. /src /ex /detail /strand_service.cpp
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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   #include "src/ex/detail/strand_impl.hpp" 10   #include "src/ex/detail/strand_impl.hpp"
11   #include <boost/capy/ex/detail/strand_service.hpp> 11   #include <boost/capy/ex/detail/strand_service.hpp>
12   #include <boost/capy/continuation.hpp> 12   #include <boost/capy/continuation.hpp>
13   #include <coroutine> 13   #include <coroutine>
14   #include <memory> 14   #include <memory>
15   #include <utility> 15   #include <utility>
16   16  
17   namespace boost { 17   namespace boost {
18   namespace capy { 18   namespace capy {
19   namespace detail { 19   namespace detail {
20   20  
21   // Sentinel stored in invoker_frame_cache_ after shutdown to prevent 21   // Sentinel stored in invoker_frame_cache_ after shutdown to prevent
22   // in-flight invokers from repopulating a freed cache slot. 22   // in-flight invokers from repopulating a freed cache slot.
23   inline void* const kCacheClosed = reinterpret_cast<void*>(1); 23   inline void* const kCacheClosed = reinterpret_cast<void*>(1);
24   24  
25   /** Concrete strand_service. 25   /** Concrete strand_service.
26   26  
27   Holds a shared mutex pool (193 entries), a linked list of live 27   Holds a shared mutex pool (193 entries), a linked list of live
28   impls (for shutdown traversal), and a single-slot invoker 28   impls (for shutdown traversal), and a single-slot invoker
29   coroutine frame cache shared across all strands of this service. 29   coroutine frame cache shared across all strands of this service.
30   30  
31   The dispatch helpers (`enqueue`, `dispatch_pending`, etc.) are 31   The dispatch helpers (`enqueue`, `dispatch_pending`, etc.) are
32   public so the namespace-scope `make_invoker` coroutine and the 32   public so the namespace-scope `make_invoker` coroutine and the
33   `strand_service` static methods can call them without friendship. 33   `strand_service` static methods can call them without friendship.
34   */ 34   */
35   class strand_service_impl : public strand_service 35   class strand_service_impl : public strand_service
36   { 36   {
37   public: 37   public:
38   static constexpr std::size_t num_mutexes = 193; 38   static constexpr std::size_t num_mutexes = 193;
39   39  
40   std::mutex mutex_; 40   std::mutex mutex_;
41   std::size_t salt_ = 0; 41   std::size_t salt_ = 0;
42   std::shared_ptr<std::mutex> mutexes_[num_mutexes]; 42   std::shared_ptr<std::mutex> mutexes_[num_mutexes];
43   intrusive_list<strand_impl> impl_list_; 43   intrusive_list<strand_impl> impl_list_;
44   std::atomic<void*> invoker_frame_cache_{nullptr}; 44   std::atomic<void*> invoker_frame_cache_{nullptr};
45   45  
46   explicit 46   explicit
HITCBC 47   34 strand_service_impl(execution_context&) 47   34 strand_service_impl(execution_context&)
HITCBC 48   34 { 48   34 {
HITCBC 49   34 } 49   34 }
50   50  
51   std::shared_ptr<strand_impl> 51   std::shared_ptr<strand_impl>
HITCBC 52   11446 create_implementation() override 52   11446 create_implementation() override
53   { 53   {
HITCBC 54   11446 auto new_impl = std::make_shared<strand_impl>(); 54   11446 auto new_impl = std::make_shared<strand_impl>();
55   55  
HITCBC 56   11446 std::lock_guard<std::mutex> lock(mutex_); 56   11446 std::lock_guard<std::mutex> lock(mutex_);
57   57  
HITCBC 58   11446 std::size_t s = salt_++; 58   11446 std::size_t s = salt_++;
HITCBC 59   11446 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get()); 59   11446 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get());
HITCBC 60   11446 idx += idx >> 3; 60   11446 idx += idx >> 3;
