GN only writes files if they've changed

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/file_util.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
@@ skipping 10 matching lines @@
 const char kWriteFile_HelpShort[] =
     "write_file: Write a file to disk.";
 const char kWriteFile_Help[] =
     "write_file: Write a file to disk.\n"
     "\n"
     "  write_file(filename, data)\n"
     "\n"
     "  If data is a list, the list will be written one-item-per-line with no\n"
     "  quoting or brackets.\n"
     "\n"
+    "  If the file exists and the contents are identical to that being\n"
+    "  written, the file will not be updated. This will prevent unnecessary\n"
+    "  rebuilds of targets that depend on this file.\n"
+    "\n"
     "  TODO(brettw) we probably need an optional third argument to control\n"
     "  list formatting.\n"
     "\n"
     "Arguments:\n"
     "\n"
     "  filename\n"
     "      Filename to write. This must be within the output directory.\n"
     "\n"
     "  data:\n"
     "      The list or string to write.\n";
@@ skipping 20 matching lines @@
 
   // Compute output.
   std::ostringstream contents;
   if (args[1].type() == Value::LIST) {
     const std::vector<Value>& list = args[1].list_value();
     for (size_t i = 0; i < list.size(); i++)
       contents << list[i].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)
+    return Value();  // Nothing to do.
 
   // Write file, creating the directory if necessary.
-  base::FilePath file_path =
-      scope->settings()->build_settings()->GetFullPath(source_file);
-  const std::string& contents_string = contents.str();
   if (!base::CreateDirectory(file_path.DirName())) {
     *err = Err(function->function(), "Unable to create directory.",
                "I was using \"" + FilePathToUTF8(file_path.DirName()) + "\".");
     return Value();
   }
-  int int_size = static_cast<int>(contents_string.size());
-  if (base::WriteFile(file_path, contents_string.c_str(), int_size)
+
+  int int_size = static_cast<int>(new_contents.size());
+  if (base::WriteFile(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