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 //
 // thread_pool.hpp
 // ~~~~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2025 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
 #ifndef BOOST_ASIO_THREAD_POOL_HPP
 #define BOOST_ASIO_THREAD_POOL_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
 #include <boost/asio/detail/atomic_count.hpp>
 #include <boost/asio/detail/scheduler.hpp>
 #include <boost/asio/detail/thread_group.hpp>
 #include <boost/asio/execution.hpp>
 #include <boost/asio/execution_context.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 namespace detail {
   struct thread_pool_bits
   {
     static constexpr unsigned int blocking_never = 1;
     static constexpr unsigned int blocking_always = 2;
     static constexpr unsigned int blocking_mask = 3;
     static constexpr unsigned int relationship_continuation = 4;
     static constexpr unsigned int outstanding_work_tracked = 8;
   };
 } // namespace detail
 
 /// A simple fixed-size thread pool.
 /**
  * The thread pool class is an execution context where functions are permitted
  * to run on one of a fixed number of threads.
  *
  * @par Thread Safety
  * @e Distinct @e objects: Safe.@n
  * @e Shared @e objects: Safe, with the specific exceptions of the join()
  * wait() and notify_fork() functions. The join() and wait() functions must not
  * be called at the same time as other calls to join() or wait() on the same
  * pool. The notify_fork() function should not be called while any thread_pool
  * function, or any function on an I/O object that is associated with the
  * thread_pool, is being called in another thread. (In effect, this means that
  * notify_fork() is safe only on a thread pool that has no internal or attached
  * threads at the time.)
  *
  * @par Submitting tasks to the pool
  *
  * To submit functions to the thread pool, use the @ref boost::asio::dispatch,
  * @ref boost::asio::post or @ref boost::asio::defer free functions.
  *
  * For example:
  *
  * @code void my_task()
  * {
  *   ...
  * }
  *
  * ...
  *
  * // Launch the pool with four threads.
  * boost::asio::thread_pool pool(4);
  *
  * // Submit a function to the pool.
  * boost::asio::post(pool, my_task);
  *
  * // Submit a lambda object to the pool.
  * boost::asio::post(pool,
  *     []()
  *     {
  *       ...
  *     });
  *
  * // Wait for all tasks in the pool to complete.
  * pool.join(); @endcode
  */
 class thread_pool
   : public execution_context
 {
 public:
   template <typename Allocator, unsigned int Bits>
   class basic_executor_type;
 
   template <typename Allocator, unsigned int Bits>
   friend class basic_executor_type;
 
   /// Executor used to submit functions to a thread pool.
   typedef basic_executor_type<std::allocator<void>, 0> executor_type;
 
 #if !defined(BOOST_ASIO_NO_TS_EXECUTORS)
   /// Constructs a pool with an automatically determined number of threads.
   BOOST_ASIO_DECL thread_pool();
 
   /// Constructs a pool with an automatically determined number of threads.
   /**
    * @param a An allocator that will be used for allocating objects that are
    * associated with the execution context, such as services and internal state
    * for I/O objects.
    */
   template <typename Allocator>
   thread_pool(allocator_arg_t, const Allocator& a);
 #endif // !defined(BOOST_ASIO_NO_TS_EXECUTORS)
 
   /// Constructs a pool with a specified number of threads.
   /**
    * @param num_threads The number of threads required.
    */
   BOOST_ASIO_DECL explicit thread_pool(std::size_t num_threads);
 
   /// Constructs a pool with a specified number of threads.
   /**
    * @param num_threads The number of threads required.
    *
    * @param a An allocator that will be used for allocating objects that are
    * associated with the execution context, such as services and internal state
    * for I/O objects.
    */
   template <typename Allocator>
   thread_pool(allocator_arg_t, const Allocator& a, std::size_t num_threads);
 
   /// Constructs a pool with a specified number of threads.
   /**
    * Construct with a service maker, to create an initial set of services that
    * will be installed into the execution context at construction time.
    *
    * @param num_threads The number of threads required.
    *
    * @param initial_services Used to create the initial services. The @c make
    * function will be called once at the end of execution_context construction.
    */
   BOOST_ASIO_DECL thread_pool(std::size_t num_threads,
       const execution_context::service_maker& initial_services);
 
