Do not wait for cancellation signal (#4664)

* Do not wait for cancelation signal

* Test

* Explicit constructor

nano/core_test/async.cpp

@@ -56,10 +56,30 @@ TEST (async, cancellation)
 
 	cancellation.emit ();
 
-	ASSERT_EQ (fut.wait_for (500ms), std::future_status::ready);
+	ASSERT_EQ (fut.wait_for (1s), std::future_status::ready);
 	ASSERT_NO_THROW (fut.get ());
 }
 
+// Test that cancellation signal behaves well when the cancellation is emitted after the task has completed
+TEST (async, cancellation_lifetime)
+{
+	test_context ctx;
+
+	nano::async::cancellation cancellation{ ctx.strand };
+	{
+		auto fut = asio::co_spawn (
+		ctx.strand,
+		[&] () -> asio::awaitable<void> {
+			co_await nano::async::sleep_for (100ms);
+		},
+		asio::bind_cancellation_slot (cancellation.slot (), asio::use_future));
+		ASSERT_EQ (fut.wait_for (1s), std::future_status::ready);
+		fut.get ();
+	}
+	auto cancel_fut = cancellation.emit ();
+	ASSERT_EQ (cancel_fut.wait_for (1s), std::future_status::ready);
+}
+
 TEST (async, task)
 {
 	nano::test::system system;

nano/lib/async.hpp

@@ -30,7 +30,7 @@ class cancellation
 public:
 	explicit cancellation (nano::async::strand & strand) :
 		strand{ strand },
-		signal{ std::make_unique<asio::cancellation_signal> () }
+		signal{ std::make_shared<asio::cancellation_signal> () }
 	{
 	}
 
@@ -49,12 +49,11 @@ class cancellation
 	};
 
 public:
-	void emit (asio::cancellation_type type = asio::cancellation_type::all)
+	auto emit (asio::cancellation_type type = asio::cancellation_type::all)
 	{
-		asio::dispatch (strand, asio::use_future ([this, type] () {
-			signal->emit (type);
-		}))
-		.wait ();
+		return asio::dispatch (strand, asio::use_future ([signal_l = signal, type] () {
+			signal_l->emit (type);
+		}));
 	}
 
 	auto slot ()
@@ -67,9 +66,8 @@ class cancellation
 	nano::async::strand & strand;
 
 private:
-	std::unique_ptr<asio::cancellation_signal> signal; // Wrap the signal in a unique_ptr to enable moving
-
-	bool slotted{ false };
+	std::shared_ptr<asio::cancellation_signal> signal;
+	bool slotted{ false }; // For debugging purposes
 };
 
 /**
@@ -82,7 +80,7 @@ class task
 	// Only thread-like void tasks are supported for now
 	using value_type = void;
 
-	task (nano::async::strand & strand) :
+	explicit task (nano::async::strand & strand) :
 		strand{ strand },
 		cancellation{ strand }
 	{