keep resettable reader code

pkg/common/accessio/reserttablereader.go

@@ -0,0 +1,168 @@
+// Copyright 2022 SAP SE or an SAP affiliate company. All rights reserved. This file is licensed under the Apache Software License, v. 2 except as noted otherwise in the LICENSE file
+//
+//  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 accessio
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+type ResettableReader struct {
+	orig  io.ReadCloser
+	buf   Buffer
+	count int
+}
+
+func NewResettableReader(orig io.ReadCloser, size int64) (*ResettableReader, error) {
+	var buf Buffer
+	var err error
+
+	if size < 0 || size > 8192 {
+		buf, err = NewFileBuffer()
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		buf = &memoryBuffer{}
+	}
+	return &ResettableReader{
+		orig: orig, buf: buf,
+	}, nil
+}
+
+func (b *ResettableReader) Read(out []byte) (int, error) {
+	n, err := b.orig.Read(out)
+	if n > 0 {
+		return b.buf.Write(out[:n])
+	}
+	return n, err
+}
+
+func (b *ResettableReader) Close() error {
+	//fmt.Printf("close resend buffer\n")
+	b.buf.Close()
+	b.buf = nil
+	return b.orig.Close()
+}
+
+func (b *ResettableReader) Reset() (io.ReadCloser, error) {
+	b.count++
+	if b.buf.Len() <= 0 {
+		return &prefixReader{
+			nil,
+			b,
+		}, nil
+	}
+	r, err := b.buf.Reader()
+	if err != nil {
+		return nil, err
+	}
+	return &prefixReader{
+		r,
+		b,
+	}, nil
+}
+
+type prefixReader struct {
+	prefix io.ReadCloser
+	resend *ResettableReader
+}
+
+func (p *prefixReader) Read(out []byte) (int, error) {
+	if p.prefix != nil {
+		n, err := p.prefix.Read(out)
+		if err == nil {
+			return n, nil
+		}
+		p.prefix.Close()
+		p.prefix = nil
+	}
+	n, err := p.resend.Read(out)
+	//fmt.Printf("blob read %d: %s\n", n, err)
+	return n, err
+}
+
+func (p *prefixReader) Close() error {
+	//fmt.Printf("close prefix reader\n")
+	return nil
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+type Buffer interface {
+	Write(out []byte) (int, error)
+	Reader() (io.ReadCloser, error)
+	Len() int
+	Close() error
+}
+
+type memoryBuffer struct {
+	bytes.Buffer
+}
+
+var _ Buffer = (*memoryBuffer)(nil)
+
+func (m *memoryBuffer) Reader() (io.ReadCloser, error) {
+	return io.NopCloser(bytes.NewReader(m.Bytes())), nil
+}
+
+func (m *memoryBuffer) Close() error {
+	return nil
+}
+
+type fileBuffer struct {
+	path string
+	file *os.File
+}
+
+var _ Buffer = (*fileBuffer)(nil)
+
+func NewFileBuffer() (*fileBuffer, error) {
+	file, err := ioutil.TempFile("", "ociblob*")
+	if err != nil {
+		return nil, err
+	}
+	return &fileBuffer{
+		path: file.Name(),
+		file: file,
+	}, nil
+}
+
+func (b *fileBuffer) Write(out []byte) (int, error) {
+	return b.file.Write(out)
+}
+
+func (b *fileBuffer) Reader() (io.ReadCloser, error) {
+	return os.Open(b.path)
+}
+
+func (b *fileBuffer) Len() int {
+	fi, err := b.file.Stat()
+	if err != nil {
+		return -1
+	}
+	return int(fi.Size())
+}
+
+func (b *fileBuffer) Close() error {
+	err := b.file.Close()
+	err2 := os.Remove(b.path)
+	if err2 != nil {
+		return err2
+	}
+	return err
+}