Source: legacy_file.go in package google.golang.org/protobuf/internal/impl


package impl

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

	"google.golang.org/protobuf/internal/filedesc"
	"google.golang.org/protobuf/reflect/protoreflect"
	"google.golang.org/protobuf/reflect/protoregistry"
)

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 legacyFileDescCache sync.Map // map[*byte]protoreflect.FileDescriptor

legacyLoadFileDesc 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.

Fast-path: check whether we already have a cached file descriptor.

	if fd, ok := legacyFileDescCache.Load(&b[0]); ok {
		return fd.(protoreflect.FileDescriptor)
	}

Slow-path: decompress and unmarshal the file descriptor proto.

	zr, err := gzip.NewReader(bytes.NewReader(b))
	if err != nil {
		panic(err)
	}
	b2, err := ioutil.ReadAll(zr)
	if err != nil {
		panic(err)
	}

	fd := filedesc.Builder{
		RawDescriptor: b2,
		FileRegistry:  resolverOnly{protoregistry.GlobalFiles}, // do not register back to global registry
	}.Build().File
	if fd, ok := legacyFileDescCache.LoadOrStore(&b[0], fd); ok {
		return fd.(protoreflect.FileDescriptor)
	}
	return fd
}

type resolverOnly struct {
	reg *protoregistry.Files
}

func (r resolverOnly) FindFileByPath(path string) (protoreflect.FileDescriptor, error) {
	return r.reg.FindFileByPath(path)
}
func (r resolverOnly) FindDescriptorByName(name protoreflect.FullName) (protoreflect.Descriptor, error) {
	return r.reg.FindDescriptorByName(name)
}
func (resolverOnly) RegisterFile(protoreflect.FileDescriptor) error {
	return nil