tracing v2: Introduce handles for safe usage of ProtoZeroMessage

components/tracing/core/proto_zero_message_handle.cc

Index: components/tracing/core/proto_zero_message_handle.cc
diff --git a/components/tracing/core/proto_zero_message_handle.cc b/components/tracing/core/proto_zero_message_handle.cc
new file mode 100644
index 0000000000000000000000000000000000000000..1906cf24721cb00d4b291963c8c3f5f3f2d3992d
--- /dev/null
+++ b/components/tracing/core/proto_zero_message_handle.cc
@@ -0,0 +1,67 @@
+// 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 "base/logging.h"
+#include "components/tracing/core/proto_zero_message.h"
+
+namespace tracing {
+namespace v2 {
+
+namespace {
+
+inline void FinalizeMessageIfSet(ProtoZeroMessage* message) {
+  if (message) {
+    message->Finalize();
+#if DCHECK_IS_ON()
+    message->set_handle(nullptr);
+#endif
+  }
+}
+
+}  // namespace
+
+ProtoZeroMessageHandleBase::ProtoZeroMessageHandleBase(
+    ProtoZeroMessage* message)
+    : message_(message) {
+#if DCHECK_IS_ON()
+  message_->set_handle(this);
+#endif
+}
+
+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. Therefore this case is deliberately ignored (if hit, it
+  // will manifest as a segfault when dereferencing |message_| below) to avoid a
+  // useless null-check.
+  message_ = other->message_;
+  other->message_ = nullptr;
+#if DCHECK_IS_ON()
+  message_->set_handle(this);
+#endif
+}
+
+}  // namespace v2
+}  // namespace tracing