dispatch returns void and resumes inline if possible

Author: vinniefalco

include/boost/corosio/basic_io_context.hpp

@@ -342,21 +342,18 @@ class basic_io_context::executor_type
     /** Dispatch a coroutine handle.
 
         This is the executor interface for capy coroutines. If called
-        from within `run()`, returns the handle for symmetric transfer.
-        Otherwise posts the handle and returns `noop_coroutine`.
+        from within `run()`, resumes the coroutine inline via a normal
+        function call. Otherwise posts the coroutine for later execution.
 
         @param h The coroutine handle to dispatch.
-
-        @return The handle for symmetric transfer, or `noop_coroutine`
-            if the handle was posted.
     */
-    capy::coro
+    void
     dispatch(capy::coro h) const
     {
         if (running_in_this_thread())
-            return h;
-        ctx_->sched_->post(h);
-        return std::noop_coroutine();
+            h.resume();
+        else
+            ctx_->sched_->post(h);
     }
 
     /** Post a coroutine for deferred execution.

include/boost/corosio/tcp_server.hpp

@@ -354,7 +354,8 @@ class BOOST_COROSIO_DECL
             Ex const&, std::stop_token) noexcept
         {
             // Dispatch to server's executor before touching shared state
-            return self_.ex_.dispatch(h);
+            self_.ex_.dispatch(h);
+            return std::noop_coroutine();
         }
 
         void await_resume() noexcept

src/corosio/src/detail/epoll/acceptors.cpp

@@ -128,7 +128,7 @@ operator()()
     capy::executor_ref saved_ex( std::move( ex ) );
     capy::coro saved_h( std::move( h ) );
     auto prevent_premature_destruction = std::move(impl_ptr);
-    saved_ex.dispatch( saved_h ).resume();
+    saved_ex.dispatch( saved_h );
 }
 
 epoll_acceptor_impl::

src/corosio/src/detail/iocp/overlapped_op.hpp

@@ -148,13 +148,13 @@ struct overlapped_op
         dwError = err;
     }
 
-    /** Complete immediately and return handle for symmetric transfer.
+    /** Complete immediately via dispatch.
 
         Use this for immediate completion paths instead of posting to
-        the scheduler. Sets output parameters and returns the coroutine
-        handle to resume via symmetric transfer.
+        the scheduler. Sets output parameters and dispatches the coroutine
+        for resumption.
     */
-    std::coroutine_handle<> complete_immediate()
+    void complete_immediate()
     {
         stop_cb.reset();
 
@@ -171,7 +171,7 @@ struct overlapped_op
         if (bytes_out)
             *bytes_out = static_cast<std::size_t>(bytes_transferred);
 
-        return d.dispatch(h);
+        d.dispatch(h);
     }
 };
 

src/corosio/src/detail/iocp/signals.cpp

@@ -157,7 +157,7 @@ operator()()
     auto* service = svc;
     svc = nullptr;
 
-    d.dispatch(h).resume();
+    d.dispatch(h);
 
     // Balance the on_work_started() from start_wait. When svc is null
     // (immediate completion from queued signal), no work tracking occurred.
@@ -216,7 +216,7 @@ wait(
             *ec = make_error_code(capy::error::canceled);
         if (signal_out)
             *signal_out = 0;
-        d.dispatch(h).resume();
+        d.dispatch(h);
         return;
     }
 
@@ -495,7 +495,7 @@ cancel_wait(win_signal_impl& impl)
             *op->ec_out = make_error_code(capy::error::canceled);
         if (op->signal_out)
             *op->signal_out = 0;
-        op->d.dispatch(op->h).resume();
+        op->d.dispatch(op->h);
         sched_.on_work_finished();
     }
 }

src/corosio/src/detail/iocp/sockets.cpp

@@ -528,16 +528,10 @@ read_some(
         DWORD err = ::WSAGetLastError();
         if (err != WSA_IO_PENDING)
         {
-            // Immediate error - must use post(), not complete_immediate().
-            // Using symmetric transfer (complete_immediate) here breaks
-            // coroutine chains that hold async mutexes: the resumed coroutine
-            // releases its lock and tries to wake the next waiter, but the
-            // symmetric transfer chain doesn't return control to io_context
-            // properly. Posting ensures the scheduler processes the completion,
-            // calling resume_coro() which explicitly calls .resume().
+            // Immediate error - dispatch inline
             svc_.work_finished();
             op.dwError = err;
-            svc_.post(&op);
+            op.complete_immediate();
             return;
         }
     }

src/corosio/src/detail/resume_coro.hpp

@@ -22,10 +22,6 @@ namespace boost::corosio::detail {
     error codes, bytes transferred) written by other threads are
     visible to the resumed coroutine before it continues execution.
 
-    Uses symmetric transfer: if dispatch returns the same handle,
-    we resume directly. If it returns noop_coroutine, the work was
-    posted to a queue and will be resumed by the scheduler.
-
     @param d The executor to dispatch through.
     @param h The coroutine handle to resume.
 */
@@ -35,9 +31,7 @@ resume_coro(capy::executor_ref d, capy::coro h)
     // I/O results may have been written by another thread (OS or worker).
     // Acquire fence ensures those writes are visible before coroutine resumes.
     std::atomic_thread_fence(std::memory_order_acquire);
-    auto resume_h = d.dispatch(h);
-    if (resume_h.address() == h.address())
-        resume_h.resume();
+    d.dispatch(h);
 }
 
 } // namespace boost::corosio::detail

src/corosio/src/detail/select/acceptors.cpp

@@ -121,7 +121,7 @@ operator()()
     capy::executor_ref saved_ex( std::move( ex ) );
     capy::coro saved_h( std::move( h ) );
     impl_ptr.reset();
-    saved_ex.dispatch( saved_h ).resume();
+    saved_ex.dispatch( saved_h );
 }
 
 select_acceptor_impl::

src/corosio/src/detail/select/op.hpp

@@ -174,7 +174,7 @@ struct select_op : scheduler_op
         capy::executor_ref saved_ex( std::move( ex ) );
         capy::coro saved_h( std::move( h ) );
         impl_ptr.reset();
-        saved_ex.dispatch( saved_h ).resume();
+        saved_ex.dispatch( saved_h );
     }
 
     virtual bool is_read_operation() const noexcept { return false; }

src/corosio/src/detail/select/sockets.cpp

@@ -101,7 +101,7 @@ operator()()
     capy::executor_ref saved_ex( std::move( ex ) );
     capy::coro saved_h( std::move( h ) );
     impl_ptr.reset();
-    saved_ex.dispatch( saved_h ).resume();
+    saved_ex.dispatch( saved_h );
 }
 
 select_socket_impl::