HITCBC 61   11446 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2); 61   11446 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2);
HITCBC 62   11446 idx %= num_mutexes; 62   11446 idx %= num_mutexes;
HITCBC 63   11446 if(!mutexes_[idx]) 63   11446 if(!mutexes_[idx])
HITCBC 64   670 mutexes_[idx] = std::make_shared<std::mutex>(); 64   669 mutexes_[idx] = std::make_shared<std::mutex>();
HITCBC 65   11446 new_impl->mutex_ = mutexes_[idx].get(); 65   11446 new_impl->mutex_ = mutexes_[idx].get();
66   66  
HITCBC 67   11446 impl_list_.push_back(new_impl.get()); 67   11446 impl_list_.push_back(new_impl.get());
HITCBC 68   11446 new_impl->service_.store(this, std::memory_order_release); 68   11446 new_impl->service_.store(this, std::memory_order_release);
69   69  
HITCBC 70   22892 return new_impl; 70   22892 return new_impl;
HITCBC 71   11446 } 71   11446 }
72   72  
73   static bool 73   static bool
HITCBC 74   30342 enqueue(strand_impl& impl, continuation& c) 74   30342 enqueue(strand_impl& impl, continuation& c)
75   { 75   {
HITCBC 76   30342 std::lock_guard<std::mutex> lock(*impl.mutex_); 76   30342 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 77   30342 impl.pending_.push(c); 77   30342 impl.pending_.push(c);
HITCBC 78   30342 if(!impl.locked_) 78   30342 if(!impl.locked_)
79   { 79   {
HITCBC 80   17963 impl.locked_ = true; 80   17760 impl.locked_ = true;
HITCBC 81   17963 return true; 81   17760 return true;
82   } 82   }
HITCBC 83   12379 return false; 83   12582 return false;
HITCBC 84   30342 } 84   30342 }
85   85  
86   static void 86   static void
HITCBC 87   19075 dispatch_pending(strand_impl& impl) 87   19057 dispatch_pending(strand_impl& impl)
88   { 88   {
HITCBC 89   19075 strand_queue::taken_batch batch; 89   19057 strand_queue::taken_batch batch;
90   { 90   {
HITCBC 91   19075 std::lock_guard<std::mutex> lock(*impl.mutex_); 91   19057 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 92   19075 batch = impl.pending_.take_all(); 92   19057 batch = impl.pending_.take_all();
HITCBC 93   19075 } 93   19057 }
HITCBC 94   19075 impl.pending_.dispatch_batch(batch); 94   19057 impl.pending_.dispatch_batch(batch);
HITCBC 95   19075 } 95   19057 }
96   96  
97   static bool 97   static bool
HITCBC 98   19075 try_unlock(strand_impl& impl) 98   19057 try_unlock(strand_impl& impl)
99   { 99   {
HITCBC 100   19075 std::lock_guard<std::mutex> lock(*impl.mutex_); 100   19057 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 101   19075 if(impl.pending_.empty()) 101   19057 if(impl.pending_.empty())
102   { 102   {
HITCBC 103   17963 impl.locked_ = false; 103   17760 impl.locked_ = false;
HITCBC 104   17963 return true; 104   17760 return true;
105   } 105   }
HITCBC 106   1112 return false; 106   1297 return false;
HITCBC 107   19075 } 107   19057 }
108   108  
109   static void 109   static void
HITCBC 110   19075 set_dispatch_thread(strand_impl& impl) noexcept 110   19057 set_dispatch_thread(strand_impl& impl) noexcept
111   { 111   {
HITCBC 112   19075 impl.dispatch_thread_.store(std::this_thread::get_id()); 112   19057 impl.dispatch_thread_.store(std::this_thread::get_id());
HITCBC 113   19075 } 113   19057 }
114   114  
115   static void 115   static void
HITCBC 116   17963 clear_dispatch_thread(strand_impl& impl) noexcept 116   17760 clear_dispatch_thread(strand_impl& impl) noexcept
117   { 117   {
HITCBC 118   17963 impl.dispatch_thread_.store(std::thread::id{}); 118   17760 impl.dispatch_thread_.store(std::thread::id{});
HITCBC 119   17963 } 119   17760 }
120   120  
121   // Defined below; needs strand_invoker complete. 121   // Defined below; needs strand_invoker complete.
