Connections now support immediate executors

Immediate completions in connection and any_connection are now correctly
    dispatched to the token's immediate executor using asio::async_immediate
    instead of plain asio::post.
Added a section on executors in the reference docs of async functions in
    connection and any_connection
Disabled TSAN connection_pool_cancel_get_connection for libc++ builds

close #301

include/boost/mysql/any_connection.hpp

@@ -389,6 +389,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_ASIO_COMPLETION_TOKEN_FOR(void(error_code))
@@ -751,6 +761,16 @@ public:
      * The handler signature for this operation is
      * `void(boost::mysql::error_code, std::size_t)`.
      *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
+     *
      * \par Object lifetimes
      * The storage that `output` references must be kept alive until the operation completes.
      */
@@ -799,6 +819,16 @@ public:
      * The handler signature for this operation is
      * `void(boost::mysql::error_code, std::size_t)`.
      *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
+     *
      * \par Object lifetimes
      * The storage that `output` references must be kept alive until the operation completes.
      */
@@ -903,6 +933,16 @@ public:
      * \n
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
@@ -1017,6 +1057,16 @@ public:
      * \n
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
@@ -1076,6 +1126,16 @@ public:
      * \details
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_ASIO_COMPLETION_TOKEN_FOR(void(error_code))
@@ -1143,6 +1203,16 @@ public:
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
      *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
+     *
      * \par Object lifetimes
      * The request and response objects must be kept alive and should not be modified
      * until the operation completes.

include/boost/mysql/connection.hpp

@@ -247,6 +247,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         typename EndpointType,
@@ -322,6 +332,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -393,6 +413,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_MYSQL_EXECUTION_REQUEST ExecutionRequest,
@@ -495,6 +525,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_MYSQL_EXECUTION_REQUEST ExecutionRequest,
@@ -576,6 +616,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code, boost::mysql::statement)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code, ::boost::mysql::statement))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -633,6 +683,16 @@ public:
      *
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -693,6 +753,16 @@ public:
      * \par Handler signature
      * The handler signature for this operation is
      * `void(boost::mysql::error_code, boost::mysql::rows_view)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code, ::boost::mysql::rows_view))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -828,6 +898,16 @@ public:
      * The handler signature for this operation is
      * `void(boost::mysql::error_code, std::size_t)`.
      *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
+     *
      * \par Object lifetimes
      * The storage that `output` references must be kept alive until the operation completes.
      */
@@ -878,6 +958,16 @@ public:
      * The handler signature for this operation is
      * `void(boost::mysql::error_code, std::size_t)`.
      *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
+     *
      * \par Object lifetimes
      * The storage that `output` references must be kept alive until the operation completes.
      */
@@ -943,6 +1033,16 @@ public:
      * \par Handler signature
      * The handler signature for this operation is
      * `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <
         BOOST_MYSQL_EXECUTION_STATE_TYPE ExecutionStateType,
@@ -999,6 +1099,16 @@ public:
      * \n
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -1071,6 +1181,16 @@ public:
      * \n
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -1128,6 +1248,16 @@ public:
      * \details
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>
@@ -1186,6 +1316,16 @@ public:
      * \details
      * \par Handler signature
      * The handler signature for this operation is `void(boost::mysql::error_code)`.
+     *
+     * \par Executor
+     * Intermediate completion handlers, as well as the final handler, are executed using
+     * `token`'s associated executor, or `this->get_executor()` if the token doesn't have an associated
+     * executor.
+     *
+     * If the final handler has an associated immediate executor, and the operation
+     * completes immediately, the final handler is dispatched to it.
+     * Otherwise, the final handler is called as if it was submitted using `asio::post`,
+     * and is never be called inline from within this function.
      */
     template <BOOST_ASIO_COMPLETION_TOKEN_FOR(void(::boost::mysql::error_code))
                   CompletionToken BOOST_ASIO_DEFAULT_COMPLETION_TOKEN_TYPE(executor_type)>

include/boost/mysql/detail/engine_impl.hpp

@@ -19,6 +19,7 @@
 #include <boost/asio/any_io_executor.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/asio/compose.hpp>
+#include <boost/asio/immediate.hpp>
 #include <boost/asio/post.hpp>
 #include <boost/assert.hpp>
 
@@ -66,7 +67,7 @@ struct run_algo_op
                         BOOST_MYSQL_YIELD(
                             resume_point_,
                             1,
-                            asio::post(stream_.get_executor(), std::move(self))
+                            asio::async_immediate(stream_.get_executor(), std::move(self))
                         )
                     }
                     self.complete(stored_ec_);

test/common/include/test_common/network_result.hpp

@@ -53,6 +53,11 @@ struct BOOST_ATTRIBUTE_NODISCARD network_result_base
 {
     error_code err;
     diagnostics diag;
+    bool was_immediate{};
+
+    network_result_base(error_code ec = {}, diagnostics d = {}) noexcept : err(ec), diag(std::move(d)) {}
+
+    void validate_immediate(bool expect_immediate, source_location loc = BOOST_MYSQL_CURRENT_LOCATION) const;
 
     void validate_no_error(source_location loc = BOOST_MYSQL_CURRENT_LOCATION) const;
 
