Add granular file tracing.

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index 89077b41c1d8d03a9aa1e33ea822b9fb9454f807..55595804be6e6891fadeb8ff311d29a84f80932f 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -18,6 +18,7 @@
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/files/file_path.h"
 #include "base/files/scoped_file.h"
 #include "base/gtest_prod_util.h"
 #include "base/move.h"
@@ -31,8 +32,6 @@ FORWARD_DECLARE_TEST(FileTest, MemoryCorruption);
 
 namespace base {
 
-class FilePath;
-
 #if defined(OS_WIN)
 typedef HANDLE PlatformFile;
 #elif defined(OS_POSIX)
@@ -162,8 +161,8 @@ class BASE_EXPORT File {
   File();
 
   // Creates or opens the given file. This will fail with 'access denied' if the
-  // |name| contains path traversal ('..') components.
-  File(const FilePath& name, uint32 flags);
+  // |path| contains path traversal ('..') components.
+  File(const FilePath& path, uint32 flags);
 
   // Takes ownership of |platform_file|.
   explicit File(PlatformFile platform_file);
@@ -180,7 +179,7 @@ class BASE_EXPORT File {
   File& operator=(RValue other);
 
   // Creates or opens the given file.
-  void Initialize(const FilePath& name, uint32 flags);
+  void Initialize(const FilePath& path, uint32 flags);
 
   bool IsValid() const;
 
@@ -191,7 +190,7 @@ class BASE_EXPORT File {
 
   // Returns the OS result of opening this file. Note that the way to verify
   // the success of the operation is to use IsValid(), not this method:
-  //   File file(name, flags);
+  //   File file(path, flags);
   //   if (!file.IsValid())
   //     return;
   Error error_details() const { return error_details_; }
@@ -305,6 +304,9 @@ class BASE_EXPORT File {
  private:
   FRIEND_TEST_ALL_PREFIXES(::FileTest, MemoryCorruption);
 
+  // The tracing group file traces will show up as.
+  static const char kTraceGroup[];

[Sami, 2015/04/28 11:10:39]

nit: Usually we just write the trace category explicitly in each trace event. It
makes it easier to grep where trace events are coming from, and the compiler
will fold the identical strings into one anyway.

[Dan Beam, 2015/04/28 15:47:58]

I guess I can do this, it's just... TRACE_DISABLED_BY_DEFAULT("file") is pretty
long.

[Sami, 2015/04/28 15:51:05]

I don't think it's a hard rule, so I'll leave it up to you to decide.

[groby-ooo-7-16, 2015/04/28 20:26:54]

I don't have a specific preference, but consider naming it kFileTraceGroup -
this way we can still grep somewhat reliably.

[Dan Beam, 2015/04/29 02:48:03]

  $ git grep \\bkTraceGroup\\b

yields only these results reliably

+
 #if defined(OS_POSIX)
   // Encloses a single ScopedFD, saving a cheap tamper resistent memory checksum
   // alongside it. This checksum is validated at every access, allowing early
@@ -350,9 +352,9 @@ class BASE_EXPORT File {
   };
 #endif
 
-  // Creates or opens the given file. Only called if |name| has no traversal
+  // Creates or opens the given file. Only called if |path_| has no traversal
   // ('..') components.
-  void DoInitialize(const FilePath& name, uint32 flags);
+  void DoInitialize(uint32 flags);
 
   // TODO(tnagel): Reintegrate into Flush() once histogram isn't needed anymore,
   // cf. issue 473337.
@@ -366,6 +368,9 @@ class BASE_EXPORT File {
   MemoryCheckingScopedFD file_;
 #endif
 

[groby-ooo-7-16, 2015/04/28 20:26:54]

|unsafe_path_|, then?

[Dan Beam, 2015/04/29 02:48:02]

Done.

+  // Path that |Initialize()| was called with. Can be unsafe (e.g. have '..').
+  FilePath path_;
+
   Error error_details_;
   bool created_;
   bool async_;