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diff --git a/src/rocksdb/third-party/folly/folly/Optional.h b/src/rocksdb/third-party/folly/folly/Optional.h
new file mode 100644
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+++ b/src/rocksdb/third-party/folly/folly/Optional.h
@@ -0,0 +1,570 @@
+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+/*
+ * Optional - For conditional initialization of values, like boost::optional,
+ * but with support for move semantics and emplacement.  Reference type support
+ * has not been included due to limited use cases and potential confusion with
+ * semantics of assignment: Assigning to an optional reference could quite
+ * reasonably copy its value or redirect the reference.
+ *
+ * Optional can be useful when a variable might or might not be needed:
+ *
+ *  Optional<Logger> maybeLogger = ...;
+ *  if (maybeLogger) {
+ *    maybeLogger->log("hello");
+ *  }
+ *
+ * Optional enables a 'null' value for types which do not otherwise have
+ * nullability, especially useful for parameter passing:
+ *
+ * void testIterator(const unique_ptr<Iterator>& it,
+ *                   initializer_list<int> idsExpected,
+ *                   Optional<initializer_list<int>> ranksExpected = none) {
+ *   for (int i = 0; it->next(); ++i) {
+ *     EXPECT_EQ(it->doc().id(), idsExpected[i]);
+ *     if (ranksExpected) {
+ *       EXPECT_EQ(it->doc().rank(), (*ranksExpected)[i]);
+ *     }
+ *   }
+ * }
+ *
+ * Optional models OptionalPointee, so calling 'get_pointer(opt)' will return a
+ * pointer to nullptr if the 'opt' is empty, and a pointer to the value if it is
+ * not:
+ *
+ *  Optional<int> maybeInt = ...;
+ *  if (int* v = get_pointer(maybeInt)) {
+ *    cout << *v << endl;
+ *  }
+ */
+
+#include <cstddef>
+#include <functional>
+#include <new>
+#include <stdexcept>
+#include <type_traits>
+#include <utility>
+
+#include <folly/CPortability.h>
+#include <folly/Traits.h>
+#include <folly/Utility.h>
+
+namespace folly {
+
+template <class Value>
+class Optional;
+
+namespace detail {
+template <class Value>
+struct OptionalPromiseReturn;
+} // namespace detail
+
+struct None {
+  enum class _secret { _token };
+
+  /**
+   * No default constructor to support both `op = {}` and `op = none`
+   * as syntax for clearing an Optional, just like std::nullopt_t.
+   */
+  constexpr explicit None(_secret) {}
+};
+constexpr None none{None::_secret::_token};
+
+class FOLLY_EXPORT OptionalEmptyException : public std::runtime_error {
+ public:
+  OptionalEmptyException()
+      : std::runtime_error("Empty Optional cannot be unwrapped") {}
+};
+
+template <class Value>
+class Optional {
+ public:
+  typedef Value value_type;
+
+  static_assert(
+      !std::is_reference<Value>::value,
+      "Optional may not be used with reference types");
+  static_assert(
+      !std::is_abstract<Value>::value,
+      "Optional may not be used with abstract types");
+
+  Optional() noexcept {}
+
+  Optional(const Optional& src) noexcept(
+      std::is_nothrow_copy_constructible<Value>::value) {
+    if (src.hasValue()) {
+      construct(src.value());
+    }
+  }
+
+  Optional(Optional&& src) noexcept(
+      std::is_nothrow_move_constructible<Value>::value) {
+    if (src.hasValue()) {
+      construct(std::move(src.value()));
+      src.clear();
+    }
+  }
+
+  /* implicit */ Optional(const None&) noexcept {}
+
+  /* implicit */ Optional(Value&& newValue) noexcept(
+      std::is_nothrow_move_constructible<Value>::value) {
+    construct(std::move(newValue));
+  }
+
+  /* implicit */ Optional(const Value& newValue) noexcept(
+      std::is_nothrow_copy_constructible<Value>::value) {
+    construct(newValue);
+  }
+
+  template <typename... Args>
+  explicit Optional(in_place_t, Args&&... args) noexcept(
+      std::is_nothrow_constructible<Value, Args...>::value)
+      : Optional{PrivateConstructor{}, std::forward<Args>(args)...} {}
+
+  template <typename U, typename... Args>
+  explicit Optional(
+      in_place_t,
