context.hpp

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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
 
#pragma once
 
#include "azure/core/azure_assert.hpp"
#include "azure/core/datetime.hpp"
#include "azure/core/dll_import_export.hpp"
#include "azure/core/rtti.hpp"
 
#include <atomic>
#include <chrono>
#include <memory>
#include <stdexcept>
#include <string>
#include <type_traits>
 
// Forward declare TracerProvider to resolve an include file dependency ordering problem.
namespace Azure { namespace Core { namespace Tracing {
  class TracerProvider;
}}} // namespace Azure::Core::Tracing
 
namespace Azure { namespace Core {
 
  class OperationCancelledException final : public std::runtime_error {
  public:
    explicit OperationCancelledException(std::string const& what) : std::runtime_error(what) {}
  };
 
  class Context final {
  public:
    class Key final {
      Key const* m_uniqueAddress;
 
    public:
      Key() : m_uniqueAddress(this) {}
 
      bool operator==(Key const& other) const
      {
        return this->m_uniqueAddress == other.m_uniqueAddress;
      }
 
      bool operator!=(Key const& other) const { return !(*this == other); }
    };
 
  private:
    struct ContextSharedState final
    {
      std::shared_ptr<ContextSharedState> Parent;
      std::atomic<DateTime::rep> Deadline;
      std::shared_ptr<Azure::Core::Tracing::TracerProvider> TraceProvider;
      Context::Key Key;
      std::shared_ptr<void> Value;
#if defined(AZ_CORE_RTTI)
      const std::type_info& ValueType;
#endif
      static constexpr DateTime::rep ToDateTimeRepresentation(DateTime const& dateTime)
      {
        return dateTime.time_since_epoch().count();
      }
 
      static constexpr DateTime FromDateTimeRepresentation(DateTime::rep dtRepresentation)
      {
        return DateTime(DateTime::time_point(DateTime::duration(dtRepresentation)));
      }
 
      explicit ContextSharedState()
          : Deadline(ToDateTimeRepresentation((DateTime::max)())), Value(nullptr)
#if defined(AZ_CORE_RTTI)
            ,
            ValueType(typeid(std::nullptr_t))
#endif
      {
      }
 
      explicit ContextSharedState(
          const std::shared_ptr<ContextSharedState>& parent,
          DateTime const& deadline)
          : Parent(parent), Deadline(ToDateTimeRepresentation(deadline)), Value(nullptr)
#if defined(AZ_CORE_RTTI)
            ,
            ValueType(typeid(std::nullptr_t))
#endif
      {
      }
 
      template <class T>
      explicit ContextSharedState(
          const std::shared_ptr<ContextSharedState>& parent,
          DateTime const& deadline,
          Context::Key const& key,
          T value) // NOTE, should this be T&&
          : Parent(parent), Deadline(ToDateTimeRepresentation(deadline)), Key(key),
            Value(std::make_shared<T>(std::move(value)))
#if defined(AZ_CORE_RTTI)
            ,
            ValueType(typeid(T))
#endif
      {
      }
    };
 
    std::shared_ptr<ContextSharedState> m_contextSharedState;
 
    explicit Context(std::shared_ptr<ContextSharedState> impl)
        : m_contextSharedState(std::move(impl))
    {
    }
 
  public:
    Context() : m_contextSharedState(std::make_shared<ContextSharedState>()) {}
 
    Context WithDeadline(DateTime const& deadline) const
    {
      return Context{std::make_shared<ContextSharedState>(m_contextSharedState, deadline)};
    }
 
    template <class T> Context WithValue(Key const& key, T&& value) const
    {
      return Context{std::make_shared<ContextSharedState>(
          m_contextSharedState, (DateTime::max)(), key, std::forward<T>(value))};
    }
 
    DateTime GetDeadline() const;
 
    template <class T> bool TryGetValue(Key const& key, T& outputValue) const
    {
      for (auto ptr = m_contextSharedState; ptr; ptr = ptr->Parent)
      {
        if (ptr->Key == key)
        {
#if defined(AZ_CORE_RTTI)
          AZURE_ASSERT_MSG(
              typeid(T) == ptr->ValueType, "Type mismatch for Context::TryGetValue().");
#endif
 
          outputValue = *reinterpret_cast<const T*>(ptr->Value.get());
          return true;
        }
      }
      return false;
    }
 
    void Cancel()
    {
      m_contextSharedState->Deadline
          = ContextSharedState::ToDateTimeRepresentation((DateTime::min)());
    }
 
    bool IsCancelled() const { return GetDeadline() < std::chrono::system_clock::now(); }
 
    void ThrowIfCancelled() const
    {
      if (IsCancelled())
      {
        throw OperationCancelledException("Request was cancelled by context.");
      }
    }
 
    static const AZ_CORE_DLLEXPORT Context ApplicationContext;
  };
}} // namespace Azure::Core