. /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   31 strand_service_impl(execution_context&) 47   31 strand_service_impl(execution_context&)
HITCBC 48   31 { 48   31 {
HITCBC 49   31 } 49   31 }
50   50  
51   std::shared_ptr<strand_impl> 51   std::shared_ptr<strand_impl>
HITCBC 52   11443 create_implementation() override 52   11443 create_implementation() override
53   { 53   {
HITCBC 54   11443 auto new_impl = std::make_shared<strand_impl>(); 54   11443 auto new_impl = std::make_shared<strand_impl>();
55   55  
HITCBC 56   11443 std::lock_guard<std::mutex> lock(mutex_); 56   11443 std::lock_guard<std::mutex> lock(mutex_);
57   57  
HITCBC 58   11443 std::size_t s = salt_++; 58   11443 std::size_t s = salt_++;
HITCBC 59   11443 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get()); 59   11443 std::size_t idx = reinterpret_cast<std::size_t>(new_impl.get());
HITCBC 60   11443 idx += idx >> 3; 60   11443 idx += idx >> 3;
HITCBC 61   11443 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2); 61   11443 idx ^= s + 0x9e3779b9 + (idx << 6) + (idx >> 2);
HITCBC 62   11443 idx %= num_mutexes; 62   11443 idx %= num_mutexes;
HITCBC 63   11443 if(!mutexes_[idx]) 63   11443 if(!mutexes_[idx])
HITCBC 64   668 mutexes_[idx] = std::make_shared<std::mutex>(); 64   678 mutexes_[idx] = std::make_shared<std::mutex>();
HITCBC 65   11443 new_impl->mutex_ = mutexes_[idx].get(); 65   11443 new_impl->mutex_ = mutexes_[idx].get();
66   66  
HITCBC 67   11443 impl_list_.push_back(new_impl.get()); 67   11443 impl_list_.push_back(new_impl.get());
HITCBC 68   11443 new_impl->service_.store(this, std::memory_order_release); 68   11443 new_impl->service_.store(this, std::memory_order_release);
69   69  
HITCBC 70   22886 return new_impl; 70   22886 return new_impl;
HITCBC 71   11443 } 71   11443 }
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   19903 impl.locked_ = true; 80   18360 impl.locked_ = true;
HITCBC 81   19903 return true; 81   18360 return true;
82   } 82   }
HITCBC 83   10437 return false; 83   11980 return false;
HITCBC 84   30340 } 84   30340 }
85   85  
86   static void 86   static void
HITCBC 87   20134 dispatch_pending(strand_impl& impl) 87   18596 dispatch_pending(strand_impl& impl)
88   { 88   {
HITCBC 89   20134 strand_queue::taken_batch batch; 89   18596 strand_queue::taken_batch batch;
90   { 90   {
HITCBC 91   20134 std::lock_guard<std::mutex> lock(*impl.mutex_); 91   18596 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 92   20134 batch = impl.pending_.take_all(); 92   18596 batch = impl.pending_.take_all();
HITCBC 93   20134 } 93   18596 }
HITCBC 94   20134 impl.pending_.dispatch_batch(batch); 94   18596 impl.pending_.dispatch_batch(batch);
HITCBC 95   20134 } 95   18596 }
96   96  
97   static bool 97   static bool
HITCBC 98   20134 try_unlock(strand_impl& impl) 98   18596 try_unlock(strand_impl& impl)
99   { 99   {
HITCBC 100   20134 std::lock_guard<std::mutex> lock(*impl.mutex_); 100   18596 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 101   20134 if(impl.pending_.empty()) 101   18596 if(impl.pending_.empty())
102   { 102   {
HITCBC 103   19903 impl.locked_ = false; 103   18360 impl.locked_ = false;
HITCBC 104   19903 return true; 104   18360 return true;
105   } 105   }
HITCBC 106   231 return false; 106   236 return false;
HITCBC 107   20134 } 107   18596 }
108   108  
109   static void 109   static void
HITCBC 110   20134 set_dispatch_thread(strand_impl& impl) noexcept 110   18596 set_dispatch_thread(strand_impl& impl) noexcept
111   { 111   {
HITCBC 112   20134 impl.dispatch_thread_.store(std::this_thread::get_id()); 112   18596 impl.dispatch_thread_.store(std::this_thread::get_id());
HITCBC 113   20134 } 113   18596 }
114   114  
115   static void 115   static void
HITCBC 116   19903 clear_dispatch_thread(strand_impl& impl) noexcept 116   18360 clear_dispatch_thread(strand_impl& impl) noexcept
117   { 117   {
HITCBC 118   19903 impl.dispatch_thread_.store(std::thread::id{}); 118   18360 impl.dispatch_thread_.store(std::thread::id{});
HITCBC 119   19903 } 119   18360 }
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   31 shutdown() override 127   31 shutdown() override
128   { 128   {
HITCBC 129   31 std::lock_guard<std::mutex> lock(mutex_); 129   31 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 130   32 while(auto* p = impl_list_.pop_front()) 130   32 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   31 void* fp = invoker_frame_cache_.exchange( 137   31 void* fp = invoker_frame_cache_.exchange(
138   kCacheClosed, std::memory_order_acq_rel); 138   kCacheClosed, std::memory_order_acq_rel);
HITCBC 139   31 if(fp) ::operator delete(fp); 139   31 if(fp) ::operator delete(fp);
HITCBC 140   31 } 140   31 }
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   19903 operator new( 164   18360 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   19903 auto* svc = impl->service_.load(std::memory_order_acquire); 168   18360 auto* svc = impl->service_.load(std::memory_order_acquire);
