Tracing V2: Fully-functional plugin.

components/tracing/core/proto_zero_message_handle.h

Index: components/tracing/core/proto_zero_message_handle.h
diff --git a/components/tracing/core/proto_zero_message_handle.h b/components/tracing/core/proto_zero_message_handle.h
index 2f83ec9638f0d400a96eec74c159c0730eab62f7..9629c3802ec850e4227fbc4ddeaf0fc19b387a47 100644
--- a/components/tracing/core/proto_zero_message_handle.h
+++ b/components/tracing/core/proto_zero_message_handle.h
@@ -5,6 +5,12 @@
 #ifndef COMPONENTS_TRACING_CORE_PROTO_ZERO_MESSAGE_HANDLE_H_
 #define COMPONENTS_TRACING_CORE_PROTO_ZERO_MESSAGE_HANDLE_H_
 
+#include <stdint.h>
+
+#include <string>
+#include <unordered_set>
+
+#include "base/logging.h"
 #include "base/macros.h"
 #include "components/tracing/tracing_export.h"
 
@@ -32,6 +38,12 @@ class TRACING_EXPORT ProtoZeroMessageHandleBase {
   ProtoZeroMessageHandleBase(ProtoZeroMessageHandleBase&& other);
   ProtoZeroMessageHandleBase& operator=(ProtoZeroMessageHandleBase&& other);
 
+  inline void SealField(uint32_t field_id) {

[Primiano Tucci (use gerrit), 2016/08/24 08:59:45]

wait why sealfield is a property of the Handle?
The handle is supposed to be just a transparent adaptor.
Please don't add anything to that. THe idea is that if the handle become a
bottleneck we can just short-circuit to ProtoZeroMessage.
Also, really, the fact that the sealed fields is here feels a hack to me.

Also note how this handle has move constructors.

+    DCHECK(sealed_fields_.count(field_id) == 0) <<
+        "Field (" + std::to_string(field_id) + ") can't be sealed again.";
+    sealed_fields_.insert(field_id);
+  }
+
  protected:
   explicit ProtoZeroMessageHandleBase(ProtoZeroMessage*);
   ProtoZeroMessage& operator*() const { return *message_; }
@@ -40,16 +52,20 @@ class TRACING_EXPORT ProtoZeroMessageHandleBase {
  private:
   friend class ProtoZeroMessage;
 
-  void reset_message() { message_ = nullptr; }
+  void reset_message() {
+    message_ = nullptr;
+    sealed_fields_.clear();
+  }
   void Move(ProtoZeroMessageHandleBase* other);
 
   ProtoZeroMessage* message_;
+  std::unordered_set<uint32_t> sealed_fields_;
 
   DISALLOW_COPY_AND_ASSIGN(ProtoZeroMessageHandleBase);
 };
 
 template <typename T>
-class ProtoZeroMessageHandle : ProtoZeroMessageHandleBase {
+class ProtoZeroMessageHandle : public ProtoZeroMessageHandleBase {
  public:
   explicit ProtoZeroMessageHandle(T* message)
       : ProtoZeroMessageHandleBase(message) {}