@@ -100,6 +105,16 @@ struct BOOST_ATTRIBUTE_NODISCARD network_result : network_result_base
     {
     }
 
+    // Allow chaining
+    network_result<R>& validate_immediate(
+        bool expect_immediate,
+        source_location loc = BOOST_MYSQL_CURRENT_LOCATION
+    )
+    {
+        network_result_base::validate_immediate(expect_immediate, loc);
+        return *this;
+    }
+
     BOOST_ATTRIBUTE_NODISCARD
     value_type get(source_location loc = BOOST_MYSQL_CURRENT_LOCATION) &&
     {
@@ -127,6 +142,7 @@ struct BOOST_ATTRIBUTE_NODISCARD runnable_network_result
             create_server_diag("network_result_v2 - diagnostics not cleared")
         };
         bool done{false};
+        bool was_immediate{false};
 
         impl_t(asio::io_context& ctx) : ctx(ctx) {}
     };

test/common/include/test_common/poll_until.hpp

@@ -28,6 +28,13 @@ void poll_until(
     source_location loc = BOOST_MYSQL_CURRENT_LOCATION
 );
 
+// run fn() in ctx, then poll until it completes
+void run_in_context(
+    asio::io_context& ctx,
+    const std::function<void()>& fn,
+    source_location loc = BOOST_MYSQL_CURRENT_LOCATION
+);
+
 }  // namespace test
 }  // namespace mysql
 }  // namespace boost

test/common/src/utils.cpp

@@ -22,6 +22,8 @@
 #include <boost/mysql/detail/access.hpp>
 
 #include <boost/asio/any_io_executor.hpp>
+#include <boost/asio/bind_executor.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/execution/blocking.hpp>
 #include <boost/asio/execution/relationship.hpp>
 #include <boost/asio/execution_context.hpp>
@@ -414,6 +416,20 @@ void boost::mysql::test::poll_until(
     }
 }
 
+void boost::mysql::test::run_in_context(
+    asio::io_context& ctx,
+    const std::function<void()>& fn,
+    source_location loc
+)
+{
+    bool finished = false;
+    asio::dispatch(asio::bind_executor(ctx.get_executor(), [fn, &finished]() {
+        fn();
+        finished = true;
+    }));
+    poll_until(ctx, &finished, loc);
+}
+
 //
 // io_context_fixture.hpp
 //
@@ -427,6 +443,13 @@ boost::mysql::test::io_context_fixture::~io_context_fixture()
 //
 // network_result.hpp
 //
+
+void boost::mysql::test::network_result_base::validate_immediate(bool expect_immediate, source_location loc)
+    const
+{
+    BOOST_TEST_CONTEXT("Called from " << loc) { BOOST_TEST(was_immediate == expect_immediate); }
+}
+
 void boost::mysql::test::network_result_base::validate_no_error(source_location loc) const
 {
     validate_error(error_code(), diagnostics(), loc);
@@ -503,6 +526,9 @@ void boost::mysql::test::test_detail::as_netres_handler_base::complete_base(
     network_result_base& netres
 ) const
 {
+    // Are we in an immediate completion?
+    bool is_immediate = is_initiation_function();
+
     // Check executor. The passed executor must be the top one in all cases.
     // Immediate completions must be dispatched through the immediate executor, too.
     // In all cases, we may encounter a bigger stack because of previous immediate completions.
@@ -512,9 +538,8 @@ void boost::mysql::test::test_detail::as_netres_handler_base::complete_base(
     };
 
     // Expected top of the executor stack
-    boost::span<const int> expected_stack_top = is_initiation_function()
-                                                    ? boost::span<const int>(stack_data_immediate)
-                                                    : boost::span<const int>(stack_data_regular);
+    boost::span<const int> expected_stack_top = is_immediate ? boost::span<const int>(stack_data_immediate)
+                                                             : boost::span<const int>(stack_data_regular);
 
     // Actual top of the executor stack
     auto actual_stack_top = executor_stack().last(
@@ -530,4 +555,7 @@ void boost::mysql::test::test_detail::as_netres_handler_base::complete_base(
         netres.diag = *diag_ptr;
     else
         netres.diag = create_server_diag("<diagnostics unavailable>");
+
+    // Record immediate-ness
+    netres.was_immediate = is_immediate;
 }

test/integration/test/any_connection.cpp

@@ -35,8 +35,8 @@
 #include "test_common/create_basic.hpp"
 #include "test_common/create_diagnostics.hpp"
 #include "test_common/network_result.hpp"
+#include "test_common/poll_until.hpp"
 #include "test_common/printing.hpp"
-#include "test_common/tracker_executor.hpp"
 #include "test_integration/any_connection_fixture.hpp"
 #include "test_integration/connect_params_builder.hpp"
 #include "test_integration/run_coro.hpp"
@@ -317,6 +317,25 @@ BOOST_FIXTURE_TEST_CASE(default_token_cancel_after, any_connection_fixture)
     });
 }
 
