tracing v2: Introduce handles for safe usage of ProtoZeroMessage

components/tracing/core/proto_zero_message.h

Index: components/tracing/core/proto_zero_message.h
diff --git a/components/tracing/core/proto_zero_message.h b/components/tracing/core/proto_zero_message.h
index 7f6509387c4289a9fc947dd955c4e7a2ae5cda0b..0b55355f7e5caec1d2e5fecb20303dfecc9e7c75 100644
--- a/components/tracing/core/proto_zero_message.h
+++ b/components/tracing/core/proto_zero_message.h
@@ -21,12 +21,17 @@
 namespace tracing {
 namespace v2 {
 
+class ProtoZeroMessageHandleBase;
+
 // Base class extended by the proto C++ stubs generated by the ProtoZero
 // compiler (see //components/tracing/tools/). This class provides the minimal
 // runtime required to support append-only operations and is desiged for
 // performance. None of the methods require any dynamic memory allocation.
 class TRACING_EXPORT ProtoZeroMessage {
  public:
+  // Clears up the state, allowing the message to be reused as a fresh one.
+  void Reset(ScatteredStreamWriter*);
+
   // Commits all the changes to the buffer (backfills the size field of this and
   // all nested messages) and seals the message. Returns the size of the message
   // (and all nested sub-messages), without taking into account any chunking.
@@ -48,12 +53,16 @@ class TRACING_EXPORT ProtoZeroMessage {
   // partial size that lies in the last chunk has to be backfilled.
   void inc_size_already_written(size_t size) { size_already_written_ += size; }
 
+  // Only for ProtoZeroMessageHandleBase.
+  void set_handle(ProtoZeroMessageHandleBase* handle) {

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

Maybe wrap the whole function in DCHECK_IS_ON(), so that usages of this have to
do the same and make it clearer at the callsite that this is just there for
debugging/asserting purposes.

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

Done.

+#if DCHECK_IS_ON()
+    handle_ = handle;
+#endif
+  }
+
  protected:
   ProtoZeroMessage();
 
-  // Clears up the state, allowing the message to be reused as a fresh one.
-  void Reset(ScatteredStreamWriter*);
-
   void AppendVarIntU64(uint32_t field_id, uint64_t value);
   void AppendVarIntU32(uint32_t field_id, uint32_t value);
   void AppendFloat(uint32_t field_id, float value);
@@ -87,6 +96,7 @@ class TRACING_EXPORT ProtoZeroMessage {
   FRIEND_TEST_ALL_PREFIXES(ProtoZeroMessageTest, BackfillSizeOnFinalization);
   FRIEND_TEST_ALL_PREFIXES(ProtoZeroMessageTest, NestedMessagesSimple);
   FRIEND_TEST_ALL_PREFIXES(ProtoZeroMessageTest, StressTest);
+  FRIEND_TEST_ALL_PREFIXES(ProtoZeroMessageTest, MessageHandle);
 
   enum : uint32_t { kMaxNestingDepth = 8 };
 
@@ -125,6 +135,8 @@ class TRACING_EXPORT ProtoZeroMessage {
   // When true, no more changes to the message are allowed. This is to DCHECK
   // attempts of writing to a message which has been Finalize()-d.
   bool sealed_;
+
+  ProtoZeroMessageHandleBase* handle_;
 #endif
 
   // Pointer to the last child message created through BeginNestedMessage(), if