Tracing V2: Fully-functional plugin.

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 b5b0a37cce1bc4b82788c62108ee7036702cb6d0..01853c75d7be43242b46de5bf6cf22fc891bf88a 100644
--- a/components/tracing/core/proto_zero_message.h
+++ b/components/tracing/core/proto_zero_message.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 
+#include <set>
 #include <type_traits>
 
 #include "base/compiler_specific.h"
@@ -22,7 +23,9 @@
 namespace tracing {
 namespace v2 {
 
+#if DCHECK_IS_ON()

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

is this causing an actual compilation error? If not please remove this #if and
always fwd declare.

 class ProtoZeroMessageHandleBase;
+#endif
 
 // Base class extended by the proto C++ stubs generated by the ProtoZero
 // compiler (see //components/tracing/tools/). This class provides the minimal
@@ -62,7 +65,7 @@ class TRACING_EXPORT ProtoZeroMessage {
  protected:
   ProtoZeroMessage();
 
-  // Proto types: uint64, uint32, int64, int32, bool, enum.
+  // Proto types: uint64, uint32, int64, int32, enum (large).
   template <typename T>
   void AppendVarInt(uint32_t field_id, T value) {
     if (nested_message_)
@@ -83,6 +86,21 @@ class TRACING_EXPORT ProtoZeroMessage {
     AppendVarInt(field_id, proto::ZigZagEncode(value));
   }
 
+  // Proto types: bool, enum (small).
+  // It could be looking weird not to optimize writing field_id, but it's

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

Honestly I feel this comment is just too apologetic. Just remove it, caused me
more time to figure out what you really meant ;-)

+  // unnecessary because compiler already optimize WriteVarInt for constexpr.
+  void AppendTinyVarInt(uint32_t field_id, int32_t value) {
+    DCHECK(0 <= value && value < 0x80);
+    if (nested_message_)
+      EndNestedMessage();
+
+    uint8_t buffer[proto::kMaxSimpleFieldEncodedSize];
+    uint8_t* pos = buffer;
+    pos = proto::WriteVarInt(proto::MakeTagVarInt(field_id), pos);
+    *pos++ = static_cast<uint8_t>(value);
+    WriteToStream(buffer, pos);
+  }
+
   // Proto types: fixed64, sfixed64, fixed32, sfixed32, double, float.
   template <typename T>
   void AppendFixed(uint32_t field_id, T value) {
@@ -99,7 +117,6 @@ class TRACING_EXPORT ProtoZeroMessage {
     WriteToStream(buffer, pos);
   }
 
-  // TODO(kraynov): Add AppendTinyInt.
   void AppendString(uint32_t field_id, const char* str);
   void AppendBytes(uint32_t field_id, const void* value, size_t size);
 
@@ -122,6 +139,14 @@ class TRACING_EXPORT ProtoZeroMessage {
     return message;
   }
 
+  // Prevent multiple appends for non-repeated fields.

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

honestly I feel this duplicated field detection is a bit too much complexity for
the benefit given 

+  inline void DCheckSealField(uint32_t field_id) {
+#if DCHECK_IS_ON()
+    DCHECK(field_id > 0);
+    SealField(field_id);
+#endif
+  }
+
  private:
   friend class ProtoZeroMessageTest;
   FRIEND_TEST_ALL_PREFIXES(ProtoZeroMessageTest, BasicTypesNoNesting);
@@ -147,6 +172,10 @@ class TRACING_EXPORT ProtoZeroMessage {
     size_ += size;
   }
 
+#if DCHECK_IS_ON()
+  void SealField(uint32_t field_id);
+#endif
+
   // Only POD fields are allowed. This class's dtor is never called.
   // See the comment on the static_assert in the the corresponding .cc file.