[wrapper-tracing] Add heap snapshot generator infrastructure

third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp

Index: third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
index 6ded783a981de097db3e6563388ac6f197bae47a..f5df87e500e46bd5cb030265d4bc500f4acefb5a 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
@@ -51,6 +51,7 @@
 #include "wtf/Vector.h"
 #include "wtf/allocator/Partitions.h"
 #include <algorithm>
+#include <unordered_set>

[haraken, 2017/01/12 16:25:17]

Remove this? You're using WTF::HashSet.

[Michael Lippautz, 2017/01/13 15:08:40]

I rewrote now everything using std:: types which can be easily passed across the
API boundary. If you feel that we should use WTF:: during processing and only
convert to std:: in the end just left me know.

 
 namespace blink {
 
@@ -148,6 +149,121 @@ class MinorGCUnmodifiedWrapperVisitor : public v8::PersistentHandleVisitor {
   v8::Isolate* m_isolate;
 };
 
+class HeapSnaphotWrapperVisitor : public ScriptWrappableVisitor {
+ public:
+  explicit HeapSnaphotWrapperVisitor(v8::Isolate* isolate)
+      : ScriptWrappableVisitor(isolate) {}
+
+  // Trace through the blink heap to find all V8 wrappers reachable from
+  // ActiveScriptWrappables.
+  v8::HeapProfiler::RetainerChildren findPendingWrappers() {
+    m_foundV8Wrappers.clear();
+
+    TracePrologue();
+    ActiveScriptWrappableBase::traceActiveScriptWrappables(m_isolate, this);
+    AdvanceTracing(
+        0,
+        v8::EmbedderHeapTracer::AdvanceTracingActions(
+            v8::EmbedderHeapTracer::ForceCompletionAction::FORCE_COMPLETION));
+    // Abort instead of Epilogue as we want to finish synchronously.
+    AbortTracing();
+
+    return std::move(m_foundV8Wrappers);
+  }
+
+  // Trace through the blink heap to find all V8 wrappers reachable from
+  // |traceable|.
+  v8::HeapProfiler::RetainerChildren findV8WrappersReachableFrom(
+      ScriptWrappable* traceable) {
+    m_foundV8Wrappers.clear();
+
+    TracePrologue();
+    traceable->wrapperTypeInfo()->traceWrappers(this, traceable);
+    AdvanceTracing(
+        0,
+        v8::EmbedderHeapTracer::AdvanceTracingActions(
+            v8::EmbedderHeapTracer::ForceCompletionAction::FORCE_COMPLETION));
+    // Abort instead of Epilogue as we want to finish synchronously.
+    AbortTracing();
+
+    return std::move(m_foundV8Wrappers);
+  }
+
+  void markWrapper(const v8::PersistentBase<v8::Value>* value) const override {
+    if (!m_tracingInProgress)
+      return;
+    m_foundV8Wrappers.insert(value);
+  }
+
+ private:
+  mutable v8::HeapProfiler::RetainerChildren m_foundV8Wrappers;
+};
+
+class HeapSnapshotHandlesVisitor : public v8::PersistentHandleVisitor {

[haraken, 2017/01/12 16:25:17]

In short term, this is fine, but I'm not sure if it's a good idea to identify a
list of root nodes by HeapSnapshotHandlesVisitor.

In theory, it's possible that there is a Node X such that X is reachable from
the root set of a V8 GC but X doesn't have a wrapper. Then X won't be detected
by v8::PersistentHandleVisitor. So, to get a full list of the root Nodes, we'll
need to use the wrapper tracing instead of v8::PersistentHandleVisitor.

[Michael Lippautz, 2017/01/13 15:08:40]

See general comment.

+ public:
+  explicit HeapSnapshotHandlesVisitor(v8::Isolate* isolate)
+      : m_isolate(isolate) {}
+
+  void VisitPersistentHandle(v8::Persistent<v8::Value>* value,
+                             uint16_t classId) override {
+    if (classId != WrapperTypeInfo::NodeClassId &&
+        classId != WrapperTypeInfo::ObjectClassId)

[haraken, 2017/01/12 16:25:17]

Remove '&& classId != WrapperTypeInfo::ObjectClassId'. Here we're interested in
only Nodes.

[Michael Lippautz, 2017/01/13 15:08:40]

Done.

+      return;
+
+    if (value->IsIndependent())

[haraken, 2017/01/12 16:25:17]

DCHECK(!value->IsIndependent()). Nodes should be dependent objects.

[Michael Lippautz, 2017/01/13 15:08:40]

Done.

+      return;
+
+    if (classId == WrapperTypeInfo::NodeClassId) {

[haraken, 2017/01/12 16:25:17]

Remove the if statement.

[Michael Lippautz, 2017/01/13 15:08:40]

Done.

+      v8::Local<v8::Object> wrapper = v8::Local<v8::Object>::New(
+          m_isolate, v8::Persistent<v8::Object>::Cast(*value));
+      DCHECK(V8Node::hasInstance(wrapper, m_isolate));
+      Node* node = V8Node::toImpl(wrapper);
+      Node* root = V8GCController::opaqueRootForGC(m_isolate, node);
+      m_nodesRequiringTracing.add(root);
+    }
+  }
+
+  WTF::HashSet<UntracedMember<Node>> nodesRequiringTracing() {
+    return std::move(m_nodesRequiringTracing);

[haraken, 2017/01/12 16:25:17]

It might be clearer to use std::unique_ptr<WTF::HashSet<UntracedMember<Node>>>.
Then you can return m_nodesRequiringTracing.get().

[Michael Lippautz, 2017/01/13 15:08:40]

Doesn't apply anymore.

+  }
+
+ private:
+  v8::Isolate* m_isolate;
+  WTF::HashSet<UntracedMember<Node>> m_nodesRequiringTracing;
+};
+
+v8::HeapProfiler::RetainerInfos V8GCController::getRetainerInfos(
+    v8::Isolate* isolate) {
+  v8::HeapProfiler::RetainerInfos infos;
+  HeapSnaphotWrapperVisitor tracer(isolate);
+  V8PerIsolateData::TemporaryScriptWrappableVisitorScope scope(isolate,
+                                                               &tracer);
+
+  // Find wrappers for pending activities.
+  {
+    v8::HeapProfiler::RetainerChildren wrappers = tracer.findPendingWrappers();
+    if (!wrappers.empty()) {
+      infos.push_back(std::make_pair(
+          new SuspendableObjectsInfo(wrappers.size()), std::move(wrappers)));
+    }
+  }
+
+  // Find wrappers for DOM trees.
+  {
+    HeapSnapshotHandlesVisitor visitor(isolate);
+    isolate->VisitHandlesWithClassIds(&visitor);

[haraken, 2017/01/12 16:25:17]

Why can't we use |&tracer|?

[Michael Lippautz, 2017/01/13 15:08:40]

Rewrote that tracer so that it implements both required interfaces
(ScriptWrappableVisitor and v8::PersistentHandleVisitor).

+    for (auto root : visitor.nodesRequiringTracing()) {
+      auto wrappers = tracer.findV8WrappersReachableFrom(root);
+      if (!wrappers.empty()) {
+        infos.push_back(
+            std::make_pair(new RetainedDOMInfo(root), std::move(wrappers)));
+      }
+    }
+  }
+
+  return infos;
+}
+
 class MajorGCWrapperVisitor : public v8::PersistentHandleVisitor {
  public:
   explicit MajorGCWrapperVisitor(v8::Isolate* isolate,