internal/legacy/file.go

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package legacy

import (
	"bytes"
	"compress/gzip"
	"io/ioutil"
	"sync"
)

// Every enum and message type generated by protoc-gen-go since commit 2fc053c5
// on February 25th, 2016 has had a method to get the raw descriptor.
// Types that were not generated by protoc-gen-go or were generated prior
// to that version are not supported.
//
// The []byte returned is the encoded form of a FileDescriptorProto message
// compressed using GZIP. The []int is the path from the top-level file
// to the specific message or enum declaration.
type (
	enumV1 interface {
		EnumDescriptor() ([]byte, []int)
	}
	messageV1 interface {
		Descriptor() ([]byte, []int)
	}
)

var fileDescCache sync.Map // map[*byte]*descriptorpb.FileDescriptorProto

// loadFileDesc unmarshals b as a compressed FileDescriptorProto message.
//
// This assumes that b is immutable and that b does not refer to part of a
// concatenated series of GZIP files (which would require shenanigans that
// rely on the concatenation properties of both protobufs and GZIP).
// File descriptors generated by protoc-gen-go do not rely on that property.
func loadFileDesc(b []byte) *fileDescriptorProto {
	// Fast-path: check whether we already have a cached file descriptor.
	if fd, ok := fileDescCache.Load(&b[0]); ok {
		return fd.(*fileDescriptorProto)
	}

	// Slow-path: decompress and unmarshal the file descriptor proto.
	zr, err := gzip.NewReader(bytes.NewReader(b))
	if err != nil {
		panic(err)
	}
	b, err = ioutil.ReadAll(zr)
	if err != nil {
		panic(err)
	}
	fd := parseFileDescProto(b)
	if fd, ok := fileDescCache.LoadOrStore(&b[0], fd); ok {
		return fd.(*fileDescriptorProto)
	}
	return fd
}