AU: Add support for persistent update engine preferences store.

BUG=5140
TEST=emerge-x86-generic, attached unit tests

Review URL: http://codereview.chromium.org/3014035

prefs.cc

diff --git a/prefs.cc b/prefs.cc
new file mode 100644
index 0000000..38129c9
--- /dev/null
+++ b/prefs.cc
@@ -0,0 +1,65 @@
+// Copyright (c) 2010 The Chromium OS 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 "update_engine/prefs.h"
+
+#include "base/file_util.h"
+#include "base/logging.h"
+#include "base/string_util.h"
+#include "update_engine/utils.h"
+
+using std::string;
+
+namespace chromeos_update_engine {
+
+bool Prefs::Init(const FilePath& prefs_dir) {
+  prefs_dir_ = prefs_dir;
+  return true;
+}
+
+bool Prefs::GetString(const string& key, string* value) {
+  LOG(INFO) << "Getting key \"" << key << "\"";
+  FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  TEST_AND_RETURN_FALSE(file_util::ReadFileToString(filename, value));
+  LOG(INFO) << "Key \"" << key << "\" value \"" << *value << "\"";
+  return true;
+}
+
+bool Prefs::SetString(const std::string& key, const std::string& value) {
+  LOG(INFO) << "Setting key \"" << key << "\" value \"" << value << "\"";
+  FilePath filename;
+  TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
+  TEST_AND_RETURN_FALSE(file_util::CreateDirectory(filename.DirName()));
+  TEST_AND_RETURN_FALSE(
+      file_util::WriteFile(filename, value.data(), value.size()) ==
+      static_cast<int>(value.size()));
+  return true;
+}
+
+bool Prefs::GetInt64(const string& key, int64_t* value) {
+  string str_value;
+  TEST_AND_RETURN_FALSE(GetString(key, &str_value));
+  TrimWhitespaceASCII(str_value, TRIM_ALL, &str_value);
+  TEST_AND_RETURN_FALSE(StringToInt64(str_value, value));
+  return true;
+}
+
+bool Prefs::SetInt64(const string& key, const int64_t value) {
+  return SetString(key, Int64ToString(value));
+}
+
+bool Prefs::GetFileNameForKey(const std::string& key, FilePath* filename) {
+  // Allows only non-empty keys containing [A-Za-z0-9_-].
+  TEST_AND_RETURN_FALSE(!key.empty());
+  for (size_t i = 0; i < key.size(); ++i) {
+    char c = key.at(i);
+    TEST_AND_RETURN_FALSE(IsAsciiAlpha(c) || IsAsciiDigit(c) ||
+                          c == '_' || c == '-');
+  }
+  *filename = prefs_dir_.Append(key);
+  return true;
+}
+
+}  // namespace chromeos_update_engine