proto: Add a descriptor subpackage.

This provides a more reasonable API for obtaining a FileDescriptorProto and
DescriptorProto for a given proto.Message — a process that is currently possible
(but undocumented) using the public proto API.

The major use case for obtaining a DescriptorProto is to decode custom message
options, which are encoded as extensions on the MessageOptions submessage.
(See https://developers.google.com/protocol-buffers/docs/proto#customoptions.)

Fixes https://github.com/golang/protobuf/issues/179.
PiperOrigin-RevId: 139356528

descriptor/descriptor.go

@@ -0,0 +1,93 @@
+// Go support for Protocol Buffers - Google's data interchange format
+//
+// Copyright 2016 The Go Authors.  All rights reserved.
+// https://github.com/golang/protobuf
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Package descriptor provides functions for obtaining protocol buffer
+// descriptors for generated Go types.
+//
+// These functions cannot go in package proto because they depend on the
+// generated protobuf descriptor messages, which themselves depend on proto.
+package descriptor
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"io/ioutil"
+
+	"github.com/golang/protobuf/proto"
+	"google.golang.org/genproto/protobuf"
+)
+
+// extractFile extracts a FileDescriptorProto from a gzip'd buffer.
+func extractFile(gz []byte) (*protobuf.FileDescriptorProto, error) {
+	r, err := gzip.NewReader(bytes.NewReader(gz))
+	if err != nil {
+		return nil, fmt.Errorf("failed to open gzip reader: %v", err)
+	}
+	defer r.Close()
+
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		return nil, fmt.Errorf("failed to uncompress descriptor: %v", err)
+	}
+
+	fd := new(protobuf.FileDescriptorProto)
+	if err := proto.Unmarshal(b, fd); err != nil {
+		return nil, fmt.Errorf("malformed FileDescriptorProto: %v", err)
+	}
+
+	return fd, nil
+}
+
+// Message is a proto.Message with a method to return its descriptor.
+//
+// Message types generated by the protocol compiler always satisfy
+// the Message interface.
+type Message interface {
+	proto.Message
+	Descriptor() ([]byte, []int)
+}
+
+// ForMessage returns a FileDescriptorProto and a DescriptorProto from within it
+// describing the given message.
+func ForMessage(msg Message) (fd *protobuf.FileDescriptorProto, md *protobuf.DescriptorProto) {
+	gz, path := msg.Descriptor()
+	fd, err := extractFile(gz)
+	if err != nil {
+		panic(fmt.Sprintf("invalid FileDescriptorProto for %T: %v", msg, err))
+	}
+
+	md = fd.MessageType[path[0]]
+	for _, i := range path[1:] {
+		md = md.NestedType[i]
+	}
+	return fd, md
+}

descriptor/descriptor_test.go

@@ -0,0 +1,32 @@
+package descriptor_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/golang/protobuf/descriptor"
+	tpb "github.com/golang/protobuf/proto/testdata"
+	"google.golang.org/genproto/protobuf"
+)
+
+func TestMessage(t *testing.T) {
+	var msg *protobuf.DescriptorProto
+	fd, md := descriptor.ForMessage(msg)
+	if pkg, want := fd.GetPackage(), "google.protobuf"; pkg != want {
+		t.Errorf("descriptor.ForMessage(%T).GetPackage() = %q; want %q", msg, pkg, want)
+	}
+	if name, want := md.GetName(), "DescriptorProto"; name != want {
+		t.Fatalf("descriptor.ForMessage(%T).GetName() = %q; want %q", msg, name, want)
+	}
+}
+
+func Example_Options() {
+	var msg *tpb.MyMessageSet
+	_, md := descriptor.ForMessage(msg)
+	if md.GetOptions().GetMessageSetWireFormat() {
+		fmt.Printf("%v uses option message_set_wire_format.\n", md.GetName())
+	}
+
+	// Output:
+	// MyMessageSet uses option message_set_wire_format.
+}