122   static void 122   static void
123   post_invoker(std::shared_ptr<strand_impl> impl, executor_ref ex); 123   post_invoker(std::shared_ptr<strand_impl> impl, executor_ref ex);
124   124  
125   protected: 125   protected:
126   void 126   void
HITCBC 127   34 shutdown() override 127   34 shutdown() override
128   { 128   {
HITCBC 129   34 std::lock_guard<std::mutex> lock(mutex_); 129   34 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 130   35 while(auto* p = impl_list_.pop_front()) 130   35 while(auto* p = impl_list_.pop_front())
131   { 131   {
HITCBC 132   1 std::lock_guard<std::mutex> impl_lock(*p->mutex_); 132   1 std::lock_guard<std::mutex> impl_lock(*p->mutex_);
HITCBC 133   1 p->locked_ = true; 133   1 p->locked_ = true;
HITCBC 134   1 p->service_.store(nullptr, std::memory_order_release); 134   1 p->service_.store(nullptr, std::memory_order_release);
HITCBC 135   1 } 135   1 }
136   136  
HITCBC 137   34 void* fp = invoker_frame_cache_.exchange( 137   34 void* fp = invoker_frame_cache_.exchange(
138   kCacheClosed, std::memory_order_acq_rel); 138   kCacheClosed, std::memory_order_acq_rel);
HITCBC 139   34 if(fp) ::operator delete(fp); 139   34 if(fp) ::operator delete(fp);
HITCBC 140   34 } 140   34 }
141   }; 141   };
142   142  
143   /** Invoker coroutine that drains a strand's pending queue. 143   /** Invoker coroutine that drains a strand's pending queue.
144   144  
145   Runs once the strand transitions from unlocked to locked. Holds 145   Runs once the strand transitions from unlocked to locked. Holds
146   the impl alive via the coroutine parameter (a shared_ptr in the 146   the impl alive via the coroutine parameter (a shared_ptr in the
147   coroutine frame), so user code may drop its strand handle while 147   coroutine frame), so user code may drop its strand handle while
148   the invoker is mid-flight. 148   the invoker is mid-flight.
149   149  
150   The frame's allocator recycles a single per-service slot. The 150   The frame's allocator recycles a single per-service slot. The
151   trailer points at the service (lifetime: execution_context), 151   trailer points at the service (lifetime: execution_context),
152   NOT the impl (lifetime: per-strand), so operator delete is 152   NOT the impl (lifetime: per-strand), so operator delete is
153   safe even after the impl has been destroyed. 153   safe even after the impl has been destroyed.
154   */ 154   */
155   struct strand_invoker 155   struct strand_invoker
156   { 156   {
157   struct promise_type 157   struct promise_type
158   { 158   {
159   // Stored in the coroutine frame so its address is stable for 159   // Stored in the coroutine frame so its address is stable for
160   // posting to the inner executor. 160   // posting to the inner executor.
161   continuation self_; 161   continuation self_;
162   162  
163   void* 163   void*
HITCBC 164   17963 operator new( 164   17760 operator new(
165   std::size_t n, 165   std::size_t n,
166   std::shared_ptr<strand_impl> const& impl) 166   std::shared_ptr<strand_impl> const& impl)
167   { 167   {
HITCBC 168   17963 auto* svc = impl->service_.load(std::memory_order_acquire); 168   17760 auto* svc = impl->service_.load(std::memory_order_acquire);
HITCBC 169   17963 constexpr auto A = alignof(strand_service_impl*); 169   17760 constexpr auto A = alignof(strand_service_impl*);
HITCBC 170   17963 std::size_t padded = (n + A - 1) & ~(A - 1); 170   17760 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 171   17963 std::size_t total = padded + sizeof(strand_service_impl*); 171   17760 std::size_t total = padded + sizeof(strand_service_impl*);
172   172  
HITCBC 173   17963 void* p = svc->invoker_frame_cache_.exchange( 173   17760 void* p = svc->invoker_frame_cache_.exchange(
174   nullptr, std::memory_order_acquire); 174   nullptr, std::memory_order_acquire);