+      std::initializer_list<U> il,
+      Args&&... args) noexcept(std::
+                                   is_nothrow_constructible<
+                                       Value,
+                                       std::initializer_list<U>,
+                                       Args...>::value)
+      : Optional{PrivateConstructor{}, il, std::forward<Args>(args)...} {}
+
+  // Used only when an Optional is used with coroutines on MSVC
+  /* implicit */ Optional(const detail::OptionalPromiseReturn<Value>& p)
+      : Optional{} {
+    p.promise_->value_ = this;
+  }
+
+  void assign(const None&) {
+    clear();
+  }
+
+  void assign(Optional&& src) {
+    if (this != &src) {
+      if (src.hasValue()) {
+        assign(std::move(src.value()));
+        src.clear();
+      } else {
+        clear();
+      }
+    }
+  }
+
+  void assign(const Optional& src) {
+    if (src.hasValue()) {
+      assign(src.value());
+    } else {
+      clear();
+    }
+  }
+
+  void assign(Value&& newValue) {
+    if (hasValue()) {
+      storage_.value = std::move(newValue);
+    } else {
+      construct(std::move(newValue));
+    }
+  }
+
+  void assign(const Value& newValue) {
+    if (hasValue()) {
+      storage_.value = newValue;
+    } else {
+      construct(newValue);
+    }
+  }
+
+  Optional& operator=(None) noexcept {
+    reset();
+    return *this;
+  }
+
+  template <class Arg>
+  Optional& operator=(Arg&& arg) {
+    assign(std::forward<Arg>(arg));
+    return *this;
+  }
+
+  Optional& operator=(Optional&& other) noexcept(
+      std::is_nothrow_move_assignable<Value>::value) {
+    assign(std::move(other));
+    return *this;
+  }
+
+  Optional& operator=(const Optional& other) noexcept(
+      std::is_nothrow_copy_assignable<Value>::value) {
+    assign(other);
+    return *this;
+  }
+
+  template <class... Args>
+  Value& emplace(Args&&... args) {
+    clear();
+    construct(std::forward<Args>(args)...);
+    return value();
+  }
+
+  template <class U, class... Args>
+  typename std::enable_if<
+      std::is_constructible<Value, std::initializer_list<U>&, Args&&...>::value,
+      Value&>::type
+  emplace(std::initializer_list<U> ilist, Args&&... args) {
+    clear();
+    construct(ilist, std::forward<Args>(args)...);
+    return value();
+  }
+
+  void reset() noexcept {
+    storage_.clear();
+  }
+
+  void clear() noexcept {
+    reset();
+  }
+
+  void swap(Optional& that) noexcept(IsNothrowSwappable<Value>::value) {
+    if (hasValue() && that.hasValue()) {
+      using std::swap;
+      swap(value(), that.value());
+    } else if (hasValue()) {
+      that.emplace(std::move(value()));
+      reset();
+    } else if (that.hasValue()) {
+      emplace(std::move(that.value()));
+      that.reset();
+    }
+  }
+
+  const Value& value() const& {
+    require_value();
+    return storage_.value;
+  }
+
+  Value& value() & {
+    require_value();
+    return storage_.value;
+  }
+
+  Value&& value() && {
+    require_value();
+    return std::move(storage_.value);
+  }
+
+  const Value&& value() const&& {
+    require_value();
+    return std::move(storage_.value);
+  }
+
+  const Value* get_pointer() const& {
+    return storage_.hasValue ? &storage_.value : nullptr;
+  }
+  Value* get_pointer() & {
+    return storage_.hasValue ? &storage_.value : nullptr;
+  }
+  Value* get_pointer() && = delete;
+
+  bool has_value() const noexcept {
+    return storage_.hasValue;
+  }
+
+  bool hasValue() const noexcept {
+    return has_value();
+  }
+
+  explicit operator bool() const noexcept {
+    return has_value();
+  }
+
+  const Value& operator*() const& {
+    return value();
+  }
+  Value& operator*() & {
+    return value();
+  }
+  const Value&& operator*() const&& {
+    return std::move(value());
+  }
+  Value&& operator*() && {
+    return std::move(value());
+  }
+
+  const Value* operator->() const {
+    return &value();
+  }
+  Value* operator->() {
+    return &value();
+  }
+
+  // Return a copy of the value if set, or a given default if not.
