SettingsImpl.h

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/*
 * Copyright 2018-present Facebook, Inc.
 *
 * 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.
 */
#pragma once

#include <memory>
#include <mutex>
#include <string>
#include <typeindex>

#include <folly/CachelinePadded.h>
#include <folly/Conv.h>
#include <folly/Range.h>
#include <folly/SharedMutex.h>
#include <folly/ThreadLocal.h>
#include <folly/experimental/settings/SettingsMetadata.h>

namespace folly {
namespace settings {
namespace detail {

template <class T>
struct IsSmallPOD
    : std::integral_constant<
          bool,
          std::is_trivial<T>::value && sizeof(T) <= sizeof(uint64_t)> {};

template <class T>
struct SettingContents {
  std::string updateReason;
  T value;

  template <class... Args>
  SettingContents(std::string _reason, Args&&... args)
      : updateReason(std::move(_reason)), value(std::forward<Args>(args)...) {}
};

class SnapshotBase;

class SettingCoreBase {
 public:
  using Key = intptr_t;
  using Version = uint64_t;

  virtual void setFromString(
      StringPiece newValue,
      StringPiece reason,
      SnapshotBase* snapshot) = 0;
  virtual std::pair<std::string, std::string> getAsString(
      const SnapshotBase* snapshot) const = 0;
  virtual void resetToDefault(SnapshotBase* snapshot) = 0;
  virtual const SettingMetadata& meta() const = 0;
  virtual ~SettingCoreBase() {}

  Key getKey() const {
    return reinterpret_cast<Key>(this);
  }
};

void registerSetting(SettingCoreBase& core);

SettingCoreBase::Version nextGlobalVersion();

template <class T>
class SettingCore;

class BoxedValue {
 public:
  BoxedValue() = default;

  template <class T>
  explicit BoxedValue(const SettingContents<T>& value)
      : value_(std::make_shared<SettingContents<T>>(value)) {}

  template <class T>
  BoxedValue(const T& value, StringPiece reason, SettingCore<T>& core)
      : value_(std::make_shared<SettingContents<T>>(reason.str(), value)),
        publish_([value = value_, &core]() {
          auto& contents = BoxedValue::unboxImpl<T>(value.get());
          core.set(contents.value, contents.updateReason);
        }) {}

  template <class T>
  const SettingContents<T>& unbox() const {
    return BoxedValue::unboxImpl<T>(value_.get());
  }

  void publish() {
    if (publish_) {
      publish_();
    }
  }

 private:
  std::shared_ptr<void> value_;
  std::function<void()> publish_;

  template <class T>
  static const SettingContents<T>& unboxImpl(void* value) {
    return *static_cast<const SettingContents<T>*>(value);
  }
};

void saveValueForOutstandingSnapshots(
    SettingCoreBase::Key settingKey,
    SettingCoreBase::Version version,
    const BoxedValue& value);

const BoxedValue* getSavedValue(
    SettingCoreBase::Key key,
    SettingCoreBase::Version at);

class SnapshotBase {
 public:
  using SettingsInfo = std::pair<std::string, std::string>;

  virtual void publish() = 0;

  virtual bool setFromString(
      StringPiece settingName,
      StringPiece newValue,
      StringPiece reason) = 0;

  virtual Optional<SettingsInfo> getAsString(StringPiece settingName) const = 0;

  virtual bool resetToDefault(StringPiece settingName) = 0;

  virtual void forEachSetting(
      const std::function<
          void(const SettingMetadata&, StringPiece, StringPiece)>& func)
      const = 0;

  virtual ~SnapshotBase();

 protected:
  detail::SettingCoreBase::Version at_;
  std::unordered_map<detail::SettingCoreBase::Key, detail::BoxedValue>
      snapshotValues_;

  template <typename T>
  friend class SettingCore;

  SnapshotBase();

  template <class T>
  const SettingContents<T>& get(const detail::SettingCore<T>& core) const {
    auto it = snapshotValues_.find(core.getKey());
    if (it != snapshotValues_.end()) {
      return it->second.template unbox<T>();
    }
    auto savedValue = detail::getSavedValue(core.getKey(), at_);
    if (savedValue) {
      return savedValue->template unbox<T>();
    }
    return core.getSlow();
  }

  template <class T>
  void set(detail::SettingCore<T>& core, const T& t, StringPiece reason) {
    snapshotValues_[core.getKey()] = detail::BoxedValue(t, reason, core);
  }
};

template <class T>
std::enable_if_t<std::is_constructible<T, StringPiece>::value, T>
convertOrConstruct(StringPiece newValue) {
  return T(newValue);
}
template <class T>
std::enable_if_t<!std::is_constructible<T, StringPiece>::value, T>
convertOrConstruct(StringPiece newValue) {
  return to<T>(newValue);
}

template <class T>
class SettingCore : public SettingCoreBase {
 public:
  using Contents = SettingContents<T>;

  void setFromString(
      StringPiece newValue,
      StringPiece reason,
      SnapshotBase* snapshot) override {
    set(convertOrConstruct<T>(newValue), reason.str(), snapshot);
  }

  std::pair<std::string, std::string> getAsString(
      const SnapshotBase* snapshot) const override {
    auto& contents = snapshot ? snapshot->get(*this) : getSlow();
    return std::make_pair(
        to<std::string>(contents.value), contents.updateReason);
  }

  void resetToDefault(SnapshotBase* snapshot) override {
    set(defaultValue_, "default", snapshot);
  }

  const SettingMetadata& meta() const override {
    return meta_;
  }

  std::conditional_t<IsSmallPOD<T>::value, T, const T&> getWithHint(
      std::atomic<uint64_t>& trivialStorage) const {
    return getImpl(IsSmallPOD<T>(), trivialStorage);
  }
  const SettingContents<T>& getSlow() const {
    return *tlValue();
  }
  /***
   * SmallPOD version: just read the global atomic
   */
  T getImpl(std::true_type, std::atomic<uint64_t>& trivialStorage) const {
    uint64_t v = trivialStorage.load();
    T t;
    std::memcpy(&t, &v, sizeof(T));
    return t;
  }

  const T& getImpl(std::false_type, std::atomic<uint64_t>& /* ignored */)
      const {
    return const_cast<SettingCore*>(this)->tlValue()->value;
  }

  void set(const T& t, StringPiece reason, SnapshotBase* snapshot = nullptr) {
    /* Check that we can still display it (will throw otherwise) */
    to<std::string>(t);

    if (snapshot) {
      snapshot->set(*this, t, reason);
      return;
    }

    SharedMutex::WriteHolder lg(globalLock_);

    if (globalValue_) {
      saveValueForOutstandingSnapshots(
          getKey(), *settingVersion_, BoxedValue(*globalValue_));
    }
    globalValue_ = std::make_shared<Contents>(reason.str(), t);
    if (IsSmallPOD<T>::value) {
      uint64_t v = 0;
      std::memcpy(&v, &t, sizeof(T));
      trivialStorage_.store(v);
    }
    *settingVersion_ = nextGlobalVersion();
  }

  const T& defaultValue() const {
    return defaultValue_;
  }

  SettingCore(
      SettingMetadata meta,
      T defaultValue,
      std::atomic<uint64_t>& trivialStorage)
      : meta_(std::move(meta)),
        defaultValue_(std::move(defaultValue)),
        trivialStorage_(trivialStorage),
        localValue_([]() {
          return new CachelinePadded<
              std::pair<Version, std::shared_ptr<Contents>>>(0, nullptr);
        }) {
    set(defaultValue_, "default");
    registerSetting(*this);
  }

 private:
  SettingMetadata meta_;
  const T defaultValue_;

  SharedMutex globalLock_;
  std::shared_ptr<Contents> globalValue_;

  std::atomic<uint64_t>& trivialStorage_;

  /* Thread local versions start at 0, this will force a read on first access.
   */
  CachelinePadded<std::atomic<Version>> settingVersion_{1};

  ThreadLocal<CachelinePadded<std::pair<Version, std::shared_ptr<Contents>>>>
      localValue_;

  FOLLY_ALWAYS_INLINE const std::shared_ptr<Contents>& tlValue() const {
    auto& value = **localValue_;
    if (LIKELY(value.first == *settingVersion_)) {
      return value.second;
    }
    return tlValueSlow();
  }
  FOLLY_NOINLINE const std::shared_ptr<Contents>& tlValueSlow() const {
    auto& value = **localValue_;
    while (value.first < *settingVersion_) {
      /* If this destroys the old value, do it without holding the lock */
      value.second.reset();
      SharedMutex::ReadHolder lg(globalLock_);
      value.first = *settingVersion_;
      value.second = globalValue_;
    }
    return value.second;
  }
};

} // namespace detail
} // namespace settings
} // namespace folly