add API to query whether alerts have been dropped or not

ChangeLog

@@ -1,3 +1,4 @@
+	* add API to query whether alerts have been dropped or not
 	* add flags()/set_flags()/unset_flags() to torrent_handle, deprecate individual functions
 	* added alert for block being sent to the send buffer
 	* drop support for windows compilers without std::wstring

include/libtorrent/alert.hpp

@@ -228,11 +228,11 @@ namespace libtorrent {
 		//			}
 		//		}
 		//	}
-		virtual int type() const = 0;
+		virtual int type() const noexcept = 0;
 
 		// returns a string literal describing the type of the alert. It does
 		// not include any information that might be bundled with the alert.
-		virtual char const* what() const = 0;
+		virtual char const* what() const noexcept = 0;
 
 		// generate a string describing the alert and the information bundled
 		// with it. This is mainly intended for debug and development use. It is not suitable
@@ -242,7 +242,7 @@ namespace libtorrent {
 		virtual std::string message() const = 0;
 
 		// returns a bitmask specifying which categories this alert belong to.
-		virtual alert_category_t category() const = 0;
+		virtual alert_category_t category() const noexcept = 0;
 
 #ifndef TORRENT_NO_DEPRECATE
 

include/libtorrent/alert_manager.hpp

@@ -37,6 +37,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/alert.hpp"
 #include "libtorrent/heterogeneous_queue.hpp"
 #include "libtorrent/stack_allocator.hpp"
+#include "libtorrent/alert_types.hpp" // for num_alert_types
 
 #include <functional>
 #include <list>
@@ -44,6 +45,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <mutex>
 #include <condition_variable>
 #include <atomic>
+#include <bitset>
 
 namespace libtorrent {
 
@@ -51,6 +53,10 @@ namespace libtorrent {
 	struct plugin;
 #endif
 
+	// this bitset is used to indicate which alert types have been dropped since
+	// last queried.
+	using dropped_alerts_t = std::bitset<num_alert_types>;
+
 	class TORRENT_EXTRA_EXPORT alert_manager
 	{
 	public:
@@ -58,8 +64,10 @@ namespace libtorrent {
 			, alert_category_t alert_mask = alert::error_notification);
 		~alert_manager();
 
+		dropped_alerts_t dropped_alerts();
+
 		template <class T, typename... Args>
-		void emplace_alert(Args&&... args)
+		void emplace_alert(Args&&... args) try
 		{
 			std::unique_lock<std::mutex> lock(m_mutex);
 
@@ -69,12 +77,8 @@ namespace libtorrent {
 			if (m_alerts[m_generation].size() >= m_queue_size_limit
 				* (1 + T::priority))
 			{
-//				if (T::priority > 0)
-//				{
-					// TODO: there should be a way for the client to detect that an
-					// alert was dropped. Maybe add a flag to each m_alerts
-					// generation
-//				}
+				// record that we dropped an alert of this type
+				m_dropped.set(T::alert_type);
 				return;
 			}
 
@@ -83,6 +87,12 @@ namespace libtorrent {
 
 			maybe_notify(&alert, lock);
 		}
+		catch (std::bad_alloc const&)
+		{
+			// record that we dropped an alert of this type
+			std::unique_lock<std::mutex> lock(m_mutex);
+			m_dropped.set(T::alert_type);
+		}
 
 		bool pending() const;
 		void get_all(std::vector<alert*>& alerts);
@@ -105,12 +115,12 @@ namespace libtorrent {
 			m_alert_mask = m;
 		}
 
-		alert_category_t alert_mask() const
+		alert_category_t alert_mask() const noexcept
 		{
 			return m_alert_mask;
 		}
 
-		int alert_queue_size_limit() const { return m_queue_size_limit; }
+		int alert_queue_size_limit() const noexcept { return m_queue_size_limit; }
 		int set_alert_queue_size_limit(int queue_size_limit_);
 
 		void set_notify_function(std::function<void()> const& fun);
@@ -133,6 +143,12 @@ namespace libtorrent {
 		std::atomic<alert_category_t> m_alert_mask;
 		int m_queue_size_limit;
 
+		// a bitfield where each bit represents an alert type. Every time we drop
+		// an alert (because the queue is full or of some other error) we set the
+		// corresponding bit in this mask, to communicate to the client that it
+		// may have missed an update.
+		dropped_alerts_t m_dropped;
+
 		// this function (if set) is called whenever the number of alerts in
 		// the alert queue goes from 0 to 1. The client is expected to wake up
 		// its main message loop for it to poll for alerts (using get_alerts()).

