From 85535cec190106111e89a459b87f78322bcf959e Mon Sep 17 00:00:00 2001
From: Christopher Kohlhoff <chris@kohlhoff.com>
Date: Wed, 26 Jun 2024 22:43:11 +1000
Subject: [PATCH] Allow bind_executor to be used as a partial completion token.

---
 include/boost/asio/bind_executor.hpp | 104 +++++++++++++++++++++++++--
 test/bind_executor.cpp               |  48 +++++++++----
 2 files changed, 136 insertions(+), 16 deletions(-)

diff --git a/include/boost/asio/bind_executor.hpp b/include/boost/asio/bind_executor.hpp
index b38d2c89..fbfcf81f 100644
--- a/include/boost/asio/bind_executor.hpp
+++ b/include/boost/asio/bind_executor.hpp
@@ -395,6 +395,49 @@ private:
     uses_executor<T, Executor>::value> base_type;
 };
 
+/// A function object type that adapts a @ref completion_token to specify that
+/// the completion handler should have the supplied executor as its associated
+/// executor.
+/**
+ * May also be used directly as a completion token, in which case it adapts the
+ * asynchronous operation's default completion token (or boost::asio::deferred
+ * if no default is available).
+ */
+template <typename Executor>
+struct partial_executor_binder
+{
+  /// Constructor that specifies associated executor.
+  explicit partial_executor_binder(const Executor& ex)
+    : executor_(ex)
+  {
+  }
+
+  /// Adapt a @ref completion_token to specify that the completion handler
+  /// should have the executor as its associated executor.
+  template <typename CompletionToken>
+  BOOST_ASIO_NODISCARD inline
+  constexpr executor_binder<decay_t<CompletionToken>, Executor>
+  operator()(CompletionToken&& completion_token) const
+  {
+    return executor_binder<decay_t<CompletionToken>, Executor>(executor_arg_t(),
+        static_cast<CompletionToken&&>(completion_token), executor_);
+  }
+
+//private:
+  Executor executor_;
+};
+
+/// Create a partial completion token that associates an executor.
+template <typename Executor>
+BOOST_ASIO_NODISCARD inline partial_executor_binder<Executor>
+bind_executor(const Executor& ex,
+    constraint_t<
+      is_executor<Executor>::value || execution::is_executor<Executor>::value
+    > = 0)
+{
+  return partial_executor_binder<Executor>(ex);
+}
+
 /// Associate an object of type @c T with an executor of type @c Executor.
 template <typename Executor, typename T>
 BOOST_ASIO_NODISCARD inline executor_binder<decay_t<T>, Executor>
@@ -407,6 +450,20 @@ bind_executor(const Executor& ex, T&& t,
       executor_arg_t(), ex, static_cast<T&&>(t));
 }
 
+/// Create a partial completion token that associates an execution context's
+/// executor.
+template <typename ExecutionContext>
+BOOST_ASIO_NODISCARD inline partial_executor_binder<
+    typename ExecutionContext::executor_type>
+bind_executor(ExecutionContext& ctx,
+    constraint_t<
+      is_convertible<ExecutionContext&, execution_context&>::value
+    > = 0)
+{
+  return partial_executor_binder<typename ExecutionContext::executor_type>(
+      ctx.get_executor());
+}
+
 /// Associate an object of type @c T with an execution context's executor.
 template <typename ExecutionContext, typename T>
 BOOST_ASIO_NODISCARD inline executor_binder<decay_t<T>,
@@ -527,11 +584,17 @@ public:
   static auto initiate(Initiation&& initiation,
       RawCompletionToken&& token, Args&&... args)
     -> decltype(
-      async_initiate<T, Signature>(
-        declval<init_wrapper<decay_t<Initiation>>>(),
-        token.get(), static_cast<Args&&>(args)...))
+      async_initiate<
+        conditional_t<
+          is_const<remove_reference_t<RawCompletionToken>>::value, const T, T>,
+        Signature>(
+          declval<init_wrapper<decay_t<Initiation>>>(),
+          token.get(), static_cast<Args&&>(args)...))
   {
-    return async_initiate<T, Signature>(
+    return async_initiate<
+      conditional_t<
+        is_const<remove_reference_t<RawCompletionToken>>::value, const T, T>,
+      Signature>(
         init_wrapper<decay_t<Initiation>>(
           token.get_executor(), static_cast<Initiation&&>(initiation)),
         token.get(), static_cast<Args&&>(args)...);
@@ -542,6 +605,39 @@ private:
   async_result& operator=(const async_result&) = delete;
 };
 
