. /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   30 strand_service_impl(execution_context&) 47   30 strand_service_impl(execution_context&)
HITCBC 48   30 { 48   30 {
HITCBC 49   30 } 49   30 }
50   50  
51   std::shared_ptr<strand_impl> 51   std::shared_ptr<strand_impl>
HITCBC 52   11442 create_implementation() override 52   11442 create_implementation() override
53   { 53   {
HITCBC 54   11442 auto new_impl = std::make_shared<strand_impl>(); 54   11442 auto new_impl = std::make_shared<strand_impl>();
55   55  
HITCBC 56   11442 std::lock_guard<std::mutex> lock(mutex_); 56   11442 std::lock_guard<std::mutex> lock(mutex_);
57   57  
HITCBC 58   11442 std::size_t s = salt_++; 58   11442 std::size_t s = salt_++;
HITCBC 59   11442 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get()); 59   11442 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get());
HITCBC 60   11442 idx += idx >> 3; 60   11442 idx += idx >> 3;
HITCBC 61   11442 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2); 61   11442 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2);
HITCBC 62   11442 idx %= num_mutexes; 62   11442 idx %= num_mutexes;
HITCBC 63   11442 if(!mutexes_[idx]) 63   11442 if(!mutexes_[idx])
HITCBC 64   673 mutexes_[idx] = std::make_shared<std::mutex>(); 64   669 mutexes_[idx] = std::make_shared<std::mutex>();
HITCBC 65   11442 new_impl->mutex_ = mutexes_[idx].get(); 65   11442 new_impl->mutex_ = mutexes_[idx].get();
66   66  
HITCBC 67   11442 impl_list_.push_back(new_impl.get()); 67   11442 impl_list_.push_back(new_impl.get());
HITCBC 68   11442 new_impl->service_.store(this, std::memory_order_release); 68   11442 new_impl->service_.store(this, std::memory_order_release);
69   69  
HITCBC 70   22884 return new_impl; 70   22884 return new_impl;
HITCBC 71   11442 } 71   11442 }
72   72  
73   static bool 73   static bool
HITCBC 74   30340 enqueue(strand_impl& impl, continuation& c) 74   30340 enqueue(strand_impl& impl, continuation& c)
75   { 75   {
HITCBC 76   30340 std::lock_guard<std::mutex> lock(*impl.mutex_); 76   30340 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 77   30340 impl.pending_.push(c); 77   30340 impl.pending_.push(c);
HITCBC 78   30340 if(!impl.locked_) 78   30340 if(!impl.locked_)
79   { 79   {
HITCBC 80   19330 impl.locked_ = true; 80   20153 impl.locked_ = true;
HITCBC 81   19330 return true; 81   20153 return true;
82   } 82   }
HITCBC 83   11010 return false; 83   10187 return false;
HITCBC 84   30340 } 84   30340 }
85   85  
86   static void 86   static void
HITCBC 87   19981 dispatch_pending(strand_impl& impl) 87   20481 dispatch_pending(strand_impl& impl)
88   { 88   {
HITCBC 89   19981 strand_queue::taken_batch batch; 89   20481 strand_queue::taken_batch batch;
90   { 90   {
HITCBC 91   19981 std::lock_guard<std::mutex> lock(*impl.mutex_); 91   20481 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 92   19981 batch = impl.pending_.take_all(); 92   20481 batch = impl.pending_.take_all();
HITCBC 93   19981 } 93   20481 }
HITCBC 94   19981 impl.pending_.dispatch_batch(batch); 94   20481 impl.pending_.dispatch_batch(batch);
HITCBC 95   19981 } 95   20481 }
96   96  
97   static bool 97   static bool
HITCBC 98   19981 try_unlock(strand_impl& impl) 98   20481 try_unlock(strand_impl& impl)
99   { 99   {
HITCBC 100   19981 std::lock_guard<std::mutex> lock(*impl.mutex_); 100   20481 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 101   19981 if(impl.pending_.empty()) 101   20481 if(impl.pending_.empty())
102   { 102   {
HITCBC 103   19330 impl.locked_ = false; 103   20153 impl.locked_ = false;
HITCBC 104   19330 return true; 104   20153 return true;
105   } 105   }
HITCBC 106   651 return false; 106   328 return false;
HITCBC 107   19981 } 107   20481 }
108   108  
109   static void 109   static void
HITCBC 110   19981 set_dispatch_thread(strand_impl& impl) noexcept 110   20481 set_dispatch_thread(strand_impl& impl) noexcept
111   { 111   {
HITCBC 112   19981 impl.dispatch_thread_.store(std::this_thread::get_id()); 112   20481 impl.dispatch_thread_.store(std::this_thread::get_id());