include/libtorrent/alert_types.hpp

@@ -161,9 +161,9 @@ namespace libtorrent {
 	name(name&&) noexcept = default; \
 	static const int priority = prio; \
 	static const int alert_type = seq; \
-	virtual int type() const override { return alert_type; } \
-	virtual alert_category_t category() const override { return static_category; } \
-	virtual char const* what() const override { return #name; }
+	virtual int type() const noexcept override { return alert_type; } \
+	virtual alert_category_t category() const noexcept override { return static_category; } \
+	virtual char const* what() const noexcept override { return #name; }
 
 #define TORRENT_DEFINE_ALERT(name, seq) \
 	TORRENT_DEFINE_ALERT_IMPL(name, seq, 0)

include/libtorrent/session_handle.hpp

@@ -45,6 +45,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/io_service.hpp"
 #include "libtorrent/session_types.hpp"
 #include "libtorrent/portmap.hpp" // for portmap_protocol
+#include "libtorrent/alert_manager.hpp" // for dropped_alerts_t
 
 #include "libtorrent/kademlia/dht_storage.hpp"
 #include "libtorrent/kademlia/dht_settings.hpp"
@@ -934,9 +935,22 @@ namespace libtorrent {
 		// retrieval of alerts should not be done in the callback. In fact, the
 		// callback should not block. It should not perform any expensive work.
 		// It really should just notify the main application thread.
+		//
+		// The ``dropped_alerts()`` function returns a ``std::bitfield``
+		// representing which types of alerts have been dropped. Dropped meaning
+		// that the alert failed to be delivered to the client. The most common
+		// cause of such failure is that the internal alert queue grew too big
+		// (controlled by alert_queue_size). This call also clears the internal
+		// bitfield, so the bitfield starts recording dropped alerts from this
+		// point forward only.
+		//
+		// The type of an alert is returned by the polymorphic function
+		// ``alert::type()`` but can also be queries from a concrete type via
+		// ``T::alert_type``, as a static constant.
 		void pop_alerts(std::vector<alert*>* alerts);
 		alert* wait_for_alert(time_duration max_wait);
 		void set_alert_notify(std::function<void()> const& fun);
+		dropped_alerts_t dropped_alerts();
 
 #ifndef TORRENT_NO_DEPRECATE
 #include "libtorrent/aux_/disable_warnings_push.hpp"

include/libtorrent/settings_pack.hpp

@@ -1442,7 +1442,11 @@ namespace libtorrent {
 
 			// ``alert_queue_size`` is the maximum number of alerts queued up
 			// internally. If alerts are not popped, the queue will eventually
-			// fill up to this level.
+			// fill up to this level. Once the alert queue is full, additional
+			// alerts will be dropped, and not delievered to the client. Once the
+			// client drains the queue, new alerts may be delivered again. In order
+			// to know that alerts have been dropped, see
+			// session_handle::dropped_alerts().
 			alert_queue_size,
 
 			// ``max_metadata_size`` is the maximum allowed size (in bytes) to be

src/alert_manager.cpp

@@ -146,4 +146,12 @@ namespace libtorrent {
 		std::swap(m_queue_size_limit, queue_size_limit_);
 		return queue_size_limit_;
 	}
+
+	dropped_alerts_t alert_manager::dropped_alerts()
+	{
+		std::unique_lock<std::mutex> lock(m_mutex);
+		dropped_alerts_t const ret = m_dropped;
+		m_dropped.reset();
+		return ret;
+	}
 }

src/session_handle.cpp

@@ -1130,6 +1130,13 @@ namespace {
 		s->alerts().set_notify_function(fun);
 	}
 
+	dropped_alerts_t session_handle::dropped_alerts()
+	{
+		std::shared_ptr<session_impl> s = m_impl.lock();
+		if (!s) aux::throw_ex<system_error>(errors::invalid_session_handle);
+		return s->alerts().dropped_alerts();
+	}
+
 #ifndef TORRENT_NO_DEPRECATE
 	void session_handle::set_severity_level(alert::severity_t s)
 	{

test/test_alert_manager.cpp

@@ -248,3 +248,23 @@ TORRENT_TEST(alert_mask)
 	TEST_CHECK(!mgr.should_post<torrent_paused_alert>());
 }
 
+TORRENT_TEST(dropped_alerts)
+{
+	alert_manager mgr(1, alert::all_categories);
+
+	// nothing has dropped yet
+	TEST_CHECK(mgr.dropped_alerts().none());
+	mgr.emplace_alert<torrent_finished_alert>(torrent_handle());
+	// still nothing, there's space for one alert
+	TEST_CHECK(mgr.dropped_alerts().none());
+	mgr.emplace_alert<torrent_finished_alert>(torrent_handle());
+	// that last alert got dropped though, since it would have brought the queue
+	// size to 2
+	auto const d = mgr.dropped_alerts();
+	TEST_CHECK(d.count() == 1);
+	TEST_CHECK(d.test(torrent_finished_alert::alert_type));
+
+	// it should have been cleared now though
+	TEST_CHECK(mgr.dropped_alerts().none());
+}
+