[wrapper-tracing] Add heap snapshot generator infrastructure

third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp

Index: third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp b/third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp
index e6e7ce8ed3835089017642f63ae9bedf72b2b30e..ccaebaffbcb8c9fd680a7d28b725a2fe65123411 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/V8PerIsolateData.cpp
@@ -397,4 +397,14 @@ void V8PerIsolateData::addActiveScriptWrappable(
   m_activeScriptWrappables->add(wrappable);
 }
 
+void V8PerIsolateData::TemporaryScriptWrappableVisitorScope::setVisitor(
+    ScriptWrappableVisitor* visitor) {
+  ScriptWrappableVisitor* current = currentVisitor(m_isolate);
+  if (current)
+    current->performCleanup();

[haraken, 2017/01/16 02:46:56]

Would you elaborate on why it's okay to call performCleanup()? Isn't it possible
that the current visitor is popped again later and tries to continue the
incremental tracing?

[Michael Lippautz, 2017/01/16 10:45:06]

We can only swap tracers when tracing is not yet started. Not started still
could mean that we have a pending cleanup task. performCleanup() conditionally
cleans up the tracer.

I added a CHECK to performCleanup that we are not in tracing mode.

[haraken, 2017/01/17 00:31:25]

How is it guaranteed that V8GCController::getRetainerInfos is called when the
tracing is not yet started? Does V8 guarantee the fact?

[Michael Lippautz, 2017/01/17 09:15:32]

Yes, V8 guarantees that.

+  V8PerIsolateData::from(m_isolate)->m_scriptWrappableVisitor.release();

[haraken, 2017/01/16 02:46:56]

Ah, you need .release() because you need to leak the pointer -- which looks a
bit nasty.

Can we make TemporaryScriptWrappableVisitorScope::m_savedVisitor a
std::unique_ptr<ScriptWrappableVisitor>? Then you can just pass an ownership to
m_savedVisitor instead of leaking.

[Michael Lippautz, 2017/01/16 10:45:06]

Ah, now I understand, thanks! Done.

+  V8PerIsolateData::from(m_isolate)->m_scriptWrappableVisitor.reset(visitor);
+  m_isolate->SetEmbedderHeapTracer(visitor);
+}
+
 }  // namespace blink