Handle the cancellation of continuables used in co_await expression correctly

* Throws a await_canceled_exception now which automatically gets
  converted to a default exception type again if it becomes unhandled in the handler.
* Ref #32

include/continuable/continuable-coroutine.hpp

@@ -36,15 +36,45 @@
 #include <continuable/detail/features.hpp>
 
 #if defined(CONTINUABLE_HAS_EXCEPTIONS)
-#include <exception>
+#  include <exception>
 #endif // CONTINUABLE_HAS_EXCEPTIONS
 
 #ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
-#include <continuable/detail/other/coroutines.hpp>
+#  include <continuable/detail/other/coroutines.hpp>
 #endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
 
+#if defined(CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE)
+namespace cti {
+#  if defined(CONTINUABLE_HAS_EXCEPTIONS)
+/// Is thrown from co_await expressions if the awaited continuable is canceled
+///
+/// Default constructed exception types that are returned by a cancelled
+/// continuable are converted automatically to await_canceled_exception when
+/// being returned by a co_await expression.
+///
+/// The await_canceled_exception gets converted again to a default constructed
+/// exception type if it becomes unhandled inside a coroutine which
+/// returns a continuable_base.
+/// ```cpp
+/// continuable<> cancelled_coroutine() {
+///   co_await make_cancelling_continuable<void>();
+///
+///   co_return;
+/// }
+///
+/// // ...
+///
+/// cancelled_coroutine().fail([](exception_t e) {
+///   assert(bool(e) == false);
+/// });
+/// ```
+///
+/// \since 4.1.0
+using await_canceled_exception = detail::awaiting::await_canceled_exception;
+#  endif // CONTINUABLE_HAS_EXCEPTIONS
+} // namespace cti
+
 /// \cond false
-#ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
 // As far as I know there is no other way to implement this specialization...
 // NOLINTNEXTLINE(cert-dcl58-cpp)
 namespace std {
@@ -54,13 +84,12 @@ struct coroutine_traits<
     cti::continuable_base<Data, cti::detail::identity<Args...>>,
     FunctionArgs...> {
 
-  using promise_type =
-      cti::detail::awaiting::promise_type<cti::continuable<Args...>,
-                                          cti::promise<Args...>, Args...>;
+  using promise_type = cti::detail::awaiting::promise_type<
+      cti::continuable<Args...>, cti::promise<Args...>, Args...>;
 };
 } // namespace experimental
 } // namespace std
+  /// \endcond
 #endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
-/// \endcond
 
 #endif // CONTINUABLE_COROUTINE_HPP_INCLUDED

include/continuable/detail/other/coroutines.hpp

@@ -45,7 +45,7 @@
 #include <continuable/detail/utility/util.hpp>
 
 #if defined(CONTINUABLE_HAS_EXCEPTIONS)
-#include <exception>
+#  include <exception>
 #endif // CONTINUABLE_HAS_EXCEPTIONS
 
 namespace cti {
@@ -54,6 +54,17 @@ namespace awaiting {
 /// We import the coroutine handle in our namespace
 using std::experimental::coroutine_handle;
 
+#if defined(CONTINUABLE_HAS_EXCEPTIONS)
+class await_canceled_exception : public std::exception {
+public:
+  await_canceled_exception() noexcept = default;
+
+  char const* what() const noexcept override {
+    return "co_await canceled due to cancellation of the continuation";
+  }
+};
+#endif // CONTINUABLE_HAS_EXCEPTIONS
+
 template <typename T>
 struct result_from_identity;
 template <typename... T>
@@ -76,7 +87,7 @@ class awaitable {
 
 public:
   explicit constexpr awaitable(Continuable&& continuable)
-      : continuable_(std::move(continuable)) {
+    : continuable_(std::move(continuable)) {
 
     // If the continuable is ready resolve the result from the
     // continuable immediately.
@@ -108,15 +119,21 @@ public:
 
   /// Resume the coroutine represented by the handle
   typename result_t::value_t await_resume() noexcept(false) {
-    if (result_) {
+    if (result_.is_value()) {
       // When the result was resolved return it
       return std::move(result_).get_value();
     }
 
