Log pref file size histogram on read rather than on write.

base/json/json_file_value_serializer.h

 // Copyright (c) 2012 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_JSON_JSON_FILE_VALUE_SERIALIZER_H_
 #define BASE_JSON_JSON_FILE_VALUE_SERIALIZER_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
 #include "base/values.h"
 
 class BASE_EXPORT JSONFileValueSerializer : public base::ValueSerializer {
  public:
-  // json_file_patch is the path of a file that will be source of the
+  explicit JSONFileValueSerializer(const base::FilePath& json_file_path)
+    : JSONFileValueSerializer(json_file_path, std::string()) {}

[Alexei Svitkine (slow), 2015/01/30 13:49:03]

Nit: Indent 2 more. Maybe run git cl format?

[gab, 2015/01/30 17:33:37]

Good point, git cl formatted.

+
+  // |json_file_path_| is the path of a file that will be source of the
   // deserialization or the destination of the serialization.
   // When deserializing, the file should exist, but when serializing, the
   // serializer will attempt to create the file at the specified location.
-  explicit JSONFileValueSerializer(const base::FilePath& json_file_path)
+  // If |histogram_suffix| is non-empty, will log a
+  // "Settings.JsonDataSizeKilobytes.|histogram_suffix|" UMA histogram with the
+  // size of the content read from disk while deserializing.
+  JSONFileValueSerializer(const base::FilePath& json_file_path,
+                          const std::string& histogram_suffix)
     : json_file_path_(json_file_path),
-      allow_trailing_comma_(false) {}
+      allow_trailing_comma_(false),
+      histogram_suffix_(histogram_suffix) {}

[Alexei Svitkine (slow), 2015/01/30 13:49:03]

Nit: The ctors and dtor should probably be defined out of line.

[gab, 2015/01/30 17:33:37]

They were already inlined so I'm not sure this CL justifies changing that, I
think the reason clang allows this is that all members are POD-types (or made
only of POD types) -- i.e., all construction/destruction can be inlined.

[Alexei Svitkine (slow), 2015/01/30 17:41:16]

But they're not POD types... a std::string is backed by a buffer on the heap and
involves a string copy...

I think the motivation for the rule is that we don't want ctors/dtors that
generate a lot of machine code (e.g. in this case, the ctors for std::string can
be inlined in this one) to be in headers, as then they end up in multiple object
files and have to be de-duped by the linker, slowing the build. I think the
above still applies to this case.

Also, since you're adding a new ctor, this change is making this existing
problem worse. So I would prefer you do actually out of line them... but won't
block you on this if you really insist...

[gab, 2015/01/30 18:22:34]

Okay you convinced me, done.

 
   ~JSONFileValueSerializer() override {}
 
   // DO NOT USE except in unit tests to verify the file was written properly.
   // We should never serialize directly to a file since this will block the
   // thread. Instead, serialize to a string and write to the file you want on
   // the file thread.
   //
   // Attempt to serialize the data structure represented by Value into
   // JSON.  If the return value is true, the result will have been written
@@ skipping 33 matching lines @@
   // be a JsonFileError.
   static const char* GetErrorMessageForCode(int error_code);
 
   void set_allow_trailing_comma(bool new_value) {
     allow_trailing_comma_ = new_value;
   }
 
  private:
   bool SerializeInternal(const base::Value& root, bool omit_binary_values);
 
-  base::FilePath json_file_path_;
+  const base::FilePath json_file_path_;
   bool allow_trailing_comma_;
+  const std::string histogram_suffix_;
 
   // A wrapper for ReadFileToString which returns a non-zero JsonFileError if
   // there were file errors.
   int ReadFileToString(std::string* json_string);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSONFileValueSerializer);
 };
 
 #endif  // BASE_JSON_JSON_FILE_VALUE_SERIALIZER_H_