   /// Constructs a pool with a specified number of threads.
   /**
    * Construct with a service maker, to create an initial set of services that
    * will be installed into the execution context at construction time.
    *
    * @param a An allocator that will be used for allocating objects that are
    * associated with the execution context, such as services and internal state
    * for I/O objects.
    *
    * @param num_threads The number of threads required.
    *
    * @param initial_services Used to create the initial services. The @c make
    * function will be called once at the end of execution_context construction.
    */
   template <typename Allocator>
   thread_pool(allocator_arg_t, const Allocator& a, std::size_t num_threads,
       const execution_context::service_maker& initial_services);
 
   /// Destructor.
   /**
    * Automatically stops and joins the pool, if not explicitly done beforehand.
    */
   BOOST_ASIO_DECL ~thread_pool();
 
   /// Obtains the executor associated with the pool.
   executor_type get_executor() noexcept;
 
   /// Obtains the executor associated with the pool.
   executor_type executor() noexcept;
 
   /// Stops the threads.
   /**
    * This function stops the threads as soon as possible. As a result of calling
    * @c stop(), pending function objects may be never be invoked.
    */
   BOOST_ASIO_DECL void stop();
 
   /// Attaches the current thread to the pool.
   /**
    * This function attaches the current thread to the pool so that it may be
    * used for executing submitted function objects. Blocks the calling thread
    * until the pool is stopped or joined and has no outstanding work.
    */
   BOOST_ASIO_DECL void attach();
 
   /// Joins the threads.
   /**
    * This function blocks until the threads in the pool have completed. If @c
    * stop() is not called prior to @c join(), the @c join() call will wait
    * until the pool has no more outstanding work.
    */
   BOOST_ASIO_DECL void join();
 
   /// Waits for threads to complete.
   /**
    * This function blocks until the threads in the pool have completed. If @c
    * stop() is not called prior to @c wait(), the @c wait() call will wait
    * until the pool has no more outstanding work.
    *
    * @note @c wait() is synonymous with @c join().
    */
   BOOST_ASIO_DECL void wait();
 
 private:
   thread_pool(const thread_pool&) = delete;
   thread_pool& operator=(const thread_pool&) = delete;
 
   struct thread_function;
 
 #if !defined(BOOST_ASIO_NO_TS_EXECUTORS)
   // Helper function to calculate the default number of threads in the pool.
   BOOST_ASIO_DECL static long default_thread_pool_size();
 #endif // !defined(BOOST_ASIO_NO_TS_EXECUTORS)
 
   // Helper function to ensure the thread pool size is not out of range.
   BOOST_ASIO_DECL static long clamp_thread_pool_size(std::size_t n);
 
   // Helper function to start all threads in the pool.
   BOOST_ASIO_DECL void start();
 
   // The underlying scheduler.
   detail::scheduler& scheduler_;
 
   // The threads in the pool.
   detail::thread_group<allocator<void>> threads_;
 
   // The current number of threads in the pool.
   detail::atomic_count num_threads_;
 
   // Whether a join call will have any effect.
   bool joinable_;
 };
 
 /// Executor implementation type used to submit functions to a thread pool.
 template <typename Allocator, unsigned int Bits>
 class thread_pool::basic_executor_type : detail::thread_pool_bits
 {
 public:
   /// Copy constructor.
   basic_executor_type(const basic_executor_type& other) noexcept
     : pool_(other.pool_),
       allocator_(other.allocator_),
       bits_(other.bits_)
   {
     if (Bits & outstanding_work_tracked)
       if (pool_)
         pool_->scheduler_.work_started();
   }
 
   /// Move constructor.
   basic_executor_type(basic_executor_type&& other) noexcept
     : pool_(other.pool_),
       allocator_(static_cast<Allocator&&>(other.allocator_)),
       bits_(other.bits_)
   {
     if (Bits & outstanding_work_tracked)
       other.pool_ = 0;
   }
 
   /// Destructor.
   ~basic_executor_type() noexcept
   {
     if (Bits & outstanding_work_tracked)
       if (pool_)
         pool_->scheduler_.work_finished();
   }
 
   /// Assignment operator.
   basic_executor_type& operator=(const basic_executor_type& other) noexcept;
 
   /// Move assignment operator.
   basic_executor_type& operator=(basic_executor_type&& other) noexcept;
 
 #if !defined(GENERATING_DOCUMENTATION)
 private:
   friend struct boost_asio_require_fn::impl;
   friend struct boost_asio_prefer_fn::impl;
 #endif // !defined(GENERATING_DOCUMENTATION)
 
   /// Obtain an executor with the @c blocking.possibly property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::blocking.possibly); @endcode
    */
   constexpr basic_executor_type<Allocator,
       BOOST_ASIO_UNSPECIFIED(Bits & ~blocking_mask)>
   require(execution::blocking_t::possibly_t) const
   {
     return basic_executor_type<Allocator, Bits & ~blocking_mask>(
         pool_, allocator_, bits_ & ~blocking_mask);
   }
 
   /// Obtain an executor with the @c blocking.always property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::blocking.always); @endcode
    */
   constexpr basic_executor_type<Allocator,
       BOOST_ASIO_UNSPECIFIED((Bits & ~blocking_mask) | blocking_always)>
   require(execution::blocking_t::always_t) const
   {
     return basic_executor_type<Allocator,
         BOOST_ASIO_UNSPECIFIED((Bits & ~blocking_mask) | blocking_always)>(
           pool_, allocator_, bits_ & ~blocking_mask);
   }
 
   /// Obtain an executor with the @c blocking.never property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::blocking.never); @endcode
    */
   constexpr basic_executor_type<Allocator,
       BOOST_ASIO_UNSPECIFIED(Bits & ~blocking_mask)>
   require(execution::blocking_t::never_t) const
   {
     return basic_executor_type<Allocator, Bits & ~blocking_mask>(
         pool_, allocator_, (bits_ & ~blocking_mask) | blocking_never);
   }
 
   /// Obtain an executor with the @c relationship.fork property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::relationship.fork); @endcode
    */
   constexpr basic_executor_type require(execution::relationship_t::fork_t) const
   {
     return basic_executor_type(pool_,
         allocator_, bits_ & ~relationship_continuation);
   }
 
   /// Obtain an executor with the @c relationship.continuation property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::relationship.continuation); @endcode
    */
   constexpr basic_executor_type require(
       execution::relationship_t::continuation_t) const
   {
     return basic_executor_type(pool_,
         allocator_, bits_ | relationship_continuation);
   }
 
   /// Obtain an executor with the @c outstanding_work.tracked property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::outstanding_work.tracked); @endcode
    */
   constexpr basic_executor_type<Allocator,
       BOOST_ASIO_UNSPECIFIED(Bits | outstanding_work_tracked)>
   require(execution::outstanding_work_t::tracked_t) const
   {
     return basic_executor_type<Allocator, Bits | outstanding_work_tracked>(
         pool_, allocator_, bits_);
   }
 
   /// Obtain an executor with the @c outstanding_work.untracked property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::outstanding_work.untracked); @endcode
    */
   constexpr basic_executor_type<Allocator,
       BOOST_ASIO_UNSPECIFIED(Bits & ~outstanding_work_tracked)>
   require(execution::outstanding_work_t::untracked_t) const
   {
     return basic_executor_type<Allocator, Bits & ~outstanding_work_tracked>(
         pool_, allocator_, bits_);
   }
 
   /// Obtain an executor with the specified @c allocator property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::allocator(my_allocator)); @endcode
    */
   template <typename OtherAllocator>
   constexpr basic_executor_type<OtherAllocator, Bits>
   require(execution::allocator_t<OtherAllocator> a) const
   {
     return basic_executor_type<OtherAllocator, Bits>(
         pool_, a.value(), bits_);
   }
 
   /// Obtain an executor with the default @c allocator property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::require customisation point.
    *
    * For example:
    * @code auto ex1 = my_thread_pool.executor();
    * auto ex2 = boost::asio::require(ex1,
    *     boost::asio::execution::allocator); @endcode
    */
   constexpr basic_executor_type<std::allocator<void>, Bits>
   require(execution::allocator_t<void>) const
   {
     return basic_executor_type<std::allocator<void>, Bits>(
         pool_, std::allocator<void>(), bits_);
   }
 
 #if !defined(GENERATING_DOCUMENTATION)
 private:
   friend struct boost_asio_query_fn::impl;
   friend struct boost::asio::execution::detail::mapping_t<0>;
   friend struct boost::asio::execution::detail::inline_exception_handling_t<0>;
   friend struct boost::asio::execution::detail::outstanding_work_t<0>;
 #endif // !defined(GENERATING_DOCUMENTATION)
 
   /// Query the current value of the @c mapping property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * if (boost::asio::query(ex, boost::asio::execution::mapping)
    *       == boost::asio::execution::mapping.thread)
    *   ... @endcode
    */
   static constexpr execution::mapping_t query(execution::mapping_t) noexcept
   {
     return execution::mapping.thread;
   }
 
   /// Query the current value of the @c inline_exception_handling property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.get_executor();
    * if (boost::asio::query(ex,
    *       boost::asio::execution::inline_exception_handling)
    *     == boost::asio::execution::inline_exception_handling.terminate)
    *   ... @endcode
    */
   static constexpr execution::inline_exception_handling_t query(
       execution::inline_exception_handling_t) noexcept
   {
     return execution::inline_exception_handling.terminate;
   }
 
   /// Query the current value of the @c context property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * boost::asio::thread_pool& pool = boost::asio::query(
    *     ex, boost::asio::execution::context); @endcode
    */
   thread_pool& query(execution::context_t) const noexcept
   {
     return *pool_;
   }
 
   /// Query the current value of the @c blocking property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * if (boost::asio::query(ex, boost::asio::execution::blocking)
    *       == boost::asio::execution::blocking.always)
    *   ... @endcode
    */
   constexpr execution::blocking_t query(execution::blocking_t) const noexcept
   {
     return (bits_ & blocking_never)
       ? execution::blocking_t(execution::blocking.never)
       : ((Bits & blocking_always)
           ? execution::blocking_t(execution::blocking.always)
           : execution::blocking_t(execution::blocking.possibly));
   }
 
   /// Query the current value of the @c relationship property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * if (boost::asio::query(ex, boost::asio::execution::relationship)
    *       == boost::asio::execution::relationship.continuation)
    *   ... @endcode
    */
   constexpr execution::relationship_t query(
       execution::relationship_t) const noexcept
   {
     return (bits_ & relationship_continuation)
       ? execution::relationship_t(execution::relationship.continuation)
       : execution::relationship_t(execution::relationship.fork);
   }
 
   /// Query the current value of the @c outstanding_work property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * if (boost::asio::query(ex, boost::asio::execution::outstanding_work)
    *       == boost::asio::execution::outstanding_work.tracked)
    *   ... @endcode
    */
   static constexpr execution::outstanding_work_t query(
       execution::outstanding_work_t) noexcept
   {
     return (Bits & outstanding_work_tracked)
       ? execution::outstanding_work_t(execution::outstanding_work.tracked)
       : execution::outstanding_work_t(execution::outstanding_work.untracked);
   }
 
   /// Query the current value of the @c allocator property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * auto alloc = boost::asio::query(ex,
    *     boost::asio::execution::allocator); @endcode
    */
   template <typename OtherAllocator>
   constexpr Allocator query(
       execution::allocator_t<OtherAllocator>) const noexcept
   {
     return allocator_;
   }
 
   /// Query the current value of the @c allocator property.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * auto alloc = boost::asio::query(ex,
    *     boost::asio::execution::allocator); @endcode
    */
   constexpr Allocator query(execution::allocator_t<void>) const noexcept
   {
     return allocator_;
   }
 
   /// Query the occupancy (recommended number of work items) for the pool.
   /**
    * Do not call this function directly. It is intended for use with the
    * boost::asio::query customisation point.
    *
    * For example:
    * @code auto ex = my_thread_pool.executor();
    * std::size_t occupancy = boost::asio::query(
    *     ex, boost::asio::execution::occupancy); @endcode
    */
   std::size_t query(execution::occupancy_t) const noexcept
   {
     return static_cast<std::size_t>(pool_->num_threads_);
   }
 
 public:
   /// Determine whether the thread pool is running in the current thread.
   /**
    * @return @c true if the current thread is running the thread pool. Otherwise
    * returns @c false.
    */
   bool running_in_this_thread() const noexcept;
 
   /// Compare two executors for equality.
   /**
    * Two executors are equal if they refer to the same underlying thread pool.
    */
   friend bool operator==(const basic_executor_type& a,
       const basic_executor_type& b) noexcept
   {
     return a.pool_ == b.pool_
       && a.allocator_ == b.allocator_
       && a.bits_ == b.bits_;
   }
 
