internal/filedesc: avoid deep-copying the options

The protoreflect.Descriptor.Options method is currently documented as
returning a reference to the options, where the user must not mutate
the returned message. This changes internal/filedesc to avoid returning
a copy of the options by caching the first unmarshal.

See golang/protobuf#877

Change-Id: I15701d33fbda7535b21b2add72628b02992c373f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185197
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

internal/filedesc/desc_lazy.go

@@ -6,6 +6,7 @@ package filedesc
 
 import (
 	"reflect"
+	"sync"
 
 	"google.golang.org/protobuf/internal/descopts"
 	"google.golang.org/protobuf/internal/encoding/wire"
@@ -675,14 +676,18 @@ func (db *DescBuilder) optionsUnmarshaler(p pref.ProtoMessage, b []byte) func()
 	if b == nil {
 		return nil
 	}
+	var opts pref.ProtoMessage
+	var once sync.Once
 	return func() pref.ProtoMessage {
-		p := reflect.New(reflect.TypeOf(p).Elem()).Interface().(pref.ProtoMessage)
-		if err := (proto.UnmarshalOptions{
-			AllowPartial: true,
-			Resolver:     db.TypeResolver,
-		}).Unmarshal(b, p); err != nil {
-			panic(err)
-		}
-		return p
+		once.Do(func() {
+			opts = reflect.New(reflect.TypeOf(p).Elem()).Interface().(pref.ProtoMessage)
+			if err := (proto.UnmarshalOptions{
+				AllowPartial: true,
+				Resolver:     db.TypeResolver,
+			}).Unmarshal(b, opts); err != nil {
+				panic(err)
+			}
+		})
+		return opts
 	}
 }

reflect/protoreflect/type.go

@@ -85,8 +85,6 @@ type Descriptor interface {
 	// For FileDescriptor, the Path and Package are also valid.
 	IsPlaceholder() bool
 
-	// TODO: Decide memory ownership of Options.
-
 	// Options returns the descriptor options. The caller must not modify
 	// the returned value.
 	//