+  template <class U>
+  Value value_or(U&& dflt) const& {
+    if (storage_.hasValue) {
+      return storage_.value;
+    }
+
+    return std::forward<U>(dflt);
+  }
+
+  template <class U>
+  Value value_or(U&& dflt) && {
+    if (storage_.hasValue) {
+      return std::move(storage_.value);
+    }
+
+    return std::forward<U>(dflt);
+  }
+
+ private:
+  template <class T>
+  friend Optional<_t<std::decay<T>>> make_optional(T&&);
+  template <class T, class... Args>
+  friend Optional<T> make_optional(Args&&... args);
+  template <class T, class U, class... As>
+  friend Optional<T> make_optional(std::initializer_list<U>, As&&...);
+
+  /**
+   * Construct the optional in place, this is duplicated as a non-explicit
+   * constructor to allow returning values that are non-movable from
+   * make_optional using list initialization.
+   *
+   * Until C++17, at which point this will become unnecessary because of
+   * specified prvalue elision.
+   */
+  struct PrivateConstructor {
+    explicit PrivateConstructor() = default;
+  };
+  template <typename... Args>
+  Optional(PrivateConstructor, Args&&... args) noexcept(
+      std::is_constructible<Value, Args&&...>::value) {
+    construct(std::forward<Args>(args)...);
+  }
+
+  void require_value() const {
+    if (!storage_.hasValue) {
+      throw OptionalEmptyException{};
+    }
+  }
+
+  template <class... Args>
+  void construct(Args&&... args) {
+    const void* ptr = &storage_.value;
+    // For supporting const types.
+    new (const_cast<void*>(ptr)) Value(std::forward<Args>(args)...);
+    storage_.hasValue = true;
+  }
+
+  struct StorageTriviallyDestructible {
+    union {
+      char emptyState;
+      Value value;
+    };
+    bool hasValue;
+
+    StorageTriviallyDestructible()
+        : emptyState('\0'), hasValue{false} {}
+    void clear() {
+      hasValue = false;
+    }
+  };
+
+  struct StorageNonTriviallyDestructible {
+    union {
+      char emptyState;
+      Value value;
+    };
+    bool hasValue;
+
+    StorageNonTriviallyDestructible() : hasValue{false} {}
+    ~StorageNonTriviallyDestructible() {
+      clear();
+    }
+
+    void clear() {
+      if (hasValue) {
+        hasValue = false;
+        value.~Value();
+      }
+    }
+  };
+
+  using Storage = typename std::conditional<
+      std::is_trivially_destructible<Value>::value,
+      StorageTriviallyDestructible,
+      StorageNonTriviallyDestructible>::type;
+
+  Storage storage_;
+};
+
+template <class T>
+const T* get_pointer(const Optional<T>& opt) {
+  return opt.get_pointer();
+}
+
+template <class T>
+T* get_pointer(Optional<T>& opt) {
+  return opt.get_pointer();
+}
+
+template <class T>
+void swap(Optional<T>& a, Optional<T>& b) noexcept(noexcept(a.swap(b))) {
+  a.swap(b);
+}
+
+template <class T>
+Optional<_t<std::decay<T>>> make_optional(T&& v) {
+  using PrivateConstructor =
+      typename folly::Optional<_t<std::decay<T>>>::PrivateConstructor;
+  return {PrivateConstructor{}, std::forward<T>(v)};
+}
+
+template <class T, class... Args>
+folly::Optional<T> make_optional(Args&&... args) {
+  using PrivateConstructor = typename folly::Optional<T>::PrivateConstructor;
+  return {PrivateConstructor{}, std::forward<Args>(args)...};
+}
+
+template <class T, class U, class... Args>
+folly::Optional<T> make_optional(
+    std::initializer_list<U> il,
+    Args&&... args) {
+  using PrivateConstructor = typename folly::Optional<T>::PrivateConstructor;
+  return {PrivateConstructor{}, il, std::forward<Args>(args)...};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Comparisons.
+
+template <class U, class V>
+bool operator==(const Optional<U>& a, const V& b) {
+  return a.hasValue() && a.value() == b;
+}
+
+template <class U, class V>
+bool operator!=(const Optional<U>& a, const V& b) {
+  return !(a == b);
+}
+
+template <class U, class V>
+bool operator==(const U& a, const Optional<V>& b) {
+  return b.hasValue() && b.value() == a;
+}
+
+template <class U, class V>
+bool operator!=(const U& a, const Optional<V>& b) {
+  return !(a == b);
+}
+
+template <class U, class V>
+bool operator==(const Optional<U>& a, const Optional<V>& b) {
+  if (a.hasValue() != b.hasValue()) {
+    return false;
+  }
+  if (a.hasValue()) {
+    return a.value() == b.value();
+  }
+  return true;
+}
+
+template <class U, class V>
+bool operator!=(const Optional<U>& a, const Optional<V>& b) {
+  return !(a == b);
+}
+
+template <class U, class V>
+bool operator<(const Optional<U>& a, const Optional<V>& b) {
+  if (a.hasValue() != b.hasValue()) {
+    return a.hasValue() < b.hasValue();
+  }
+  if (a.hasValue()) {
+    return a.value() < b.value();
+  }
+  return false;
+}
+
+template <class U, class V>
+bool operator>(const Optional<U>& a, const Optional<V>& b) {
+  return b < a;
+}
+
+template <class U, class V>
+bool operator<=(const Optional<U>& a, const Optional<V>& b) {
+  return !(b < a);
+}
+
+template <class U, class V>
+bool operator>=(const Optional<U>& a, const Optional<V>& b) {
+  return !(a < b);
+}
+
+// Suppress comparability of Optional<T> with T, despite implicit conversion.
+template <class V>
+bool operator<(const Optional<V>&, const V& other) = delete;
+template <class V>
+bool operator<=(const Optional<V>&, const V& other) = delete;
+template <class V>
+bool operator>=(const Optional<V>&, const V& other) = delete;
+template <class V>
+bool operator>(const Optional<V>&, const V& other) = delete;
+template <class V>
+bool operator<(const V& other, const Optional<V>&) = delete;
+template <class V>
+bool operator<=(const V& other, const Optional<V>&) = delete;
+template <class V>
+bool operator>=(const V& other, const Optional<V>&) = delete;
+template <class V>
+bool operator>(const V& other, const Optional<V>&) = delete;
+
+// Comparisons with none
+template <class V>
+bool operator==(const Optional<V>& a, None) noexcept {
+  return !a.hasValue();
+}
+template <class V>
+bool operator==(None, const Optional<V>& a) noexcept {
+  return !a.hasValue();
+}
+template <class V>
+bool operator<(const Optional<V>&, None) noexcept {
+  return false;
+}
+template <class V>
+bool operator<(None, const Optional<V>& a) noexcept {
+  return a.hasValue();
+}
+template <class V>
+bool operator>(const Optional<V>& a, None) noexcept {
+  return a.hasValue();
+}
+template <class V>
+bool operator>(None, const Optional<V>&) noexcept {
+  return false;
+}
+template <class V>
+bool operator<=(None, const Optional<V>&) noexcept {
+  return true;
+}
+template <class V>
+bool operator<=(const Optional<V>& a, None) noexcept {
+  return !a.hasValue();
+}
+template <class V>
+bool operator>=(const Optional<V>&, None) noexcept {
+  return true;
+}
+template <class V>
+bool operator>=(None, const Optional<V>& a) noexcept {
+  return !a.hasValue();
+}
+
+///////////////////////////////////////////////////////////////////////////////
+
+} // namespace folly