[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..2ceca81677dbc68d0e041271ea9702ec1c3302c0 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -687,6 +687,86 @@ std::string GetOutputSubdirName(const Label& toolchain_label, bool is_default) {
   return toolchain_label.name() + "/";
 }
 
+bool ContentsEqual(const base::FilePath& file_path, const std::string& data) {
+  // Compare file and stream sizes first. Quick and will save us some time if
+  // they are different sizes.
+  int64_t file_size;
+  if (!base::GetFileSize(file_path, &file_size) ||
+      static_cast<size_t>(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;
+}
+
+bool WriteFileIfChanged(const base::FilePath& file_path,
+                        const std::string& data,
+                        Err* err) {
+  if (ContentsEqual(file_path, data))
+    return true;
+
+  // Create the directory if necessary.
+  if (!base::CreateDirectory(file_path.DirName())) {
+    if (err) {
+      *err =
+          Err(Location(), "Unable to create directory.",
+              "I was using \"" + FilePathToUTF8(file_path.DirName()) + "\".");
+    }
+    return false;
+  }
+
+  int size = static_cast<int>(data.size());
+  bool write_success = false;
+
+#if defined(OS_WIN)
+  // On Windows, provide a custom implementation of base::WriteFile. Sometimes
+  // the base version fails, especially on the bots. The guess is that Windows
+  // Defender or other antivirus programs still have the file open (after
+  // checking for the read) when the write happens immediately after. This
+  // version opens with FILE_SHARE_READ (normally not what you want when
+  // replacing the entire contents of the file) which lets us continue even if
+  // another program has the file open for reading. See http://crbug.com/468437
+  base::win::ScopedHandle file(::CreateFile(file_path.value().c_str(),
+                                            GENERIC_WRITE, FILE_SHARE_READ,
+                                            NULL, CREATE_ALWAYS, 0, NULL));
+  if (file.IsValid()) {
+    DWORD written;
+    BOOL result = ::WriteFile(file.Get(), data.c_str(), size, &written, NULL);
+    if (result) {
+      if (static_cast<int>(written) == size) {
+        write_success = true;
+      } else {
+        // Didn't write all the bytes.
+        LOG(ERROR) << "wrote" << written << " bytes to "
+                   << base::UTF16ToUTF8(file_path.value()) << " expected "
+                   << size;
+      }
+    } else {
+      // WriteFile failed.
+      PLOG(ERROR) << "writing file " << base::UTF16ToUTF8(file_path.value())
+                  << " failed";
+    }
+  } else {
+    PLOG(ERROR) << "CreateFile failed for path "
+                << base::UTF16ToUTF8(file_path.value());
+  }
+#else
+  write_success = base::WriteFile(file_path, data.c_str(), size) == size;
+#endif
+
+  if (!write_success && err) {
+    *err = Err(Location(), "Unable to write file.",
+               "I was writing \"" + FilePathToUTF8(file_path) + "\".");
+  }
+
+  return write_success;
+}
+
 SourceDir GetToolchainOutputDir(const Settings* settings) {
   return settings->toolchain_output_subdir().AsSourceDir(
       settings->build_settings());