update_engine: Implement Excluder Class + Tests

Excluder persists the exclusion state for excluding certain names. This
will be used to exclude update Payloads which are contiuously faulty.

BUG=chromium:928805
TEST=FEATURES=test emerge-$B update_engine
TEST=USE="${USE} -dlc" FEATURES=test emerge-$B update_engine

Change-Id: I780a9cf2ad979833382a832e01833211ec2ccf7d
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2172074
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/excluder_chromeos.cc b/excluder_chromeos.cc
new file mode 100644
index 0000000..bfd6f04
--- /dev/null
+++ b/excluder_chromeos.cc
@@ -0,0 +1,63 @@
+//
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "update_engine/excluder_chromeos.h"
+
+#include <memory>
+#include <vector>
+
+#include <base/logging.h>
+#include <base/strings/string_piece.h>
+#include <base/strings/string_number_conversions.h>
+#include <base/strings/string_split.h>
+
+#include "update_engine/common/constants.h"
+#include "update_engine/common/prefs.h"
+#include "update_engine/system_state.h"
+
+using std::string;
+using std::vector;
+
+namespace chromeos_update_engine {
+
+std::unique_ptr<ExcluderInterface> CreateExcluder(PrefsInterface* prefs) {
+  return std::make_unique<ExcluderChromeOS>(prefs);
+}
+
+ExcluderChromeOS::ExcluderChromeOS(PrefsInterface* prefs) : prefs_(prefs) {}
+
+bool ExcluderChromeOS::Exclude(const string& name) {
+  auto key = prefs_->CreateSubKey({kExclusionPrefsSubDir, name});
+  return prefs_->SetString(key, "");
+}
+
+bool ExcluderChromeOS::IsExcluded(const string& name) {
+  auto key = prefs_->CreateSubKey({kExclusionPrefsSubDir, name});
+  return prefs_->Exists(key);
+}
+
+bool ExcluderChromeOS::Reset() {
+  bool ret = true;
+  vector<string> keys;
+  if (!prefs_->GetSubKeys(kExclusionPrefsSubDir, &keys))
+    return false;
+  for (const auto& key : keys)
+    if (!(ret &= prefs_->Delete(key)))
+      LOG(ERROR) << "Failed to delete exclusion pref for " << key;
+  return ret;
+}
+
+}  // namespace chromeos_update_engine