   /// Compare two executors for inequality.
   /**
    * Two executors are equal if they refer to the same underlying thread pool.
    */
   friend bool operator!=(const basic_executor_type& a,
       const basic_executor_type& b) noexcept
   {
     return a.pool_ != b.pool_
       || a.allocator_ != b.allocator_
       || a.bits_ != b.bits_;
   }
 
   /// Execution function.
   template <typename Function>
   void execute(Function&& f) const
   {
     this->do_execute(static_cast<Function&&>(f),
         integral_constant<bool, (Bits & blocking_always) != 0>());
   }
 
 public:
 #if !defined(BOOST_ASIO_NO_TS_EXECUTORS)
   /// Obtain the underlying execution context.
   thread_pool& context() const noexcept;
 
   /// Inform the thread pool that it has some outstanding work to do.
   /**
    * This function is used to inform the thread pool that some work has begun.
    * This ensures that the thread pool's join() function will not return while
    * the work is underway.
    */
   void on_work_started() const noexcept;
 
   /// Inform the thread pool that some work is no longer outstanding.
   /**
    * This function is used to inform the thread pool that some work has
    * finished. Once the count of unfinished work reaches zero, the thread
    * pool's join() function is permitted to exit.
    */
   void on_work_finished() const noexcept;
 
   /// Request the thread pool to invoke the given function object.
   /**
    * This function is used to ask the thread pool to execute the given function
    * object. If the current thread belongs to the pool, @c dispatch() executes
    * the function before returning. Otherwise, the function will be scheduled
    * to run on the thread pool.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename OtherAllocator>
   void dispatch(Function&& f, const OtherAllocator& a) const;
 
   /// Request the thread pool to invoke the given function object.
   /**
    * This function is used to ask the thread pool to execute the given function
    * object. The function object will never be executed inside @c post().
    * Instead, it will be scheduled to run on the thread pool.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename OtherAllocator>
   void post(Function&& f, const OtherAllocator& a) const;
 
   /// Request the thread pool to invoke the given function object.
   /**
    * This function is used to ask the thread pool to execute the given function
    * object. The function object will never be executed inside @c defer().
    * Instead, it will be scheduled to run on the thread pool.
    *
    * If the current thread belongs to the thread pool, @c defer() will delay
    * scheduling the function object until the current thread returns control to
    * the pool.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename OtherAllocator>
   void defer(Function&& f, const OtherAllocator& a) const;
 #endif // !defined(BOOST_ASIO_NO_TS_EXECUTORS)
 
 private:
   friend class thread_pool;
   template <typename, unsigned int> friend class basic_executor_type;
 
   // Constructor used by thread_pool::get_executor().
   explicit basic_executor_type(thread_pool& p) noexcept
     : pool_(&p),
       allocator_(),
       bits_(0)
   {
     if (Bits & outstanding_work_tracked)
       pool_->scheduler_.work_started();
   }
 
   // Constructor used by require().
   basic_executor_type(thread_pool* p,
       const Allocator& a, unsigned int bits) noexcept
     : pool_(p),
       allocator_(a),
       bits_(bits)
   {
     if (Bits & outstanding_work_tracked)
       if (pool_)
         pool_->scheduler_.work_started();
   }
 
   /// Execution helper implementation for possibly and never blocking.
   template <typename Function>
   void do_execute(Function&& f, false_type) const;
 
   /// Execution helper implementation for always blocking.
   template <typename Function>
   void do_execute(Function&& f, true_type) const;
 
   // The underlying thread pool.
   thread_pool* pool_;
 
   // The allocator used for execution functions.
   Allocator allocator_;
 
   // The runtime-switched properties of the thread pool executor.
   unsigned int bits_;
 };
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 namespace traits {
 
 #if !defined(BOOST_ASIO_HAS_DEDUCED_EQUALITY_COMPARABLE_TRAIT)
 
 template <typename Allocator, unsigned int Bits>
 struct equality_comparable<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
 };
 
 #endif // !defined(BOOST_ASIO_HAS_DEDUCED_EQUALITY_COMPARABLE_TRAIT)
 
 #if !defined(BOOST_ASIO_HAS_DEDUCED_EXECUTE_MEMBER_TRAIT)
 
 template <typename Allocator, unsigned int Bits, typename Function>
 struct execute_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Function
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef void result_type;
 };
 
