backend/fs: introduce a lazy implementation of ReadSeekCloser

internal/backend/fs.go

@@ -285,18 +285,12 @@ func (s *storageFS) getObject(bucketName, objectName string) (StreamingObject, e
 }
 
 func openObjectAndSetSize(obj *StreamingObject, path string) error {
-	// file is expected to be closed by the caller by calling obj.Close()
-	file, err := os.Open(path)
+	info, err := os.Stat(path)
 	if err != nil {
 		return err
 	}
 
-	info, err := file.Stat()
-	if err != nil {
-		return err
-	}
-
-	obj.Content = file
+	obj.Content = newLazyReader(path)
 	obj.Size = info.Size()
 
 	return nil

internal/backend/lazy_file.go

@@ -0,0 +1,53 @@
+// Copyright 2022 Francisco Souza. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package backend
+
+import (
+	"io"
+	"os"
+	"sync"
+)
+
+type lazyReader struct {
+	filename string
+	once     *sync.Once
+	f        *os.File
+	err      error
+}
+
+func newLazyReader(filename string) io.ReadSeekCloser {
+	return &lazyReader{
+		filename: filename,
+		once:     &sync.Once{},
+	}
+}
+
+func (r *lazyReader) open() {
+	r.f, r.err = os.Open(r.filename)
+}
+
+func (r *lazyReader) Read(p []byte) (int, error) {
+	r.once.Do(r.open)
+	if r.err != nil {
+		return 0, r.err
+	}
+	return r.f.Read(p)
+}
+
+func (r *lazyReader) Seek(offset int64, whence int) (int64, error) {
+	r.once.Do(r.open)
+	if r.err != nil {
+		return 0, r.err
+	}
+	return r.f.Seek(offset, whence)
+}
+
+func (r *lazyReader) Close() error {
+	r.once.Do(r.open)
+	if r.err != nil {
+		return r.err
+	}
+	return r.f.Close()
+}