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..70a5370f8e5fe43549ca9d1b1240e2a75ef6831c
--- /dev/null
+++ b/base/files/file_tracing.h
@@ -0,0 +1,94 @@
+// 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/base_export.h"
+#include "base/basictypes.h"
+#include "base/macros.h"
+
+#define FILE_TRACING_PREFIX "File"
+
+#define SCOPED_FILE_TRACE_WITH_SIZE(name, size) \
+    FileTracing::ScopedTrace scoped_file_trace; \
+    if (scoped_file_trace.ShouldInitialize()) \
+      scoped_file_trace.Initialize(FILE_TRACING_PREFIX "::" name, this, size)
+
+#define SCOPED_FILE_TRACE(name) SCOPED_FILE_TRACE_WITH_SIZE(name, 0)
+
+namespace base {
+
+class File;
+class FilePath;
+
+class BASE_EXPORT FileTracing {
+ public:
+  class Provider {
+   public:
+    // Whether the file tracing category is currently enabled.
+    virtual bool FileTracingCategoryIsEnabled() const = 0;
+
+    // Enables file tracing for |id|. Must be called before recording events.
+    virtual void FileTracingEnable(void* id) = 0;
+
+    // Disables file tracing for |id|.
+    virtual void FileTracingDisable(void* id) = 0;
+
+    // Begins an event for |id| with |name|. |path| tells where in the directory
+    // structure the event is happening (and may be blank). |size| is reported
+    // if not 0.
+    virtual void FileTracingEventBegin(
+        const char* name, void* id, const FilePath& path, int64 size) = 0;
+
+    // Ends an event for |id| with |name|. |path| tells where in the directory
+    // structure the event is happening (and may be blank). |size| is reported
+    // if not 0.
+    virtual void FileTracingEventEnd(
+        const char* name, void* id, const FilePath& path, int64 size) = 0;
+  };
+
+  static Provider* g_provider;

[Lei Zhang, 2015/05/08 17:40:19]

Can you have a static SetProvider() method instead of exposing this?

[Dan Beam, 2015/05/08 18:54:03]

Done.

+
+  // Enables file tracing while in scope.
+  class ScopedEnabler {
+   public:
+    ScopedEnabler();
+    ~ScopedEnabler();
+  };
+
+  class ScopedTrace {
+   public:
+    ScopedTrace();
+    ~ScopedTrace();
+
+    // Whether this trace should be initialized or not.
+    bool ShouldInitialize() const;

[Lei Zhang, 2015/05/08 17:40:19]

Any reason to have this separated from Initialize()?

[Dan Beam, 2015/05/08 18:54:03]

less work is done in the common case that the category isn't enabled (only the
category is checked and 1 boolean is initialized).

also, it'd be odd to call Initialize() and have initialized_ be false later.  i
could change things around, but I don't see why it's better...

+
+    // 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 "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(ScopedTrace);
+  };
+
+  DISALLOW_COPY_AND_ASSIGN(FileTracing);
+};
+
+}  // namespace base
+
+#endif  // BASE_FILES_FILE_TRACING_H_