Add granular file tracing.

base/files/file_tracing.h

Index: base/files/file_tracing.h
diff --git a/base/files/file_tracing.h b/base/files/file_tracing.h
new file mode 100644
index 0000000000000000000000000000000000000000..fe63ee6e90c5ce413809ca6a9e9a2ab5371940c7
--- /dev/null
+++ b/base/files/file_tracing.h
@@ -0,0 +1,65 @@
+// Copyright 2015 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 BASE_FILES_FILE_TRACING_H_
+#define BASE_FILES_FILE_TRACING_H_
+
+#include "base/basictypes.h"
+#include "base/macros.h"
+#include "base/trace_event/trace_event.h"
+
+#define FILE_TRACING_PREFIX "base::File"

[oystein (OOO til 10th of July), 2015/05/01 18:15:31]

I'm not sure if "base" is really adding any useful information to these events,
but maybe I'm missing something. What was the reason for adding this?

[Dan Beam, 2015/05/01 23:29:04]

just to match code, removed (noticed many places omit namespace)

+
+#define FILE_TRACING_BEGIN() \
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0( \
+        base::kFileTracingEventCategoryGroup, FILE_TRACING_PREFIX, this);
+
+#define FILE_TRACING_END() \
+    TRACE_EVENT_NESTABLE_ASYNC_END0( \
+        base::kFileTracingEventCategoryGroup, FILE_TRACING_PREFIX, this);
+
+#define SCOPED_FILE_TRACE_WITH_SIZE(name, size) \
+    base::ScopedFileTrace trace; \
+    bool category_enabled; \

[oystein (OOO til 10th of July), 2015/05/01 18:15:31]

nit: call the scope something other than "trace" (prefix with an underscore
maybe?) and wrap the rest in {} so the macro doesn't pollute the function
namespace unnecessarily.

[Dan Beam, 2015/05/01 23:29:04]

Done.

+    TRACE_EVENT_CATEGORY_GROUP_ENABLED( \
+        base::kFileTracingEventCategoryGroup, &category_enabled); \
+    if (category_enabled) \
+      trace.Initialize(FILE_TRACING_PREFIX "::" name, this, size);
+
+#define SCOPED_FILE_TRACE(name) SCOPED_FILE_TRACE_WITH_SIZE(name, 0)
+
+namespace base {
+
+class File;
+
+extern const char kFileTracingEventCategoryGroup[];
+
+class ScopedFileTrace {
+ public:
+  ScopedFileTrace();
+  ~ScopedFileTrace();
+
+  // Called only if the tracing category is enabled.
+  void Initialize(const char* event, File* file, int64 size);
+
+ private:
+  // True if |Initialize()| has been called. Don't touch |path_|, |event_|,
+  // or |bytes_| if |initialized_| is false.
+  bool initialized_;
+
+  // The event name to trace (e.g. "Read", "Write"). Prefixed with "base::File".
+  const char* name_;
+
+  // The file being traced. Must outlive this class.
+  File* file_;
+
+  // The size (in bytes) of this trace. Not reported if 0.
+  int64 size_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFileTrace);
+};
+
+}  // namespace base
+
+#endif  // BASE_FILES_FILE_TRACING_H_