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

tools/gn/filesystem_utils_unittest.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 "base/files/file_path.h"
+#include "base/files/file_util.h"
+#include "base/files/scoped_temp_dir.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/threading/platform_thread.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/target.h"
 
 TEST(FilesystemUtils, FileExtensionOffset) {
   EXPECT_EQ(std::string::npos, FindExtensionOffset(""));
   EXPECT_EQ(std::string::npos, FindExtensionOffset("foo/bar/baz"));
   EXPECT_EQ(4u, FindExtensionOffset("foo."));
   EXPECT_EQ(4u, FindExtensionOffset("f.o.bar"));
@@ skipping 529 matching lines @@
   EXPECT_EQ("/SOURCE/foo/bar/", SourceDirForPath(root,
             base::FilePath("/SOURCE/foo/bar/")).value());
 
   // Empty source dir.
   base::FilePath empty;
   EXPECT_EQ("/source/foo/",
             SourceDirForPath(empty, base::FilePath("/source/foo")).value());
 #endif
 }
 
+TEST(FilesystemUtils, ContentsEqual) {
+  base::ScopedTempDir temp_dir;
+  ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
+
+  std::string data = "foo";
+
+  base::FilePath file_path = temp_dir.path().AppendASCII("foo.txt");
+  base::WriteFile(file_path, data.c_str(), static_cast<int>(data.size()));
+
+  EXPECT_TRUE(ContentsEqual(file_path, data));
+
+  // Different length and contents.
+  data += "bar";
+  EXPECT_FALSE(ContentsEqual(file_path, data));
+
+  // The same length, different contents.
+  EXPECT_FALSE(ContentsEqual(file_path, "bar"));
+}
+
+TEST(FilesystemUtils, WriteFileIfChanged) {
+  base::ScopedTempDir temp_dir;
+  ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
+
+  std::string data = "foo";
+
+  // Write if file doesn't exist. Create also directory.
+  base::FilePath file_path =
+      temp_dir.path().AppendASCII("bar").AppendASCII("foo.txt");
+  EXPECT_TRUE(WriteFileIfChanged(file_path, data, nullptr));
+
+  base::File::Info file_info;
+  ASSERT_TRUE(base::GetFileInfo(file_path, &file_info));
+  base::Time last_modified = file_info.last_modified;
+  base::PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(1));
+
+  // Don't write if contents is the same.
+  EXPECT_TRUE(WriteFileIfChanged(file_path, data, nullptr));
+  ASSERT_TRUE(base::GetFileInfo(file_path, &file_info));
+  EXPECT_EQ(last_modified, file_info.last_modified);
+
+  // Write if contents changed.
+  EXPECT_TRUE(WriteFileIfChanged(file_path, "bar", nullptr));
+  ASSERT_TRUE(base::GetFileInfo(file_path, &file_info));
+  EXPECT_NE(last_modified, file_info.last_modified);
+}
+
 TEST(FilesystemUtils, GetToolchainDirs) {
   BuildSettings build_settings;
   build_settings.SetBuildDir(SourceDir("//out/Debug/"));
 
   // The default toolchain.
   Settings default_settings(&build_settings, "");
   Label default_toolchain_label(SourceDir("//toolchain/"), "default");
   default_settings.set_toolchain_label(default_toolchain_label);
   default_settings.set_default_toolchain_label(default_toolchain_label);
 
@@ skipping 154 matching lines @@
   EXPECT_EQ("gen/", GetToolchainGenDirAsOutputFile(&settings).value());
   EXPECT_EQ("//obj/",
             GetOutputDirForSourceDir(&settings, SourceDir("//")).value());
   EXPECT_EQ("obj/",
             GetOutputDirForSourceDirAsOutputFile(
                 &settings, SourceDir("//")).value());
   EXPECT_EQ("gen/",
             GetGenDirForSourceDirAsOutputFile(
                 &settings, SourceDir("//")).value());
 }