+    assert(result_.is_exception());
+
 #if defined(CONTINUABLE_HAS_EXCEPTIONS)
-    std::rethrow_exception(result_.get_exception());
+    if (exception_t e = result_.get_exception()) {
+      std::rethrow_exception(std::move(e));
+    } else {
+      throw await_canceled_exception();
+    }
 #else  // CONTINUABLE_HAS_EXCEPTIONS
-    // Returning error types in await isn't supported as of now
+    // Returning error types from co_await isn't supported!
     CTI_DETAIL_TRAP();
 #endif // CONTINUABLE_HAS_EXCEPTIONS
   }
@@ -141,8 +158,7 @@ struct handle_takeover {
     handle_ = handle;
   }
 
-  void await_resume() noexcept {
-  }
+  void await_resume() noexcept {}
 };
 
 /// The type which is passed to the compiler that describes the properties
@@ -179,7 +195,7 @@ struct promise_resolver_base<promise_type<Continuable, Promise, Args...>> {
 
 template <typename Continuable, typename Promise, typename... Args>
 struct promise_type
-    : promise_resolver_base<promise_type<Continuable, Promise, Args...>> {
+  : promise_resolver_base<promise_type<Continuable, Promise, Args...>> {
 
   coroutine_handle<> handle_;
   Promise promise_;
@@ -203,9 +219,15 @@ struct promise_type
 
   void unhandled_exception() noexcept {
 #if defined(CONTINUABLE_HAS_EXCEPTIONS)
-    promise_.set_exception(std::current_exception());
+    try {
+      std::rethrow_exception(std::current_exception());
+    } catch (await_canceled_exception const&) {
+      promise_.set_canceled();
+    } catch (...) {
+      promise_.set_exception(std::current_exception());
+    }
 #else  // CONTINUABLE_HAS_EXCEPTIONS
-       // Returning error types from coroutines isn't supported
+    // Returning exception types from a coroutine isn't supported
     CTI_DETAIL_TRAP();
 #endif // CONTINUABLE_HAS_EXCEPTIONS
   }

test/unit-test/multi/test-continuable-await.cpp

@@ -26,11 +26,11 @@
 #include <continuable/detail/features.hpp>
 #ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
 
-#ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
-#include <exception>
-#endif // CONTINUABLE_WITH_NO_EXCEPTIONS
+#  ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
+#    include <exception>
+#  endif // CONTINUABLE_WITH_NO_EXCEPTIONS
 
-#include <tuple>
+#  include <tuple>
 
 /// Resolves the given promise asynchonously
 template <typename S>
@@ -76,7 +76,7 @@ TYPED_TEST(single_dimension_tests, are_awaitable) {
   EXPECT_ASYNC_RESULT(resolve_async_multiple(supply), 0, 1, 2, 3);
 }
 
-#ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
+#  ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
 
 struct await_exception : std::exception {
   char const* what() const noexcept override {
@@ -105,7 +105,9 @@ cti::continuable<> resolve_async_exceptional(S&& supplier) {
   // GTest ASSERT_THROW isn't co_await friendly yet:
   // clang: 'return statement not allowed in coroutine; did you mean
   //        'co_return'?'
-  EXPECT_THROW(co_await supplier().then([] { throw await_exception{}; }),
+  EXPECT_THROW(co_await supplier().then([] {
+    throw await_exception{};
+  }),
                await_exception);
 
   co_return;
@@ -141,6 +143,31 @@ TYPED_TEST(single_dimension_tests, are_awaitable_with_exceptions_from_coro) {
                                 await_exception{})
 }
 
-#endif // CONTINUABLE_WITH_NO_EXCEPTIONS
+template <typename S>
+cti::continuable<> resolve_coro_canceled(S&& supplier) {
+  // Pseudo wait
+  co_await supplier();
+
+  try {
+    co_await cti::make_cancelling_continuable<void>();
+  } catch (cti::await_canceled_exception const& e) {
+    std::rethrow_exception(std::make_exception_ptr(e));
+  } catch (...) {
+    EXPECT_TRUE(false);
+  }
+
+  co_return;
+}
+
+TYPED_TEST(single_dimension_tests, are_awaitable_with_cancellation_from_coro) {
+  auto const& supply = [&](auto&&... args) {
+    // Supplies the current tested continuable
+    return this->supply(std::forward<decltype(args)>(args)...);
+  };
+
+  ASSERT_ASYNC_CANCELLATION(resolve_coro_canceled(supply))
+}
+
+#  endif // CONTINUABLE_WITH_NO_EXCEPTIONS
 
 #endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE