Implement io_service::process_events_until_complete

Closes lewissbaker#30

include/cppcoro/io_service.hpp

@@ -79,6 +79,13 @@ namespace cppcoro
 			const std::chrono::duration<REP, PERIOD>& delay,
 			cancellation_token cancellationToken = {}) noexcept;
 
+		/// Process events until the task completes.
+		///
+		/// \return
+		/// Result of the co_await task.
+		template<typename TASK>
+		decltype(auto) process_events_until_complete(TASK&& task);
+
 		/// Process events until the io_service is stopped.
 		///
 		/// \return
@@ -178,6 +185,26 @@ namespace cppcoro
 
 	};
 
+	template<typename TASK>
+	decltype(auto) io_service::process_events_until_complete(TASK&& task)
+	{
+		if (!task.is_ready())
+		{
+			auto callback = [](void* io) noexcept
+			{
+				static_cast<io_service*>(io)->stop();
+			};
+
+			auto starter = task.get_starter();
+			starter.start(cppcoro::detail::continuation{ callback, this });
+
+			process_events();
+			reset();
+		}
+
+		return std::forward<TASK>(task).operator co_await().await_resume();
+	}
+
 	class io_service::schedule_operation
 	{
 	public:

test/io_service_tests.cpp

@@ -226,4 +226,36 @@ TEST_CASE_FIXTURE(io_service_fixture_with_threads<1>, "Many concurrent timers")
 		<< "ms");
 }
 
+TEST_CASE("io_service::process_events_until_complete(task<T>)")
+{
+	cppcoro::io_service ioService;
+
+	auto makeTask = [](cppcoro::io_service& io) -> cppcoro::task<std::string>
+	{
+		co_await io.schedule();
+		co_return "foo";
+	};
+
+	auto task = makeTask(ioService);
+
+	CHECK(ioService.process_events_until_complete(task) == "foo");
+	CHECK(ioService.process_events_until_complete(makeTask(ioService)) == "foo");
+}
+
+TEST_CASE("io_service::process_events_until_complete(shared_task<T>)")
+{
+	cppcoro::io_service ioService;
+
+	auto makeTask = [](cppcoro::io_service& io) -> cppcoro::shared_task<std::string>
+	{
+		co_await io.schedule();
+		co_return "foo";
+	};
+
+	auto task = makeTask(ioService);
+
+	CHECK(ioService.process_events_until_complete(task) == "foo");
+	CHECK(ioService.process_events_until_complete(makeTask(ioService)) == "foo");
+}
+
 TEST_SUITE_END();