. /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   671 mutexes_[idx] = std::make_shared<std::mutex>(); 64   671 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   18094 impl.locked_ = true; 80   16526 impl.locked_ = true;
HITCBC 81   18094 return true; 81   16526 return true;
82   } 82   }
HITCBC 83   12248 return false; 83   13816 return false;
HITCBC 84   30342 } 84   30342 }
85   85  
86   static void 86   static void
HITCBC 87   18324 dispatch_pending(strand_impl& impl) 87   16656 dispatch_pending(strand_impl& impl)
88   { 88   {
HITCBC 89   18324 strand_queue::taken_batch batch; 89   16656 strand_queue::taken_batch batch;
90   { 90   {
HITCBC 91   18324 std::lock_guard<std::mutex> lock(*impl.mutex_); 91   16656 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 92   18324 batch = impl.pending_.take_all(); 92   16656 batch = impl.pending_.take_all();
HITCBC 93   18324 } 93   16656 }
HITCBC 94   18324 impl.pending_.dispatch_batch(batch); 94   16656 impl.pending_.dispatch_batch(batch);
HITCBC 95   18324 } 95   16656 }
96   96  
97   static bool 97   static bool
HITCBC 98   18324 try_unlock(strand_impl& impl) 98   16656 try_unlock(strand_impl& impl)
99   { 99   {
HITCBC 100   18324 std::lock_guard<std::mutex> lock(*impl.mutex_); 100   16656 std::lock_guard<std::mutex> lock(*impl.mutex_);
HITCBC 101   18324 if(impl.pending_.empty()) 101   16656 if(impl.pending_.empty())
102   { 102   {
HITCBC 103   18094 impl.locked_ = false; 103   16526 impl.locked_ = false;
HITCBC 104   18094 return true; 104   16526 return true;
105   } 105   }
HITCBC 106   230 return false; 106   130 return false;
HITCBC 107   18324 } 107   16656 }
108   108  
109   static void 109   static void
HITCBC 110   18324 set_dispatch_thread(strand_impl& impl) noexcept 110   16656 set_dispatch_thread(strand_impl& impl) noexcept
111   { 111   {
HITCBC 112   18324 impl.dispatch_thread_.store(std::this_thread::get_id()); 112   16656 impl.dispatch_thread_.store(std::this_thread::get_id());
HITCBC 113   18324 } 113   16656 }
114   114  
115   static void 115   static void
HITCBC 116   18094 clear_dispatch_thread(strand_impl& impl) noexcept 116   16526 clear_dispatch_thread(strand_impl& impl) noexcept
117   { 117   {
HITCBC 118   18094 impl.dispatch_thread_.store(std::thread::id{}); 118   16526 impl.dispatch_thread_.store(std::thread::id{});
HITCBC 119   18094 } 119   16526 }
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   18094 operator new( 164   16526 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   18094 auto* svc = impl->service_.load(std::memory_order_acquire); 168   16526 auto* svc = impl->service_.load(std::memory_order_acquire);
HITCBC 169   18094 constexpr auto A = alignof(strand_service_impl*); 169   16526 constexpr auto A = alignof(strand_service_impl*);
HITCBC 170   18094 std::size_t padded = (n + A - 1) & ~(A - 1); 170   16526 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 171   18094 std::size_t total = padded + sizeof(strand_service_impl*); 171   16526 std::size_t total = padded + sizeof(strand_service_impl*);
172   172  
HITCBC 173   18094 void* p = svc->invoker_frame_cache_.exchange( 173   16526 void* p = svc->invoker_frame_cache_.exchange(
174   nullptr, std::memory_order_acquire); 174   nullptr, std::memory_order_acquire);
HITCBC 175   18094 if(!p || p == kCacheClosed) 175   16526 if(!p || p == kCacheClosed)
HITCBC 176   6046 p = ::operator new(total); 176   6783 p = ::operator new(total);
177   177  
HITCBC 178   18094 *reinterpret_cast<strand_service_impl**>( 178   16526 *reinterpret_cast<strand_service_impl**>(
HITCBC 179   18094 static_cast<char*>(p) + padded) = svc; 179   16526 static_cast<char*>(p) + padded) = svc;
HITCBC 180   18094 return p; 180   16526 return p;
181   } 181   }
182   182  
183   void 183   void
HITCBC 184   18094 operator delete(void* p, std::size_t n) noexcept 184   16526 operator delete(void* p, std::size_t n) noexcept
185   { 185   {
HITCBC 186   18094 constexpr auto A = alignof(strand_service_impl*); 186   16526 constexpr auto A = alignof(strand_service_impl*);
HITCBC 187   18094 std::size_t padded = (n + A - 1) & ~(A - 1); 187   16526 std::size_t padded = (n + A - 1) & ~(A - 1);
HITCBC 188   18094 auto* svc = *reinterpret_cast<strand_service_impl**>( 188   16526 auto* svc = *reinterpret_cast<strand_service_impl**>(
189   static_cast<char*>(p) + padded); 189   static_cast<char*>(p) + padded);
190   190  
HITCBC 191   18094 void* expected = nullptr; 191   16526 void* expected = nullptr;
HITCBC 192   18094 if(!svc->invoker_frame_cache_.compare_exchange_strong( 192   16526 if(!svc->invoker_frame_cache_.compare_exchange_strong(
193   expected, p, std::memory_order_release)) 193   expected, p, std::memory_order_release))
HITCBC 194   6027 ::operator delete(p); 194   6764 ::operator delete(p);
HITCBC 195   18094 } 195   16526 }
196   196  
197   strand_invoker 197   strand_invoker
HITCBC 198   18094 get_return_object() noexcept 198   16526 get_return_object() noexcept
199   { 199   {
HITCBC 200   18094 return {std::coroutine_handle<promise_type>::from_promise(*this)}; 200   16526 return {std::coroutine_handle<promise_type>::from_promise(*this)};
201   } 201   }
202   202  
HITCBC 203   18094 std::suspend_always initial_suspend() noexcept { return {}; } 203   16526 std::suspend_always initial_suspend() noexcept { return {}; }
HITCBC 204   18094 std::suspend_never final_suspend() noexcept { return {}; } 204   16526 std::suspend_never final_suspend() noexcept { return {}; }
HITCBC 205   18094 void return_void() noexcept {} 205   16526 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   18094 make_invoker(std::shared_ptr<strand_impl> impl) 217   16526 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   36188 } 230   33052 }
231   231  
232   void 232   void
HITCBC 233   18094 strand_service_impl::post_invoker( 233   16526 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   18094 auto invoker = make_invoker(std::move(impl)); 237   16526 auto invoker = make_invoker(std::move(impl));
HITCBC 238   18094 auto& self = invoker.h_.promise().self_; 238   16526 auto& self = invoker.h_.promise().self_;
HITCBC 239   18094 self.h = invoker.h_; 239   16526 self.h = invoker.h_;
HITCBC 240   18094 ex.post(self); 240   16526 ex.post(self);
HITCBC 241   18094 } 241   16526 }
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   18088 strand_service_impl::post_invoker(impl, ex); 290   16520 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