HITCBC 169   19903 constexpr auto A = alignof(strand_service_impl*); 169   18360 constexpr auto A = alignof(strand_service_impl*);
HITCBC 170   19903 std::size_t padded = (n + A - 1) & ~(A - 1); 170   18360 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 171   19903 std::size_t total = padded + sizeof(strand_service_impl*); 171   18360 std::size_t total = padded + sizeof(strand_service_impl*);
172   172  
HITCBC 173   19903 void* p = svc->invoker_frame_cache_.exchange( 173   18360 void* p = svc->invoker_frame_cache_.exchange(
174   nullptr, std::memory_order_acquire); 174   nullptr, std::memory_order_acquire);
HITCBC 175   19903 if(!p || p == kCacheClosed) 175   18360 if(!p || p == kCacheClosed)
HITCBC 176   9025 p = ::operator new(total); 176   7841 p = ::operator new(total);
177   177  
HITCBC 178   19903 *reinterpret_cast<strand_service_impl**>( 178   18360 *reinterpret_cast<strand_service_impl**>(
HITCBC 179   19903 static_cast<char*>(p) + padded) = svc; 179   18360 static_cast<char*>(p) + padded) = svc;
HITCBC 180   19903 return p; 180   18360 return p;
181   } 181   }
182   182  
183   void 183   void
HITCBC 184   19903 operator delete(void* p, std::size_t n) noexcept 184   18360 operator delete(void* p, std::size_t n) noexcept
185   { 185   {
HITCBC 186   19903 constexpr auto A = alignof(strand_service_impl*); 186   18360 constexpr auto A = alignof(strand_service_impl*);
HITCBC 187   19903 std::size_t padded = (n + A - 1) & ~(A - 1); 187   18360 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 188   19903 auto* svc = *reinterpret_cast<strand_service_impl**>( 188   18360 auto* svc = *reinterpret_cast<strand_service_impl**>(
189   static_cast<char*>(p) + padded); 189   static_cast<char*>(p) + padded);
190   190  
HITCBC 191   19903 void* expected = nullptr; 191   18360 void* expected = nullptr;
HITCBC 192   19903 if(!svc->invoker_frame_cache_.compare_exchange_strong( 192   18360 if(!svc->invoker_frame_cache_.compare_exchange_strong(
193   expected, p, std::memory_order_release)) 193   expected, p, std::memory_order_release))
HITCBC 194   9008 ::operator delete(p); 194   7824 ::operator delete(p);
HITCBC 195   19903 } 195   18360 }
196   196  
197   strand_invoker 197   strand_invoker
HITCBC 198   19903 get_return_object() noexcept 198   18360 get_return_object() noexcept
199   { 199   {
HITCBC 200   19903 return {std::coroutine_handle<promise_type>::from_promise(*this)}; 200   18360 return {std::coroutine_handle<promise_type>::from_promise(*this)};
201   } 201   }
202   202  
HITCBC 203   19903 std::suspend_always initial_suspend() noexcept { return {}; } 203   18360 std::suspend_always initial_suspend() noexcept { return {}; }
HITCBC 204   19903 std::suspend_never final_suspend() noexcept { return {}; } 204   18360 std::suspend_never final_suspend() noexcept { return {}; }
HITCBC 205   19903 void return_void() noexcept {} 205   18360 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   19903 make_invoker(std::shared_ptr<strand_impl> impl) 217   18360 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   39806 } 230   36720 }
231   231  
232   void 232   void
HITCBC 233   19903 strand_service_impl::post_invoker( 233   18360 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   19903 auto invoker = make_invoker(std::move(impl)); 237   18360 auto invoker = make_invoker(std::move(impl));
HITCBC 238   19903 auto& self = invoker.h_.promise().self_; 238   18360 auto& self = invoker.h_.promise().self_;
HITCBC 239   19903 self.h = invoker.h_; 239   18360 self.h = invoker.h_;
HITCBC 240   19903 ex.post(self); 240   18360 ex.post(self);
HITCBC 241   19903 } 241   18360 }
242   242  
HITCBC 243   22885 strand_impl::~strand_impl() 243   22885 strand_impl::~strand_impl()
244   { 244   {
HITCBC 245   11443 auto* svc = service_.load(std::memory_order_acquire); 245   11443 auto* svc = service_.load(std::memory_order_acquire);
HITCBC 246   11443 if(!svc) return; 246   11443 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   11443 } 249   11443 }
250   250  
HITCBC 251   31 strand_service:: 251   31 strand_service::
HITCBC 252   31 strand_service() 252   31 strand_service()
HITCBC 253   31 : service() 253   31 : service()
254   { 254   {
HITCBC 255   31 } 255   31 }
256   256  
HITCBC 257   31 strand_service:: 257   31 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   19898 strand_service_impl::post_invoker(impl, ex); 290   18355 strand_service_impl::post_invoker(impl, ex);
HITCBC 291   30335 } 291   30335 }
292   292  
293   strand_service& 293   strand_service&
HITCBC 294   11443 get_strand_service(execution_context& ctx) 294   11443 get_strand_service(execution_context& ctx)
295   { 295   {
HITCBC 296   11443 return ctx.use_service<strand_service_impl>(); 296   11443 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