[GN] Don't rewrite files with the same contents

tools/gn/function_write_file.cc

 // Copyright (c) 2013 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.
 
 #include <iostream>
 #include <sstream>
 
 #include "base/files/file_util.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
@@ skipping 112 matching lines @@
 
   // Track how to recreate this file, since we write it a gen time.
   // Note this is a hack since the correct output is not a dependency proper,
   // but an addition of this file to the output of the gn rule that writes it.
   // This dependency will, however, cause the gen step to be re-run and the
   // build restarted if the file is missing.
   g_scheduler->AddGenDependency(
       scope->settings()->build_settings()->GetFullPath(source_file));
 
   // Compute output.
-  std::ostringstream contents;
+  std::stringstream contents;
   if (args[1].type() == Value::LIST) {
     const std::vector<Value>& list = args[1].list_value();
     for (const auto& cur : list)
       contents << cur.ToString(false) << std::endl;
   } else {
     contents << args[1].ToString(false);
   }
-  const std::string& new_contents = contents.str();
+
   base::FilePath file_path =
       scope->settings()->build_settings()->GetFullPath(source_file);
 
   // Make sure we're not replacing the same contents.
-  std::string existing_contents;
-  if (base::ReadFileToString(file_path, &existing_contents) &&
-      existing_contents == new_contents)
+  if (ContentsEqual(file_path, &contents))
     return Value();  // Nothing to do.
 
   // Write file, creating the directory if necessary.

[brettw, 2016/02/02 20:50:23]

Can the rest of this function and DoWriteFile all be merged into
WriteFileIfChanged?

[Tomasz Moniuszko, 2016/02/03 10:20:30]

Done.

   if (!base::CreateDirectory(file_path.DirName())) {
     *err = Err(function->function(), "Unable to create directory.",
                "I was using \"" + FilePathToUTF8(file_path.DirName()) + "\".");
     return Value();
   }
 
+  const std::string& new_contents = contents.str();
   int int_size = static_cast<int>(new_contents.size());
   if (DoWriteFile(file_path, new_contents.c_str(), int_size)
       != int_size) {
     *err = Err(function->function(), "Unable to write file.",
                "I was writing \"" + FilePathToUTF8(file_path) + "\".");
     return Value();
   }
   return Value();
 }
 
 }  // namespace functions