File manager (#183)

* Add OpenReaderIfExists to FileManager

* Add ReadDirEntryNames to FileManager

fileutil/fileutil.go

@@ -1,13 +1,18 @@
 package fileutil
 
 import (
+	"errors"
+	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 )
 
 // FileManager ...
 type FileManager interface {
 	Open(path string) (*os.File, error)
+	OpenReaderIfExists(path string) (io.Reader, error)
+	ReadDirEntryNames(path string) ([]string, error)
 	Remove(path string) error
 	RemoveAll(path string) error
 	Write(path string, value string, perm os.FileMode) error
@@ -22,11 +27,38 @@ func NewFileManager() FileManager {
 	return fileManager{}
 }
 
+// ReadDirEntryNames reads the named directory using os.ReadDir and returns the dir entries' names.
+func (fileManager) ReadDirEntryNames(path string) ([]string, error) {
+	entries, err := os.ReadDir(path)
+	if err != nil {
+		return nil, err
+	}
+	var names []string
+	for _, entry := range entries {
+		names = append(names, entry.Name())
+	}
+	return names, nil
+}
+
 // Open ...
 func (fileManager) Open(path string) (*os.File, error) {
 	return os.Open(path)
 }
 
+// OpenReaderIfExists opens the named file using os.Open and returns an io.Reader.
+// An ErrNotExist error is absorbed and the returned io.Reader will be nil,
+// other errors from os.Open are returned as is.
+func (fileManager) OpenReaderIfExists(path string) (io.Reader, error) {
+	file, err := os.Open(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return file, nil
+}
+
 // Remove ...
 func (fileManager) Remove(path string) error {
 	return os.Remove(path)