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

tools/gn/filesystem_utils.cc

Index: tools/gn/filesystem_utils.cc
diff --git a/tools/gn/filesystem_utils.cc b/tools/gn/filesystem_utils.cc
index 5905cdb9f1ec73749a03e15428d5109542712b22..870438405934402d537746fbeef6ee6c60b59fd6 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -687,6 +687,33 @@ std::string GetOutputSubdirName(const Label& toolchain_label, bool is_default) {
   return toolchain_label.name() + "/";
 }
 
+bool ContentsEqual(const base::FilePath& file_path, std::stringstream* data) {

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

I'm thinking it might be better if this took a std::string as input so we can
avoid making a string twice out of the stringstream (these will be big strings
so we want to avoid duplicates)

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

Done.

+  // Compare file and stream sizes first. Quick and will save us some time if
+  // they are different sizes.
+  int64_t data_size = data->tellp();
+
+  int64_t file_size;
+  if (!base::GetFileSize(file_path, &file_size) || file_size != data_size)
+    return false;
+
+  std::string file_data;
+  file_data.resize(file_size);
+  if (!base::ReadFileToString(file_path, &file_data))
+    return false;
+
+  return file_data == data->str();
+}
+
+bool WriteFileIfChanged(const base::FilePath& file_path,
+                        std::stringstream* data) {
+  if (ContentsEqual(file_path, data))
+    return true;
+
+  std::string data_str = data->str();
+  int size = static_cast<int>(data_str.size());
+  return base::WriteFile(file_path, data_str.c_str(), size) == size;
+}
+
 SourceDir GetToolchainOutputDir(const Settings* settings) {
   return settings->toolchain_output_subdir().AsSourceDir(
       settings->build_settings());