 #endif // !defined(BOOST_ASIO_HAS_DEDUCED_EXECUTE_MEMBER_TRAIT)
 
 #if !defined(BOOST_ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::blocking_t::possibly_t
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits & ~blocking_mask> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::blocking_t::always_t
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<Allocator,
       (Bits & ~blocking_mask) | blocking_always> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::blocking_t::never_t
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits & ~blocking_mask> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::relationship_t::fork_t
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::relationship_t::continuation_t
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::outstanding_work_t::tracked_t
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits | outstanding_work_tracked> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::outstanding_work_t::untracked_t
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       Allocator, Bits & ~outstanding_work_tracked> result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::allocator_t<void>
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       std::allocator<void>, Bits> result_type;
 };
 
 template <unsigned int Bits,
     typename Allocator, typename OtherAllocator>
 struct require_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::allocator_t<OtherAllocator>
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = false;
   typedef boost::asio::thread_pool::basic_executor_type<
       OtherAllocator, Bits> result_type;
 };
 
 #endif // !defined(BOOST_ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
 
 #if !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
 
 template <typename Allocator, unsigned int Bits, typename Property>
 struct query_static_constexpr_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Property,
     typename boost::asio::enable_if<
       boost::asio::is_convertible<
         Property,
         boost::asio::execution::outstanding_work_t
       >::value
     >::type
   > : boost::asio::detail::thread_pool_bits
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::execution::outstanding_work_t result_type;
 
   static constexpr result_type value() noexcept
   {
     return (Bits & outstanding_work_tracked)
       ? execution::outstanding_work_t(execution::outstanding_work.tracked)
       : execution::outstanding_work_t(execution::outstanding_work.untracked);
   }
 };
 
 template <typename Allocator, unsigned int Bits, typename Property>
 struct query_static_constexpr_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Property,
     typename boost::asio::enable_if<
       boost::asio::is_convertible<
         Property,
         boost::asio::execution::mapping_t
       >::value
     >::type
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::execution::mapping_t::thread_t result_type;
 
   static constexpr result_type value() noexcept
   {
     return result_type();
   }
 };
 
 template <typename Allocator, unsigned int Bits, typename Property>
 struct query_static_constexpr_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Property,
     typename boost::asio::enable_if<
       boost::asio::is_convertible<
         Property,
         boost::asio::execution::inline_exception_handling_t
       >::value
     >::type
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::execution::inline_exception_handling_t::terminate_t
     result_type;
 
   static constexpr result_type value() noexcept
   {
     return result_type();
   }
 };
 
 #endif // !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
 
 #if !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT)
 
 template <typename Allocator, unsigned int Bits, typename Property>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Property,
     typename boost::asio::enable_if<
       boost::asio::is_convertible<
         Property,
         boost::asio::execution::blocking_t
       >::value
     >::type
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::execution::blocking_t result_type;
 };
 
 template <typename Allocator, unsigned int Bits, typename Property>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     Property,
     typename boost::asio::enable_if<
       boost::asio::is_convertible<
         Property,
         boost::asio::execution::relationship_t
       >::value
     >::type
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::execution::relationship_t result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::occupancy_t
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef std::size_t result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::context_t
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef boost::asio::thread_pool& result_type;
 };
 
 template <typename Allocator, unsigned int Bits>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::allocator_t<void>
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef Allocator result_type;
 };
 
 template <typename Allocator, unsigned int Bits, typename OtherAllocator>
 struct query_member<
     boost::asio::thread_pool::basic_executor_type<Allocator, Bits>,
     boost::asio::execution::allocator_t<OtherAllocator>
   >
 {
   static constexpr bool is_valid = true;
   static constexpr bool is_noexcept = true;
   typedef Allocator result_type;
 };
 
 #endif // !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT)
 
 } // namespace traits
 
 namespace execution {
 
 template <>
 struct is_executor<thread_pool> : false_type
 {
 };
 
 } // namespace execution
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #include <boost/asio/impl/thread_pool.hpp>
 #if defined(BOOST_ASIO_HEADER_ONLY)
 # include <boost/asio/impl/thread_pool.ipp>
 #endif // defined(BOOST_ASIO_HEADER_ONLY)
 
 #endif // BOOST_ASIO_THREAD_POOL_HPP

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