tracing v2: minor refactoring to TraceRingBuffer test helpers

components/tracing/core/trace_ring_buffer.cc

Index: components/tracing/core/trace_ring_buffer.cc
diff --git a/components/tracing/core/trace_ring_buffer.cc b/components/tracing/core/trace_ring_buffer.cc
index 2a52bf61db1b64350f7d3e20a7f5acb4da63b2c1..6abcdd22081e777f5932596af71ab91e4a0f0b4b 100644
--- a/components/tracing/core/trace_ring_buffer.cc
+++ b/components/tracing/core/trace_ring_buffer.cc
@@ -10,7 +10,9 @@ namespace tracing {
 namespace v2 {
 
 TraceRingBuffer::TraceRingBuffer(uint8_t* begin, size_t size)
-    : num_chunks_(size / Chunk::kSize), current_chunk_idx_(0) {
+    : num_chunks_(size / Chunk::kSize),
+      num_chunks_taken_(0),
+      current_chunk_idx_(0) {
   DCHECK_GT(num_chunks_, 0u);
   DCHECK_EQ(0ul, reinterpret_cast<uintptr_t>(begin) % sizeof(uintptr_t));
   chunks_.reset(new Chunk[num_chunks_]);
@@ -23,7 +25,7 @@ TraceRingBuffer::TraceRingBuffer(uint8_t* begin, size_t size)
 
 TraceRingBuffer::~TraceRingBuffer() {}
 
-TraceRingBuffer::Chunk* TraceRingBuffer::TakeChunk() {
+TraceRingBuffer::Chunk* TraceRingBuffer::TakeChunk(uint32_t writer_id) {
   base::AutoLock lock(lock_);
   DCHECK_GT(num_chunks_, 0ul);
   DCHECK_LT(current_chunk_idx_, num_chunks_);
@@ -32,7 +34,9 @@ TraceRingBuffer::Chunk* TraceRingBuffer::TakeChunk() {
     current_chunk_idx_ = (current_chunk_idx_ + 1) % num_chunks_;
     if (!chunk->is_owned()) {
       chunk->Clear();
-      chunk->set_owner(base::PlatformThread::CurrentId());
+      DCHECK_NE(0u, writer_id);
+      chunk->set_owner(writer_id);
+      num_chunks_taken_++;
       return chunk;
     }
   }
@@ -46,20 +50,41 @@ TraceRingBuffer::Chunk* TraceRingBuffer::TakeChunk() {
   return &bankrupcy_chunk_;
 }
 
-void TraceRingBuffer::ReturnChunk(TraceRingBuffer::Chunk* chunk,
-                                  uint32_t used_size) {
+void TraceRingBuffer::ReturnChunk(TraceRingBuffer::Chunk* chunk) {
+  // Returning a chunk without using it is quite odd, very likely a bug.
+  DCHECK_GT(chunk->used_size(), 0u);
+
+  // The caller should never return chunks which are part of a retaining chain.
+  DCHECK(!chunk->next_in_owner_list());
+
+  if (chunk == &bankrupcy_chunk_)
+    return;
+
   base::AutoLock lock(lock_);
-  chunk->set_used_size(used_size);
   chunk->clear_owner();

[alph, 2016/07/30 07:41:14]

If you make the owner_ atomic you could omit locking on returning the chunk.

[Primiano Tucci (use gerrit), 2016/08/04 15:26:54]

I was thinking to this but I'd wait to have the code that reads this before.
The think is: either these can become acquire/release semantic (which are free
on x86 and lighter on arm), but for that I have to see clearly what the reader
is doing.
If I just make atomic exchanges, two atomic exchanges (or increment) are going
to be as expensive as a lock (and still prone to be misused).
Added a TODO

+  num_chunks_taken_--;

[alph, 2016/07/30 07:41:14]

ditto

+}
+
+bool TraceRingBuffer::IsBankrupcyChunkForTesting(const Chunk* chunk) const {
+  return chunk->begin() >= chunks_[num_chunks_ - 1].end() ||

[alph, 2016/07/30 07:41:14]

I believe unrelated pointer comparison is an undefined behavior. E.g. if chunk
eventually happens to be the bankrupcy_chunk_

[Primiano Tucci (use gerrit), 2016/08/04 15:26:54]

Hmm not sure I follow. this is comparing just uint8_t* pointers of chunks.begin
/ chunk.end ?
In the case of chunk = bankrupcy_chunk that must have a begin / end that is not
within the chunk's storage area.
Where is the undefined behavior?

[Primiano Tucci (use gerrit), 2016/08/04 15:33:50]

Thinking more, I think this would be UB if I was comparing the chunk vs
&chunks_[x] (the chunks pointer themselves).
But here I am comparing really the value of **fields** of three chunks, which is
just a member variable set on construction. I don't think this is any different
then begin() and end() being an int value rather than uint8_t*

+         chunk->end() <= chunks_[0].begin();
+}
+
+size_t TraceRingBuffer::GetNumChunksTaken() const {
+  base::AutoLock lock(lock_);
+  return num_chunks_taken_;
 }
 
 TraceRingBuffer::Chunk::Chunk()
-    : begin_(nullptr), owner_(base::kInvalidThreadId) {}
+    : begin_(nullptr),
+      owner_(base::kInvalidThreadId),

[alph, 2016/07/30 07:41:14]

owner_(0)

[Primiano Tucci (use gerrit), 2016/08/04 15:26:54]

Oops.

+      next_in_owner_list_(nullptr) {}
 
 TraceRingBuffer::Chunk::~Chunk() {}
 
 void TraceRingBuffer::Chunk::Clear() {
   set_used_size(0);
+  set_next_in_owner_list(nullptr);
 }
 
 void TraceRingBuffer::Chunk::Initialize(uint8_t* begin) {