+template <typename Executor, typename... Signatures>
+struct async_result<partial_executor_binder<Executor>, Signatures...>
+{
+  template <typename Initiation, typename RawCompletionToken, typename... Args>
+  static auto initiate(Initiation&& initiation,
+      RawCompletionToken&& token, Args&&... args)
+    -> decltype(
+      async_initiate<
+        const executor_binder<
+          default_completion_token_t<associated_executor_t<Initiation>>,
+          Executor>&,
+        Signatures...>(
+          static_cast<Initiation&&>(initiation),
+          executor_binder<
+            default_completion_token_t<associated_executor_t<Initiation>>,
+            Executor>(executor_arg_t(), token.executor_,
+              default_completion_token_t<associated_executor_t<Initiation>>{}),
+          static_cast<Args&&>(args)...))
+  {
+    return async_initiate<
+      const executor_binder<
+        default_completion_token_t<associated_executor_t<Initiation>>,
+        Executor>&,
+      Signatures...>(
+        static_cast<Initiation&&>(initiation),
+        executor_binder<
+          default_completion_token_t<associated_executor_t<Initiation>>,
+          Executor>(executor_arg_t(), token.executor_,
+            default_completion_token_t<associated_executor_t<Initiation>>{}),
+        static_cast<Args&&>(args)...);
+  }
+};
+
 template <template <typename, typename> class Associator,
     typename T, typename Executor, typename DefaultCandidate>
 struct associator<Associator, executor_binder<T, Executor>, DefaultCandidate>
diff --git a/test/bind_executor.cpp b/test/bind_executor.cpp
index e2802633..fc37784f 100644
--- a/test/bind_executor.cpp
+++ b/test/bind_executor.cpp
@@ -21,22 +21,10 @@
 #include <boost/asio/steady_timer.hpp>
 #include "unit_test.hpp"
 
-#if defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
-# include <boost/asio/deadline_timer.hpp>
-#else // defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
-# include <boost/asio/steady_timer.hpp>
-#endif // defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
-
 using namespace boost::asio;
 namespace bindns = std;
-
-#if defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
-typedef deadline_timer timer;
-namespace chronons = boost::posix_time;
-#else // defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
 typedef steady_timer timer;
 namespace chronons = boost::asio::chrono;
-#endif // defined(BOOST_ASIO_HAS_BOOST_DATE_TIME)
 
 void increment(int* count)
 {
@@ -184,10 +172,46 @@ void bind_executor_to_completion_token_v2_test()
   BOOST_ASIO_CHECK(count == 1);
 }
 
+void partial_bind_executor_test()
+{
+  io_context ioc1;
+  io_context ioc2;
+
+  int count = 0;
+
+  timer t(ioc1, chronons::seconds(1));
+  t.async_wait(bind_executor(ioc2.get_executor()))(
+      bindns::bind(&increment, &count));
+
+  ioc1.run();
+
+  BOOST_ASIO_CHECK(count == 0);
+
+  ioc2.run();
+
+  BOOST_ASIO_CHECK(count == 1);
+
+  t.expires_after(chronons::seconds(1));
+  t.async_wait()(
+      bind_executor(ioc2.get_executor()))(
+        incrementer_token_v2(&count));
+
+  ioc1.restart();
+  ioc1.run();
+
+  BOOST_ASIO_CHECK(count == 1);
+
+  ioc2.restart();
+  ioc2.run();
+
+  BOOST_ASIO_CHECK(count == 2);
+}
+
 BOOST_ASIO_TEST_SUITE
 (
   "bind_executor",
   BOOST_ASIO_TEST_CASE(bind_executor_to_function_object_test)
   BOOST_ASIO_TEST_CASE(bind_executor_to_completion_token_v1_test)
   BOOST_ASIO_TEST_CASE(bind_executor_to_completion_token_v2_test)
+  BOOST_ASIO_TEST_CASE(partial_bind_executor_test)
 )