Initial Commit

include/asio/detail/impl/strand_service.ipp

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+//
+// detail/impl/strand_service.ipp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2025 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_IMPL_STRAND_SERVICE_IPP
+#define ASIO_DETAIL_IMPL_STRAND_SERVICE_IPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include "asio/detail/call_stack.hpp"
+#include "asio/detail/strand_service.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+struct strand_service::on_do_complete_exit
+{
+  io_context_impl* owner_;
+  strand_impl* impl_;
+
+  ~on_do_complete_exit()
+  {
+    impl_->mutex_.lock();
+    impl_->ready_queue_.push(impl_->waiting_queue_);
+    bool more_handlers = impl_->locked_ = !impl_->ready_queue_.empty();
+    impl_->mutex_.unlock();
+
+    if (more_handlers)
+      owner_->post_immediate_completion(impl_, true);
+  }
+};
+
+strand_service::strand_service(asio::io_context& io_context)
+  : asio::detail::service_base<strand_service>(io_context),
+    io_context_(io_context),
+    io_context_impl_(asio::use_service<io_context_impl>(io_context)),
+    mutex_(),
+    salt_(0)
+{
+}
+
+void strand_service::shutdown()
+{
+  op_queue<operation> ops;
+
+  asio::detail::mutex::scoped_lock lock(mutex_);
+
+  for (std::size_t i = 0; i < num_implementations; ++i)
+  {
+    if (strand_impl* impl = implementations_[i].get())
+    {
+      ops.push(impl->waiting_queue_);
+      ops.push(impl->ready_queue_);
+    }
+  }
+}
+
+void strand_service::construct(strand_service::implementation_type& impl)
+{
+  asio::detail::mutex::scoped_lock lock(mutex_);
+
+  std::size_t salt = salt_++;
+#if defined(ASIO_ENABLE_SEQUENTIAL_STRAND_ALLOCATION)
+  std::size_t index = salt;
+#else // defined(ASIO_ENABLE_SEQUENTIAL_STRAND_ALLOCATION)
+  std::size_t index = reinterpret_cast<std::size_t>(&impl);
+  index += (reinterpret_cast<std::size_t>(&impl) >> 3);
+  index ^= salt + 0x9e3779b9 + (index << 6) + (index >> 2);
+#endif // defined(ASIO_ENABLE_SEQUENTIAL_STRAND_ALLOCATION)
+  index = index % num_implementations;
+
+  if (!implementations_[index])
+  {
+    execution_context::allocator<void> alloc(context());
+    implementations_[index] = allocate_shared<strand_impl>(alloc);
+  }
+  impl = implementations_[index].get();
+}
+
+bool strand_service::running_in_this_thread(
+    const implementation_type& impl) const
+{
+  return call_stack<strand_impl>::contains(impl) != 0;
+}
+
+struct strand_service::on_dispatch_exit
+{
+  io_context_impl* io_context_impl_;
+  strand_impl* impl_;
+
+  ~on_dispatch_exit()
+  {
+    impl_->mutex_.lock();
+    impl_->ready_queue_.push(impl_->waiting_queue_);
+    bool more_handlers = impl_->locked_ = !impl_->ready_queue_.empty();
+    impl_->mutex_.unlock();
+
+    if (more_handlers)
+      io_context_impl_->post_immediate_completion(impl_, false);
+  }
+};
+
+void strand_service::do_dispatch(implementation_type& impl, operation* op)
+{
+  // If we are running inside the io_context, and no other handler already
+  // holds the strand lock, then the handler can run immediately.
+  bool can_dispatch = io_context_impl_.can_dispatch();
+  impl->mutex_.lock();
+  if (can_dispatch && !impl->locked_)
+  {
+    // Immediate invocation is allowed.
+    impl->locked_ = true;
+    impl->mutex_.unlock();
+
+    // Indicate that this strand is executing on the current thread.
+    call_stack<strand_impl>::context ctx(impl);
+
+    // Ensure the next handler, if any, is scheduled on block exit.
+    on_dispatch_exit on_exit = { &io_context_impl_, impl };
+    (void)on_exit;
+
+    op->complete(&io_context_impl_, asio::error_code(), 0);
+    return;
+  }
+
+  if (impl->locked_)
+  {
+    // Some other handler already holds the strand lock. Enqueue for later.
+    impl->waiting_queue_.push(op);
+    impl->mutex_.unlock();
+  }
+  else
+  {
+    // The handler is acquiring the strand lock and so is responsible for
+    // scheduling the strand.
+    impl->locked_ = true;
+    impl->mutex_.unlock();
+    impl->ready_queue_.push(op);
+    io_context_impl_.post_immediate_completion(impl, false);
+  }
+}
+
+void strand_service::do_post(implementation_type& impl,
+    operation* op, bool is_continuation)
+{
+  impl->mutex_.lock();
+  if (impl->locked_)
+  {
+    // Some other handler already holds the strand lock. Enqueue for later.
+    impl->waiting_queue_.push(op);
+    impl->mutex_.unlock();
+  }
+  else
+  {
+    // The handler is acquiring the strand lock and so is responsible for
+    // scheduling the strand.
+    impl->locked_ = true;
+    impl->mutex_.unlock();
+    impl->ready_queue_.push(op);
+    io_context_impl_.post_immediate_completion(impl, is_continuation);
+  }
+}
+
+void strand_service::do_complete(void* owner, operation* base,
+    const asio::error_code& ec, std::size_t /*bytes_transferred*/)
+{
+  if (owner)
+  {
+    strand_impl* impl = static_cast<strand_impl*>(base);
+
+    // Indicate that this strand is executing on the current thread.
+    call_stack<strand_impl>::context ctx(impl);
+
+    // Ensure the next handler, if any, is scheduled on block exit.
+    on_do_complete_exit on_exit;
+    on_exit.owner_ = static_cast<io_context_impl*>(owner);
+    on_exit.impl_ = impl;
+
+    // Run all ready handlers. No lock is required since the ready queue is
+    // accessed only within the strand.
+    while (operation* o = impl->ready_queue_.front())
+    {
+      impl->ready_queue_.pop();
+      o->complete(owner, ec, 0);
+    }
+  }
+}
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_DETAIL_IMPL_STRAND_SERVICE_IPP