HITCBC 175   17963 if(!p || p == kCacheClosed) 175   17760 if(!p || p == kCacheClosed)
HITCBC 176   7734 p = ::operator new(total); 176   6952 p = ::operator new(total);
177   177  
HITCBC 178   17963 *reinterpret_cast<strand_service_impl**>( 178   17760 *reinterpret_cast<strand_service_impl**>(
HITCBC 179   17963 static_cast<char*>(p) + padded) = svc; 179   17760 static_cast<char*>(p) + padded) = svc;
HITCBC 180   17963 return p; 180   17760 return p;
181   } 181   }
182   182  
183   void 183   void
HITCBC 184   17963 operator delete(void* p, std::size_t n) noexcept 184   17760 operator delete(void* p, std::size_t n) noexcept
185   { 185   {
HITCBC 186   17963 constexpr auto A = alignof(strand_service_impl*); 186   17760 constexpr auto A = alignof(strand_service_impl*);
HITCBC 187   17963 std::size_t padded = (n + A - 1) & ~(A - 1); 187   17760 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 188   17963 auto* svc = *reinterpret_cast<strand_service_impl**>( 188   17760 auto* svc = *reinterpret_cast<strand_service_impl**>(
189   static_cast<char*>(p) + padded); 189   static_cast<char*>(p) + padded);
190   190  
HITCBC 191   17963 void* expected = nullptr; 191   17760 void* expected = nullptr;
HITCBC 192   17963 if(!svc->invoker_frame_cache_.compare_exchange_strong( 192   17760 if(!svc->invoker_frame_cache_.compare_exchange_strong(
193   expected, p, std::memory_order_release)) 193   expected, p, std::memory_order_release))
HITCBC 194   7715 ::operator delete(p); 194   6933 ::operator delete(p);
HITCBC 195   17963 } 195   17760 }
196   196  
197   strand_invoker 197   strand_invoker
HITCBC 198   17963 get_return_object() noexcept 198   17760 get_return_object() noexcept
199   { 199   {
HITCBC 200   17963 return {std::coroutine_handle<promise_type>::from_promise(*this)}; 200   17760 return {std::coroutine_handle<promise_type>::from_promise(*this)};
201   } 201   }
202   202  
HITCBC 203   17963 std::suspend_always initial_suspend() noexcept { return {}; } 203   17760 std::suspend_always initial_suspend() noexcept { return {}; }
HITCBC 204   17963 std::suspend_never final_suspend() noexcept { return {}; } 204   17760 std::suspend_never final_suspend() noexcept { return {}; }
HITCBC 205   17963 void return_void() noexcept {} 205   17760 void return_void() noexcept {}
206   void unhandled_exception() { std::terminate(); } // LCOV_EXCL_LINE terminates the process 206   void unhandled_exception() { std::terminate(); } // LCOV_EXCL_LINE terminates the process
207   }; 207   };
208   208  
209   std::coroutine_handle<promise_type> h_; 209   std::coroutine_handle<promise_type> h_;
210   }; 210   };
211   211  
212   // The by-value parameter lives in the coroutine frame for the 212   // The by-value parameter lives in the coroutine frame for the
213   // invoker's lifetime, keeping the impl alive past any user-side 213   // invoker's lifetime, keeping the impl alive past any user-side
214   // strand drop. 214   // strand drop.
215   static 215   static
216   strand_invoker 216   strand_invoker
HITCBC 217   17963 make_invoker(std::shared_ptr<strand_impl> impl) 217   17760 make_invoker(std::shared_ptr<strand_impl> impl)
218   { 218   {
219   auto* p = impl.get(); 219   auto* p = impl.get();
220   for(;;) 220   for(;;)
221   { 221   {
222   strand_service_impl::set_dispatch_thread(*p); 222   strand_service_impl::set_dispatch_thread(*p);
223   strand_service_impl::dispatch_pending(*p); 223   strand_service_impl::dispatch_pending(*p);
224   if(strand_service_impl::try_unlock(*p)) 224   if(strand_service_impl::try_unlock(*p))
225   { 225   {
226   strand_service_impl::clear_dispatch_thread(*p); 226   strand_service_impl::clear_dispatch_thread(*p);
227   co_return; 227   co_return;
228   } 228   }
229   } 229   }
HITCBC 230   35926 } 230   35520 }
231   231  
232   void 232   void
