. /include/boost/capy /ex /strand.hpp
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1   // 1   //
2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com) 2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   // 3   //
4   // Distributed under the Boost Software License, Version 1.0. (See accompanying 4   // Distributed under the Boost Software License, Version 1.0. (See accompanying
5   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 5   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6   // 6   //
7   // Official repository: https://github.com/cppalliance/capy 7   // Official repository: https://github.com/cppalliance/capy
8   // 8   //
9   9  
10   #ifndef BOOST_CAPY_EX_STRAND_HPP 10   #ifndef BOOST_CAPY_EX_STRAND_HPP
11   #define BOOST_CAPY_EX_STRAND_HPP 11   #define BOOST_CAPY_EX_STRAND_HPP
12   12  
13   #include <boost/capy/detail/config.hpp> 13   #include <boost/capy/detail/config.hpp>
14   #include <boost/capy/continuation.hpp> 14   #include <boost/capy/continuation.hpp>
15   #include <coroutine> 15   #include <coroutine>
16   #include <boost/capy/ex/detail/strand_service.hpp> 16   #include <boost/capy/ex/detail/strand_service.hpp>
17   17  
18   #include <type_traits> 18   #include <type_traits>
19   19  
20   namespace boost { 20   namespace boost {
21   namespace capy { 21   namespace capy {
22   22  
23   /** Provides serialized coroutine execution for any executor type. 23   /** Provides serialized coroutine execution for any executor type.
24   24  
25   A strand wraps an inner executor and ensures that coroutines 25   A strand wraps an inner executor and ensures that coroutines
26   dispatched through it never run concurrently. At most one 26   dispatched through it never run concurrently. At most one
27   coroutine executes at a time within a strand, even when the 27   coroutine executes at a time within a strand, even when the
28   underlying executor runs on multiple threads. 28   underlying executor runs on multiple threads.
29   29  
30   Strands are lightweight handles that can be copied freely. 30   Strands are lightweight handles that can be copied freely.
31   Copies share the same internal serialization state, so 31   Copies share the same internal serialization state, so
32   coroutines dispatched through any copy are serialized with 32   coroutines dispatched through any copy are serialized with
33   respect to all other copies. 33   respect to all other copies.
34   34  
35   @par Invariant 35   @par Invariant
36   Coroutines resumed through a strand shall not run concurrently. 36   Coroutines resumed through a strand shall not run concurrently.
37   37  
38   @par Implementation 38   @par Implementation
39   The strand uses a service-based architecture with a fixed pool 39   The strand uses a service-based architecture with a fixed pool
40   of 211 implementation objects. New strands hash to select an 40   of 211 implementation objects. New strands hash to select an
41   impl from the pool. Strands that hash to the same index share 41   impl from the pool. Strands that hash to the same index share
42   serialization, which is harmless (just extra serialization) 42   serialization, which is harmless (just extra serialization)
43   and rare with 211 buckets. 43   and rare with 211 buckets.
44   44  
45   @par Executor Concept 45   @par Executor Concept
46   This class satisfies the `Executor` concept, providing: 46   This class satisfies the `Executor` concept, providing:
47   - `context()` - Returns the underlying execution context 47   - `context()` - Returns the underlying execution context
48   - `on_work_started()` / `on_work_finished()` - Work tracking 48   - `on_work_started()` / `on_work_finished()` - Work tracking
49   - `dispatch(continuation&)` - May run immediately if strand is idle 49   - `dispatch(continuation&)` - May run immediately if strand is idle
50   - `post(continuation&)` - Always queues for later execution 50   - `post(continuation&)` - Always queues for later execution
51   51  
52   @par Thread Safety 52   @par Thread Safety
53   Distinct objects: Safe. 53   Distinct objects: Safe.
54   Shared objects: Safe. 54   Shared objects: Safe.
55   55  
56   @par Example 56   @par Example
57   @code 57   @code
58   thread_pool pool(4); 58   thread_pool pool(4);
59   auto strand = make_strand(pool.get_executor()); 59   auto strand = make_strand(pool.get_executor());
60   60  
61   // These continuations will never run concurrently 61   // These continuations will never run concurrently
62   continuation c1{h1}, c2{h2}, c3{h3}; 62   continuation c1{h1}, c2{h2}, c3{h3};
63   strand.post(c1); 63   strand.post(c1);
64   strand.post(c2); 64   strand.post(c2);
65   strand.post(c3); 65   strand.post(c3);
66   @endcode 66   @endcode
67   67  
68   @tparam E The type of the underlying executor. Must 68   @tparam E The type of the underlying executor. Must
69   satisfy the `Executor` concept. 69   satisfy the `Executor` concept.
70   70  
71   @see make_strand, Executor 71   @see make_strand, Executor
72   */ 72   */
73   template<typename Ex> 73   template<typename Ex>
74   class strand 74   class strand
75   { 75   {
76   detail::strand_impl* impl_; 76   detail::strand_impl* impl_;
77   Ex ex_; 77   Ex ex_;
78   78  
79   public: 79   public:
80   /** The type of the underlying executor. 80   /** The type of the underlying executor.
