[heap] Create a thin wrapper around wrapper tracing in V8 to avoid misuse

src/heap/embedder-tracing.cc

Index: src/heap/embedder-tracing.cc
diff --git a/src/heap/embedder-tracing.cc b/src/heap/embedder-tracing.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8d5e54090f93e9d660f2525ef20eb3b23579fb18
--- /dev/null
+++ b/src/heap/embedder-tracing.cc
@@ -0,0 +1,71 @@
+// Copyright 2016 the V8 project 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 "src/heap/embedder-tracing.h"
+
+#include "src/base/logging.h"
+
+namespace v8 {
+namespace internal {
+
+void LocalEmbedderHeapTracer::TracePrologue() {
+  if (!InUse()) return;
+
+  CHECK(cached_wrappers_to_trace_.empty());
+  remote_tracer_->TracePrologue();
+}
+
+void LocalEmbedderHeapTracer::TraceEpilogue() {
+  if (!InUse()) return;
+
+  CHECK(cached_wrappers_to_trace_.empty());
+  remote_tracer_->TraceEpilogue();
+}
+
+void LocalEmbedderHeapTracer::AbortTracing() {
+  if (!InUse()) return;
+
+  cached_wrappers_to_trace_.clear();
+  remote_tracer_->AbortTracing();
+}
+
+void LocalEmbedderHeapTracer::EnterFinalPause() {
+  if (!InUse()) return;
+
+  remote_tracer_->EnterFinalPause();
+}
+
+bool LocalEmbedderHeapTracer::Trace(
+    double deadline, EmbedderHeapTracer::AdvanceTracingActions actions) {
+  if (!InUse()) return false;
+
+  RegisterWrappersWithRemoteTracer();
+  return remote_tracer_->AdvanceTracing(deadline, actions);
+}
+
+size_t LocalEmbedderHeapTracer::NumberOfWrappersToTrace() {
+  return (InUse())
+             ? cached_wrappers_to_trace_.size() +
+                   remote_tracer_->NumberOfWrappersToTrace()
+             : 0;
+}
+
+void LocalEmbedderHeapTracer::RegisterWrappersWithRemoteTracer() {
+  if (!InUse()) return;
+
+  if (cached_wrappers_to_trace_.empty()) {
+    return;
+  }
+
+  remote_tracer_->RegisterV8References(cached_wrappers_to_trace_);
+  cached_wrappers_to_trace_.clear();
+}
+
+bool LocalEmbedderHeapTracer::RequiresImmediateWrapperProcessing() {
+  const size_t kTooManyWrappers = 16000;
+  return cached_wrappers_to_trace_.size() > kTooManyWrappers;
+}
+
+}  // namespace internal
+}  // namespace v8