HITCBC 113   19981 } 113   20481 }
114   114  
115   static void 115   static void
HITCBC 116   19330 clear_dispatch_thread(strand_impl& impl) noexcept 116   20153 clear_dispatch_thread(strand_impl& impl) noexcept
117   { 117   {
HITCBC 118   19330 impl.dispatch_thread_.store(std::thread::id{}); 118   20153 impl.dispatch_thread_.store(std::thread::id{});
HITCBC 119   19330 } 119   20153 }
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   30 shutdown() override 127   30 shutdown() override
128   { 128   {
HITCBC 129   30 std::lock_guard<std::mutex> lock(mutex_); 129   30 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 130   30 while(auto* p = impl_list_.pop_front()) 130   30 while(auto* p = impl_list_.pop_front())
131   { 131   {
MISUBC 132   ✗ std::lock_guard<std::mutex> impl_lock(*p->mutex_); 132   ✗ std::lock_guard<std::mutex> impl_lock(*p->mutex_);
MISUBC 133   ✗ p->locked_ = true; 133   ✗ p->locked_ = true;
MISUBC 134   ✗ p->service_.store(nullptr, std::memory_order_release); 134   ✗ p->service_.store(nullptr, std::memory_order_release);
MISUBC 135   ✗ } 135   ✗ }
136   136  
HITCBC 137   30 void* fp = invoker_frame_cache_.exchange( 137   30 void* fp = invoker_frame_cache_.exchange(
138   kCacheClosed, std::memory_order_acq_rel); 138   kCacheClosed, std::memory_order_acq_rel);
HITCBC 139   30 if(fp) ::operator delete(fp); 139   30 if(fp) ::operator delete(fp);
HITCBC 140   30 } 140   30 }
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   19330 operator new( 164   20153 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   19330 auto* svc = impl->service_.load(std::memory_order_acquire); 168   20153 auto* svc = impl->service_.load(std::memory_order_acquire);
HITCBC 169   19330 constexpr auto A = alignof(strand_service_impl*); 169   20153 constexpr auto A = alignof(strand_service_impl*);
HITCBC 170   19330 std::size_t padded = (n + A - 1) & ~(A - 1); 170   20153 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 171   19330 std::size_t total = padded + sizeof(strand_service_impl*); 171   20153 std::size_t total = padded + sizeof(strand_service_impl*);
172   172  
HITCBC 173   19330 void* p = svc->invoker_frame_cache_.exchange( 173   20153 void* p = svc->invoker_frame_cache_.exchange(
174   nullptr, std::memory_order_acquire); 174   nullptr, std::memory_order_acquire);
HITCBC 175   19330 if(!p || p == kCacheClosed) 175   20153 if(!p || p == kCacheClosed)
HITCBC 176   7168 p = ::operator new(total); 176   6974 p = ::operator new(total);
177   177  
HITCBC 178   19330 *reinterpret_cast<strand_service_impl**>( 178   20153 *reinterpret_cast<strand_service_impl**>(
HITCBC 179   19330 static_cast<char*>(p) + padded) = svc; 179   20153 static_cast<char*>(p) + padded) = svc;
HITCBC 180   19330 return p; 180   20153 return p;
181   } 181   }
182   182  
183   void 183   void
HITCBC 184   19330 operator delete(void* p, std::size_t n) noexcept 184   20153 operator delete(void* p, std::size_t n) noexcept
185   { 185   {
HITCBC 186   19330 constexpr auto A = alignof(strand_service_impl*); 186   20153 constexpr auto A = alignof(strand_service_impl*);
HITCBC 187   19330 std::size_t padded = (n + A - 1) & ~(A - 1); 187   20153 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 188   19330 auto* svc = *reinterpret_cast<strand_service_impl**>( 188   20153 auto* svc = *reinterpret_cast<strand_service_impl**>(
189   static_cast<char*>(p) + padded); 189   static_cast<char*>(p) + padded);
190   190  
HITCBC 191   19330 void* expected = nullptr; 191   20153 void* expected = nullptr;
HITCBC 192   19330 if(!svc->invoker_frame_cache_.compare_exchange_strong( 192   20153 if(!svc->invoker_frame_cache_.compare_exchange_strong(
193   expected, p, std::memory_order_release)) 193   expected, p, std::memory_order_release))
HITCBC 194   7151 ::operator delete(p); 194   6957 ::operator delete(p);
HITCBC 195   19330 } 195   20153 }
196   196  
197   strand_invoker 197   strand_invoker
HITCBC 198   19330 get_return_object() noexcept 198   20153 get_return_object() noexcept
199   { 199   {