81   */ 81   */
82   using inner_executor_type = Ex; 82   using inner_executor_type = Ex;
83   83  
84   /** Construct a strand for the specified executor. 84   /** Construct a strand for the specified executor.
85   85  
86   Obtains a strand implementation from the service associated 86   Obtains a strand implementation from the service associated
87   with the executor's context. The implementation is selected 87   with the executor's context. The implementation is selected
88   from a fixed pool using a hash function. 88   from a fixed pool using a hash function.
89   89  
90   @param ex The inner executor to wrap. Coroutines will 90   @param ex The inner executor to wrap. Coroutines will
91   ultimately be dispatched through this executor. 91   ultimately be dispatched through this executor.
92   92  
93   @note This constructor is disabled if the argument is a 93   @note This constructor is disabled if the argument is a
94   strand type, to prevent strand-of-strand wrapping. 94   strand type, to prevent strand-of-strand wrapping.
95   */ 95   */
96   template<typename Ex1, 96   template<typename Ex1,
97   typename = std::enable_if_t< 97   typename = std::enable_if_t<
98   !std::is_same_v<std::decay_t<Ex1>, strand> && 98   !std::is_same_v<std::decay_t<Ex1>, strand> &&
99   !detail::is_strand<std::decay_t<Ex1>>::value && 99   !detail::is_strand<std::decay_t<Ex1>>::value &&
100   std::is_convertible_v<Ex1, Ex>>> 100   std::is_convertible_v<Ex1, Ex>>>
101   explicit 101   explicit
HITCBC 102   27 strand(Ex1&& ex) 102   29 strand(Ex1&& ex)
HITCBC 103   27 : impl_(detail::get_strand_service(ex.context()) 103   29 : impl_(detail::get_strand_service(ex.context())
HITCBC 104   27 .get_implementation()) 104   29 .get_implementation())
HITCBC 105   27 , ex_(std::forward<Ex1>(ex)) 105   29 , ex_(std::forward<Ex1>(ex))
106   { 106   {
HITCBC 107   27 } 107   29 }
108   108  
109   /** Construct a copy. 109   /** Construct a copy.
110   110  
111   Creates a strand that shares serialization state with 111   Creates a strand that shares serialization state with
112   the original. Coroutines dispatched through either strand 112   the original. Coroutines dispatched through either strand
113   will be serialized with respect to each other. 113   will be serialized with respect to each other.
114   */ 114   */
HITCBC 115   1 strand(strand const&) = default; 115   1 strand(strand const&) = default;
116   116  
117   /** Construct by moving. 117   /** Construct by moving.
118   118  
119   @note A moved-from strand is only safe to destroy 119   @note A moved-from strand is only safe to destroy
120   or reassign. 120   or reassign.
121   */ 121   */
122   strand(strand&&) = default; 122   strand(strand&&) = default;
123   123  
124   /** Assign by copying. 124   /** Assign by copying.
125   */ 125   */
126   strand& operator=(strand const&) = default; 126   strand& operator=(strand const&) = default;
127   127  
128   /** Assign by moving. 128   /** Assign by moving.
129   129  
130   @note A moved-from strand is only safe to destroy 130   @note A moved-from strand is only safe to destroy
131   or reassign. 131   or reassign.
132   */ 132   */
133   strand& operator=(strand&&) = default; 133   strand& operator=(strand&&) = default;
134   134  
135   /** Return the underlying executor. 135   /** Return the underlying executor.
136   136  
137   @return A const reference to the inner executor. 137   @return A const reference to the inner executor.
138   */ 138   */
139   Ex const& 139   Ex const&
HITCBC 140   1 get_inner_executor() const noexcept 140   1 get_inner_executor() const noexcept
141   { 141   {
HITCBC 142   1 return ex_; 142   1 return ex_;
143   } 143   }
144   144  
145   /** Return the underlying execution context. 145   /** Return the underlying execution context.
146   146  
147   @return A reference to the execution context associated 147   @return A reference to the execution context associated
148   with the inner executor. 148   with the inner executor.
149   */ 149   */
150   auto& 150   auto&
HITCBC 151   4 context() const noexcept 151   5 context() const noexcept
152   { 152   {
HITCBC 153   4 return ex_.context(); 153   5 return ex_.context();
154   } 154   }
155   155  
156   /** Notify that work has started. 156   /** Notify that work has started.
157   157  
158   Delegates to the inner executor's `on_work_started()`. 158   Delegates to the inner executor's `on_work_started()`.
159   This is a no-op for most executor types. 159   This is a no-op for most executor types.
160   */ 160   */
161   void 161   void
HITCBC 162   5 on_work_started() const noexcept 162   6 on_work_started() const noexcept
163   { 163   {
HITCBC 164   5 ex_.on_work_started(); 164   6 ex_.on_work_started();
HITCBC 165   5 } 165   6 }
166   166  
167   /** Notify that work has finished. 167   /** Notify that work has finished.
168   168  
169   Delegates to the inner executor's `on_work_finished()`. 169   Delegates to the inner executor's `on_work_finished()`.
