tracing v2: minor refactoring to TraceRingBuffer test helpers

components/tracing/core/trace_ring_buffer.cc

 // 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/trace_ring_buffer.h"
 
 #include "base/threading/platform_thread.h"
 
 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_]);
   uint8_t* chunk_begin = begin;
   for (size_t i = 0; i < num_chunks_; ++i) {
     chunks_[i].Initialize(chunk_begin);
     chunk_begin += Chunk::kSize;
   }
 }
 
 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_);
   for (size_t i = 0; i < num_chunks_; ++i) {
     Chunk* chunk = &chunks_[current_chunk_idx_];
     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;
     }
   }
 
   // Bankrupcy: there are more threads than chunks. All chunks were in flight.
   if (!bankrupcy_chunk_storage_) {
     bankrupcy_chunk_storage_.reset(new uint8_t[Chunk::kSize]);
     bankrupcy_chunk_.Initialize(&bankrupcy_chunk_storage_.get()[0]);
   }
   bankrupcy_chunk_.Clear();
   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;
+
+  // TODO(primiano): this lock might be removed in favor of acquire/release
+  // semantics on the two vars below. Check once the reader of these are landed.
   base::AutoLock lock(lock_);
-  chunk->set_used_size(used_size);
   chunk->clear_owner();
+  num_chunks_taken_--;
+}
+
+bool TraceRingBuffer::IsBankrupcyChunkForTesting(const Chunk* chunk) const {
+  return chunk == &bankrupcy_chunk_;
+}
+
+size_t TraceRingBuffer::GetNumChunksTaken() const {
+  base::AutoLock lock(lock_);
+  return num_chunks_taken_;
 }
 
 TraceRingBuffer::Chunk::Chunk()
-    : begin_(nullptr), owner_(base::kInvalidThreadId) {}
+    : begin_(nullptr), owner_(kNoChunkOwner), 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) {
   DCHECK_EQ(0ul, reinterpret_cast<uintptr_t>(begin) % sizeof(uintptr_t));
   begin_ = begin;
 }
 
 }  // namespace v2
 }  // namespace tracing