HITCBC 200   19330 return {std::coroutine_handle<promise_type>::from_promise(*this)}; 200   20153 return {std::coroutine_handle<promise_type>::from_promise(*this)};
201   } 201   }
202   202  
HITCBC 203   19330 std::suspend_always initial_suspend() noexcept { return {}; } 203   20153 std::suspend_always initial_suspend() noexcept { return {}; }
HITCBC 204   19330 std::suspend_never final_suspend() noexcept { return {}; } 204   20153 std::suspend_never final_suspend() noexcept { return {}; }
HITCBC 205   19330 void return_void() noexcept {} 205   20153 void return_void() noexcept {}
MISUBC 206   ✗ void unhandled_exception() { std::terminate(); } 206   ✗ void unhandled_exception() { std::terminate(); }
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   19330 make_invoker(std::shared_ptr<strand_impl> impl) 217   20153 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   38660 } 230   40306 }
231   231  
232   void 232   void
HITCBC 233   19330 strand_service_impl::post_invoker( 233   20153 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   19330 auto invoker = make_invoker(std::move(impl)); 237   20153 auto invoker = make_invoker(std::move(impl));
HITCBC 238   19330 auto& self = invoker.h_.promise().self_; 238   20153 auto& self = invoker.h_.promise().self_;
HITCBC 239   19330 self.h = invoker.h_; 239   20153 self.h = invoker.h_;
HITCBC 240   19330 ex.post(self); 240   20153 ex.post(self);
HITCBC 241   19330 } 241   20153 }
242   242  
HITCBC 243   22884 strand_impl::~strand_impl() 243   22884 strand_impl::~strand_impl()
244   { 244   {
HITCBC 245   11442 auto* svc = service_.load(std::memory_order_acquire); 245   11442 auto* svc = service_.load(std::memory_order_acquire);
HITCBC 246   11442 if(!svc) return; 246   11442 if(!svc) return;
HITCBC 247   11442 std::lock_guard<std::mutex> lock(svc->mutex_); 247   11442 std::lock_guard<std::mutex> lock(svc->mutex_);
HITCBC 248   11442 svc->impl_list_.remove(this); 248   11442 svc->impl_list_.remove(this);
HITCBC 249   11442 } 249   11442 }
250   250  
HITCBC 251   30 strand_service:: 251   30 strand_service::
HITCBC 252   30 strand_service() 252   30 strand_service()
HITCBC 253   30 : service() 253   30 : service()
254   { 254   {
HITCBC 255   30 } 255   30 }
256   256  
HITCBC 257   30 strand_service:: 257   30 strand_service::
258   ~strand_service() = default; 258   ~strand_service() = default;
259   259  
260   bool 260   bool
HITCBC 261   12 strand_service:: 261   12 strand_service::
262   running_in_this_thread(strand_impl& impl) noexcept 262   running_in_this_thread(strand_impl& impl) noexcept
263   { 263   {
HITCBC 264   12 return impl.dispatch_thread_.load() == std::this_thread::get_id(); 264   12 return impl.dispatch_thread_.load() == std::this_thread::get_id();
265   } 265   }
266   266  
267   std::coroutine_handle<> 267   std::coroutine_handle<>
HITCBC 268   8 strand_service:: 268   8 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   8 if(running_in_this_thread(*impl)) 274   8 if(running_in_this_thread(*impl))
HITCBC 275   3 return c.h; 275   3 return c.h;
276   276  
HITCBC 277   5 if(strand_service_impl::enqueue(*impl, c)) 277   5 if(strand_service_impl::enqueue(*impl, c))
HITCBC 278   5 strand_service_impl::post_invoker(impl, ex); 278   5 strand_service_impl::post_invoker(impl, ex);
HITCBC 279   5 return std::noop_coroutine(); 279   5 return std::noop_coroutine();
280   } 280   }
281   281  
282   void 282   void
HITCBC 283   30335 strand_service:: 283   30335 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   30335 if(strand_service_impl::enqueue(*impl, c)) 289   30335 if(strand_service_impl::enqueue(*impl, c))
HITCBC 290   19325 strand_service_impl::post_invoker(impl, ex); 290   20148 strand_service_impl::post_invoker(impl, ex);
HITCBC 291   30335 } 291   30335 }
292   292  
293   strand_service& 293   strand_service&
HITCBC 294   11442 get_strand_service(execution_context& ctx) 294   11442 get_strand_service(execution_context& ctx)
295   { 295   {
HITCBC 296   11442 return ctx.use_service<strand_service_impl>(); 296   11442 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