170   This is a no-op for most executor types. 170   This is a no-op for most executor types.
171   */ 171   */
172   void 172   void
HITCBC 173   5 on_work_finished() const noexcept 173   6 on_work_finished() const noexcept
174   { 174   {
HITCBC 175   5 ex_.on_work_finished(); 175   6 ex_.on_work_finished();
HITCBC 176   5 } 176   6 }
177   177  
178   /** Determine whether the strand is running in the current thread. 178   /** Determine whether the strand is running in the current thread.
179   179  
180   @return true if the current thread is executing a coroutine 180   @return true if the current thread is executing a coroutine
181   within this strand's dispatch loop. 181   within this strand's dispatch loop.
182   */ 182   */
183   bool 183   bool
HITCBC 184   2 running_in_this_thread() const noexcept 184   4 running_in_this_thread() const noexcept
185   { 185   {
HITCBC 186   2 return detail::strand_service::running_in_this_thread(*impl_); 186   4 return detail::strand_service::running_in_this_thread(*impl_);
187   } 187   }
188   188  
189   /** Compare two strands for equality. 189   /** Compare two strands for equality.
190   190  
191   Two strands are equal if they share the same internal 191   Two strands are equal if they share the same internal
192   serialization state. Equal strands serialize coroutines 192   serialization state. Equal strands serialize coroutines
193   with respect to each other. 193   with respect to each other.
194   194  
195   @param other The strand to compare against. 195   @param other The strand to compare against.
196   @return true if both strands share the same implementation. 196   @return true if both strands share the same implementation.
197   */ 197   */
198   bool 198   bool
HITCBC 199   4 operator==(strand const& other) const noexcept 199   4 operator==(strand const& other) const noexcept
200   { 200   {
HITCBC 201   4 return impl_ == other.impl_; 201   4 return impl_ == other.impl_;
202   } 202   }
203   203  
204   /** Post a continuation to the strand. 204   /** Post a continuation to the strand.
205   205  
206   The continuation is always queued for execution, never resumed 206   The continuation is always queued for execution, never resumed
207   immediately. When the strand becomes available, queued 207   immediately. When the strand becomes available, queued
208   work executes in FIFO order on the underlying executor. 208   work executes in FIFO order on the underlying executor.
209   209  
210   @par Ordering 210   @par Ordering
211   Guarantees strict FIFO ordering relative to other post() calls. 211   Guarantees strict FIFO ordering relative to other post() calls.
212   Use this instead of dispatch() when ordering matters. 212   Use this instead of dispatch() when ordering matters.
213   213  
214   @param c The continuation to post. The caller retains 214   @param c The continuation to post. The caller retains
215   ownership; the continuation must remain valid until 215   ownership; the continuation must remain valid until
216   it is dequeued and resumed. 216   it is dequeued and resumed.
217   */ 217   */
218   void 218   void
HITCBC 219   327 post(continuation& c) const 219   330 post(continuation& c) const
220   { 220   {
HITCBC 221   327 detail::strand_service::post(*impl_, executor_ref(ex_), c.h); 221   330 detail::strand_service::post(*impl_, executor_ref(ex_), c.h);
HITCBC 222   327 } 222   330 }
223   223  
224   /** Dispatch a continuation through the strand. 224   /** Dispatch a continuation through the strand.
225   225  
226   Returns a handle for symmetric transfer. If the calling 226   Returns a handle for symmetric transfer. If the calling
227   thread is already executing within this strand, returns `c.h`. 227   thread is already executing within this strand, returns `c.h`.
228   Otherwise, the continuation is queued and 228   Otherwise, the continuation is queued and
229   `std::noop_coroutine()` is returned. 229   `std::noop_coroutine()` is returned.
230   230  
231   @par Ordering 231   @par Ordering
232   Callers requiring strict FIFO ordering should use post() 232   Callers requiring strict FIFO ordering should use post()
233   instead, which always queues the continuation. 233   instead, which always queues the continuation.
234   234  
235   @param c The continuation to dispatch. The caller retains 235   @param c The continuation to dispatch. The caller retains
236   ownership; the continuation must remain valid until 236   ownership; the continuation must remain valid until
237   it is dequeued and resumed. 237   it is dequeued and resumed.
238   238  
239   @return A handle for symmetric transfer or `std::noop_coroutine()`. 239   @return A handle for symmetric transfer or `std::noop_coroutine()`.
240   */ 240   */
241   std::coroutine_handle<> 241   std::coroutine_handle<>
HITCBC 242   8 dispatch(continuation& c) const 242   8 dispatch(continuation& c) const
243   { 243   {
HITCBC 244   8 return detail::strand_service::dispatch(*impl_, executor_ref(ex_), c.h); 244   8 return detail::strand_service::dispatch(*impl_, executor_ref(ex_), c.h);
245   } 245   }
246   }; 246   };
247   247  
248   // Deduction guide 248   // Deduction guide
249   template<typename Ex> 249   template<typename Ex>
250   strand(Ex) -> strand<Ex>; 250   strand(Ex) -> strand<Ex>;
251   251  
252   } // namespace capy 252   } // namespace capy
253   } // namespace boost 253   } // namespace boost
254   254  
255   #endif 255   #endif