HITCBC 233   17963 strand_service_impl::post_invoker( 233   17760 strand_service_impl::post_invoker(
234   std::shared_ptr<strand_impl> impl, 234   std::shared_ptr<strand_impl> impl,
235   executor_ref ex) 235   executor_ref ex)
236   { 236   {
HITCBC 237   17963 auto invoker = make_invoker(std::move(impl)); 237   17760 auto invoker = make_invoker(std::move(impl));
HITCBC 238   17963 auto& self = invoker.h_.promise().self_; 238   17760 auto& self = invoker.h_.promise().self_;
HITCBC 239   17963 self.h = invoker.h_; 239   17760 self.h = invoker.h_;
HITCBC 240   17963 ex.post(self); 240   17760 ex.post(self);
HITCBC 241   17963 } 241   17760 }
242   242  
HITCBC 243   22891 strand_impl::~strand_impl() 243   22891 strand_impl::~strand_impl()
244   { 244   {
HITCBC 245   11446 auto* svc = service_.load(std::memory_order_acquire); 245   11446 auto* svc = service_.load(std::memory_order_acquire);
HITCBC 246   11446 if(!svc) return; 246   11446 if(!svc) return;
HITCBC 247   11445 std::lock_guard<std::mutex> lock(svc->mutex_); 247   11445 std::lock_guard<std::mutex> lock(svc->mutex_);
HITCBC 248   11445 svc->impl_list_.remove(this); 248   11445 svc->impl_list_.remove(this);
HITCBC 249   11446 } 249   11446 }
250   250  
HITCBC 251   34 strand_service:: 251   34 strand_service::
HITCBC 252   34 strand_service() 252   34 strand_service()
HITCBC 253   34 : service() 253   34 : service()
254   { 254   {
HITCBC 255   34 } 255   34 }
256   256  
HITCBC 257   34 strand_service:: 257   34 strand_service::
258   ~strand_service() = default; 258   ~strand_service() = default;
259   259  
260   bool 260   bool
HITCBC 261   13 strand_service:: 261   13 strand_service::
262   running_in_this_thread(strand_impl& impl) noexcept 262   running_in_this_thread(strand_impl& impl) noexcept
263   { 263   {
HITCBC 264   13 return impl.dispatch_thread_.load() == std::this_thread::get_id(); 264   13 return impl.dispatch_thread_.load() == std::this_thread::get_id();
265   } 265   }
266   266  
267   std::coroutine_handle<> 267   std::coroutine_handle<>
HITCBC 268   9 strand_service:: 268   9 strand_service::
269   dispatch( 269   dispatch(
270   std::shared_ptr<strand_impl> const& impl, 270   std::shared_ptr<strand_impl> const& impl,
271   executor_ref ex, 271   executor_ref ex,
272   continuation& c) 272   continuation& c)
273   { 273   {
HITCBC 274   9 if(running_in_this_thread(*impl)) 274   9 if(running_in_this_thread(*impl))
HITCBC 275   3 return c.h; 275   3 return c.h;
276   276  
HITCBC 277   6 if(strand_service_impl::enqueue(*impl, c)) 277   6 if(strand_service_impl::enqueue(*impl, c))
HITCBC 278   6 strand_service_impl::post_invoker(impl, ex); 278   6 strand_service_impl::post_invoker(impl, ex);
HITCBC 279   6 return std::noop_coroutine(); 279   6 return std::noop_coroutine();
280   } 280   }
281   281  
282   void 282   void
HITCBC 283   30336 strand_service:: 283   30336 strand_service::
284   post( 284   post(
285   std::shared_ptr<strand_impl> const& impl, 285   std::shared_ptr<strand_impl> const& impl,
286   executor_ref ex, 286   executor_ref ex,
287   continuation& c) 287   continuation& c)
288   { 288   {
HITCBC 289   30336 if(strand_service_impl::enqueue(*impl, c)) 289   30336 if(strand_service_impl::enqueue(*impl, c))
HITCBC 290   17957 strand_service_impl::post_invoker(impl, ex); 290   17754 strand_service_impl::post_invoker(impl, ex);
HITCBC 291   30336 } 291   30336 }
292   292  
293   strand_service& 293   strand_service&
HITCBC 294   11446 get_strand_service(execution_context& ctx) 294   11446 get_strand_service(execution_context& ctx)
295   { 295   {
HITCBC 296   11446 return ctx.use_service<strand_service_impl>(); 296   11446 return ctx.use_service<strand_service_impl>();
297   } 297   }
298   298  
299   } // namespace detail 299   } // namespace detail
300   } // namespace capy 300   } // namespace capy
301   } // namespace boost 301   } // namespace boost