add stats counter for the number of outstanding async_accept calls

include/libtorrent/performance_counters.hpp

@@ -465,6 +465,8 @@ namespace libtorrent {
 			num_utp_close_wait,
 			num_utp_deleted,
 
+			num_outstanding_accept,
+
 			num_counters,
 			num_gauges_counters = num_counters - num_stats_counters
 		};

src/session_impl.cpp

@@ -2533,6 +2533,7 @@ namespace aux {
 #endif
 
 		std::weak_ptr<tcp::acceptor> ls(listener);
+		m_stats_counters.inc_stats_counter(counters::num_outstanding_accept);
 		listener->async_accept(*str, [this, c, ls, ssl] (error_code const& ec)
 			{ return this->wrap(&session_impl::on_accept_connection, c, ls, ec, ssl); });
 	}
@@ -2543,6 +2544,8 @@ namespace aux {
 	{
 		COMPLETE_ASYNC("session_impl::on_accept_connection");
 		m_stats_counters.inc_stats_counter(counters::on_accept_counter);
+		m_stats_counters.inc_stats_counter(counters::num_outstanding_accept, -1);
+
 		TORRENT_ASSERT(is_single_thread());
 		std::shared_ptr<tcp::acceptor> listener = listen_socket.lock();
 		if (!listener) return;

src/session_stats.cpp

@@ -252,6 +252,10 @@ namespace {
 		// the number of allowed unchoked peers
 		METRIC(ses, num_unchoke_slots)
 
+		// the number of listen sockets that are currently accepting incoming
+		// connections
+		METRIC(ses, num_outstanding_accept)
+
 		// bittorrent message counters. These counters are incremented
 		// every time a message of the corresponding type is received from
 		// or sent to a bittorrent peer.

tools/parse_session_stats.py

@@ -624,6 +624,9 @@ reports = [
         'picker.piece_picker_rand_loops', \
         'picker.piece_picker_busy_loops' \
     ], {'type': stacked}),
+    ('async_accept', 'number of outstanding accept calls', '', '', [ \
+        'ses.num_outstanding_accept' \
+    ]),
 
     # ('picker_full_partials_distribution', 'full pieces', '', '', ['num full partial pieces'],
     #  {'type': histogram, 'binwidth': 5, 'numbins': 120}),