+// Spotcheck: immediate completions dispatched to the immediate executor
+BOOST_FIXTURE_TEST_CASE(immediate_completions, any_connection_fixture)
+{
+    run_in_context(ctx, [this]() {
+        // Setup
+        connect();
+        results r;
+
+        // Prepare a statement
+        auto stmt = conn.async_prepare_statement("SELECT 1", as_netresult).get();
+
+        // Executing with the wrong number of params is an immediate error
+        conn.async_execute(stmt.bind(0), r, as_netresult)
+            .run()
+            .validate_immediate(true)
+            .validate_error(client_errc::wrong_num_params);
+    });
+}
+
 #endif
 
 BOOST_AUTO_TEST_SUITE_END()

test/thread_safety/Jamfile

@@ -24,6 +24,12 @@ local tests =
 
 for test in $(tests)
 {
+    # TSAN seems to raise a false positive with libc++ shared_ptr/weak_ptr, used in this test
+    local libcpp_exclusion ;
+    if $(test) = "connection_pool_cancel_get_connection" {
+        libcpp_exclusion = <stdlib>libc++:<build>no ;
+    }
+
     # In OSX, tsan reports a race condition in Asio's kqueue reactor. Ignore this for now.
     # Net TS executors use std::atomic_thread_fence, unsupported by TSAN, which yields false positives.
     # gcc-5 seems to emit incorrect tsan instrumentation code for static initializations
@@ -36,6 +42,7 @@ for test in $(tests)
             <target-os>darwin:<build>no
             <boost.mysql.use-ts-executor>on:<build>no
             <toolset>gcc-5:<build>no
+            $(libcpp_exclusion)
             <thread-sanitizer>norecover
         : target-name $(test)
     ;

test/unit/test/detail/engine_impl.cpp

@@ -16,9 +16,11 @@
 #include <boost/asio/any_completion_handler.hpp>
 #include <boost/asio/any_io_executor.hpp>
 #include <boost/asio/async_result.hpp>
+#include <boost/asio/bind_executor.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/asio/coroutine.hpp>
 #include <boost/asio/deferred.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/error.hpp>
 #include <boost/asio/post.hpp>
 #include <boost/test/unit_test.hpp>
@@ -32,6 +34,7 @@
 #include "test_common/io_context_fixture.hpp"
 #include "test_common/netfun_maker.hpp"
 #include "test_common/network_result.hpp"
+#include "test_common/poll_until.hpp"
 #include "test_unit/printing.hpp"
 
 using namespace boost::mysql::test;
@@ -417,18 +420,15 @@ BOOST_AUTO_TEST_CASE(stream_errors)
 }
 
 // Returning an error or next_action() from resume in the first call works correctly
-BOOST_AUTO_TEST_CASE(resume_error_immediate)
+BOOST_AUTO_TEST_CASE(resume_error_immediate_sync)
 {
     struct
     {
         const char* name;
-        signature_t fn;
         error_code ec;
     } test_cases[] = {
-        {"success_sync",  sync_fn,  error_code()        },
-        {"success_async", async_fn, error_code()        },
-        {"error_sync",    sync_fn,  asio::error::no_data},
-        {"error_async",   async_fn, asio::error::no_data},
+        {"success", error_code()        },
+        {"error",   asio::error::no_data},
     };
 
     for (const auto& tc : test_cases)
@@ -440,13 +440,51 @@ BOOST_AUTO_TEST_CASE(resume_error_immediate)
             mock_algo algo(next_action(tc.ec));
             test_engine eng{fix.ctx.get_executor()};
 
-            tc.fn(eng, any_resumable_ref(algo))
+            sync_fn(eng, any_resumable_ref(algo))
                 .validate_error(tc.ec, create_server_diag("<diagnostics unavailable>"));
             BOOST_TEST(eng.value.stream().calls.size() == 0u);
             algo.check_calls({
                 {error_code(), 0u}
             });
-            // Note: the testing infrastructure already checks that we post correctly in the async versions
+        }
+    }
+}
+
+BOOST_AUTO_TEST_CASE(resume_error_immediate_async)
+{
+    struct
+    {
+        const char* name;
+        error_code ec;
+    } test_cases[] = {
+        {"success", error_code()        },
+        {"error",   asio::error::no_data},
+    };
+
+    for (const auto& tc : test_cases)
+    {
+        BOOST_TEST_CONTEXT(tc.name)
+        {
+            // We need to call the initiation function from a context thread
+            // to get immediate completions
+            io_context_fixture fix;
+            run_in_context(fix.ctx, [&]() {
+                // Setup
+                mock_algo algo(next_action(tc.ec));
+                test_engine eng{fix.ctx.get_executor()};
+
+                // Run the function
+                eng.async_run(any_resumable_ref(algo), as_netresult)
+                    .run()
+                    .validate_immediate(true)
+                    .validate_error(tc.ec, create_server_diag("<diagnostics unavailable>"));
+
+                // Check
+                BOOST_TEST(eng.value.stream().calls.size() == 0u);
+                algo.check_calls({
+                    {error_code(), 0u}
+                });
+            });
         }
     }
 }