.Join(dir, sub), 0777))
 
 	src, err := os.Open(filepath.Join("test", "testdata", "ppd-debit.ach"))