bfs：实现FileHeader的lazy load

internal/utils/bfs/file_header_lazy.go

@@ -0,0 +1,56 @@
+// Copyright 2024 GoEdge CDN goedge.cdn@gmail.com. All rights reserved. Official site: https://goedge.cn .
+
+package bfs
+
+import (
+	"bytes"
+	"encoding/json"
+	"github.com/klauspost/compress/gzip"
+)
+
+// LazyFileHeader load file header lazily to save memory
+type LazyFileHeader struct {
+	rawData    []byte
+	fileHeader *FileHeader
+}
+
+func NewLazyFileHeaderFromData(rawData []byte) *LazyFileHeader {
+	return &LazyFileHeader{
+		rawData: rawData,
+	}
+}
+
+func NewLazyFileHeader(fileHeader *FileHeader) *LazyFileHeader {
+	return &LazyFileHeader{
+		fileHeader: fileHeader,
+	}
+}
+
+func (this *LazyFileHeader) FileHeaderUnsafe() (*FileHeader, error) {
+	if this.fileHeader != nil {
+		return this.fileHeader, nil
+	}
+
+	// TODO 使用pool管理gzip
+	gzReader, err := gzip.NewReader(bytes.NewBuffer(this.rawData))
+	if err != nil {
+		return nil, err
+	}
+
+	defer func() {
+		_ = gzReader.Close()
+	}()
+
+	var header = &FileHeader{}
+	err = json.NewDecoder(gzReader).Decode(header)
+	if err != nil {
+		return nil, err
+	}
+
+	header.IsWriting = false
+
+	this.fileHeader = header
+	this.rawData = nil
+
+	return header, nil
+}