tracing v2: Introduce handles for safe usage of ProtoZeroMessage

components/tracing/core/proto_zero_message_handle.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/tracing/core/proto_zero_message_handle.h"
+
+#include "components/tracing/core/proto_zero_message.h"
+
+namespace tracing {
+namespace v2 {
+
+namespace {
+
+inline void FinalizeMessageIfSet(ProtoZeroMessage* message) {
+  if (message) {
+    message->Finalize();
+    message->set_handle(nullptr);
+  }
+}
+
+}  // namespace
+
+ProtoZeroMessageHandleBase::ProtoZeroMessageHandleBase(
+    ProtoZeroMessage* message)
+    : message_(message) {
+  message_->set_handle(this);
+}
+
+ProtoZeroMessageHandleBase::~ProtoZeroMessageHandleBase() {
+  FinalizeMessageIfSet(message_);
+}
+
+ProtoZeroMessageHandleBase::ProtoZeroMessageHandleBase(
+    ProtoZeroMessageHandleBase&& other) {
+  Move(&other);
+}
+
+ProtoZeroMessageHandleBase& ProtoZeroMessageHandleBase::operator=(
+    ProtoZeroMessageHandleBase&& other) {
+  Move(&other);
+  return *this;
+}
+
+void ProtoZeroMessageHandleBase::Move(ProtoZeroMessageHandleBase* other) {
+  // If the current handle was pointing to a message and is being reset to a new
+  // one, finalize the old message.
+  FinalizeMessageIfSet(message_);
+
+  // In theory other->message_ could be nullptr, if |other| is a handle that has
+  // been std::move-d (and hence empty). There isn't a legitimate use case for
+  // doing so, though. Threfore this case is deliberately ignored (if hit, it

[oystein (OOO til 10th of July), 2016/07/20 21:19:37]

typo: Therefore

[Primiano Tucci (use gerrit), 2016/07/21 10:50:03]

Done.

+  // will manifest as a segfault when dereferencing |message_| below) to avoid a
+  // useles null-check.

[alph, 2016/07/20 18:35:00]

typo: useless

[Primiano Tucci (use gerrit), 2016/07/21 10:50:03]

Done.

+  message_ = other->message_;
+  other->message_ = nullptr;
+  message_->set_handle(this);
+}
+
+}  // namespace v2
+}  // namespace tracing