Introduce browser side FrameTreeNodeBlameContext

content/browser/frame_host/frame_tree_node_blame_context.h

Index: content/browser/frame_host/frame_tree_node_blame_context.h
diff --git a/content/browser/frame_host/frame_tree_node_blame_context.h b/content/browser/frame_host/frame_tree_node_blame_context.h
new file mode 100644
index 0000000000000000000000000000000000000000..9042ef31021c6a2abcadf21d24e3baedfaaff852
--- /dev/null
+++ b/content/browser/frame_host/frame_tree_node_blame_context.h
@@ -0,0 +1,79 @@
+// 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.
+
+#ifndef CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_BLAME_CONTEXT_H
+#define CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_BLAME_CONTEXT_H
+
+#include "base/macros.h"
+#include "base/synchronization/lock.h"
+#include "base/trace_event/blame_context.h"
+#include "url/gurl.h"
+
+namespace base {
+namespace trace_event {
+class TracedValue;
+}
+}
+
+namespace content {
+
+class FrameTreeNode;
+
+// FrameTreeNodeBlameContext is a helper class for tracing snapshots of each
+// FrameTreeNode and attributing browser activities to frames (when possible),
+// in the framework of FrameBlamer (crbug.com/546021).
+//
+//
+// *** LIFECYCLE ***
+//
+// Each FrameTreeNodeBlameContext is owned by a FrameTreeNode, and each
+// FrameTreeNode owns at most one FrameTreeNodeBlameContext.  The lifecycle
+// of each FrameTreeNodeBlameContext is managed by its owner node.
+// Creation: For each root FrameTreeNode, its blame context is created right
+// after the node's intialization.  For each other node, its blame context is
+// created after the node is attached to a parent node. If the owner node is
+// reattached to a different parent node, its original blame context gets
+// destroyed and replaced by a new one.
+// Update: Whenever the owner node navigates, it calls UpdateArguments() of
+// its blame context to update its value. The is the only way to update the
+// value of a FrameTreeNodeBlameContext.
+// Deletion: A FrameTreeNodeBlameContext is deleted when its owner node is
+// deleted or reattached to a different parent node.
+//
+//
+// *** THREAD SAFETY ***
+//
+// FrameTreeNodeBlameContexts live in the main thread, with their lifecycle
+// managed by their owner FrameTreeNodes.  The creation, update and deletion
+// can only be done in the main thread.  However, a FrameTreeNodeBlameContext
+// can be read from a different thread, e.g., from the File thread at tracing
+// startup.  To prevent data race while minimizing architectural impact, locks
+// are applied so that AsValueInto() will be blocked when the main thread is
+// running UpdateArguments().
+
+class FrameTreeNodeBlameContext : public base::trace_event::BlameContext {
+ public:
+  // Public functions are callable only from the main thread by the owner node.
+  FrameTreeNodeBlameContext(FrameTreeNode* node);

[dcheng, 2016/04/26 20:06:29]

Nit: explicit

+  ~FrameTreeNodeBlameContext() override;
+  void UpdateArguments();
+
+ private:
+  void AsValueInto(base::trace_event::TracedValue* value) override;
+
+  // Buffered information of the owner node, maintained by UpdateArguments().
+  int process_id_;
+  int routing_id_;
+  GURL url_;
+
+  FrameTreeNode* const owner_;
+
+  mutable base::Lock lock_;
+
+  DISALLOW_COPY_AND_ASSIGN(FrameTreeNodeBlameContext);
+};
+
+}  // content
+
+#endif  // CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_BLAME_CONTEXT_H