when stopping a torrent, never perform a name lookup on the tracker (#2021)

when stopping a torrent, never perform a name lookup on the tracker, only announce to trackers whose IP we already know. This is expected to make shutdowns not hang

ChangeLog

@@ -1,3 +1,4 @@
+	* improve shutdown time by only announcing to trackers whose IP we know
 	* fix python3 portability issue in python binding
 	* delay 5 seconds before reconnecting socks5 proxy for UDP ASSOCIATE
 	* fix NAT-PMP crash when removing a mapping at the wrong time

include/libtorrent/resolver_interface.hpp

@@ -53,10 +53,11 @@ struct TORRENT_EXTRA_EXPORT resolver_interface
 
 	enum flags_t
 	{
-		// this flag will make async_resolve() always use the cache if we have an
-		// entry, regardless of how old it is. This is usefull when completing the
-		// lookup quickly is more important than accuracy
-		prefer_cache = 1,
+		// this flag will make async_resolve() only use the cache and fail if we
+		// don't have a cache entry, regardless of how old it is. This is usefull
+		// when completing the lookup quickly is more important than accuracy,
+		// like on shutdown
+		cache_only = 1,
 
 		// set this flag for lookups that are not critical during shutdown. i.e.
 		// for looking up tracker names _except_ when stopping a tracker.

src/http_tracker_connection.cpp

@@ -229,8 +229,8 @@ namespace libtorrent
 			, ps.proxy_tracker_connections ? &ps : NULL
 			, 5, user_agent, bind_interface()
 			, tracker_req().event == tracker_request::stopped
-				? resolver_interface::prefer_cache
-				: resolver_interface::abort_on_shutdown
+				? resolver_interface::cache_only : 0
+				| resolver_interface::abort_on_shutdown
 #ifndef TORRENT_NO_DEPRECATE
 			, tracker_req().auth
 #else

src/resolver.cpp

@@ -87,22 +87,9 @@ namespace libtorrent
 		}
 	}
 
-	void resolver::async_resolve(std::string const& host, int flags
+	void resolver::async_resolve(std::string const& host, int const flags
 		, resolver_interface::callback_t const& h)
 	{
-		cache_t::iterator i = m_cache.find(host);
-		if (i != m_cache.end())
-		{
-			// keep cache entries valid for m_timeout seconds
-			if ((flags & resolver_interface::prefer_cache)
-				|| i->second.last_seen + m_timeout >= aux::time_now())
-			{
-				error_code ec;
-				m_ios.post(boost::bind(h, ec, i->second.addresses));
-				return;
-			}
-		}
-
 		// special handling for raw IP addresses. There's no need to get in line
 		// behind actual lookups if we can just resolve it immediately.
 		error_code ec;
@@ -114,6 +101,27 @@ namespace libtorrent
 			m_ios.post(boost::bind(h, ec, addresses));
 			return;
 		}
+		ec.clear();
+
+		cache_t::iterator i = m_cache.find(host);
+		if (i != m_cache.end())
+		{
+			// keep cache entries valid for m_timeout seconds
+			if ((flags & resolver_interface::cache_only)
+				|| i->second.last_seen + m_timeout >= aux::time_now())
+			{
+				m_ios.post(boost::bind(h, ec, i->second.addresses));
+				return;
+			}
+		}
+
+		if (flags & resolver_interface::cache_only)
+		{
+			// we did not find a cache entry, fail the lookup
+			m_ios.post(boost::bind(h, boost::asio::error::host_not_found
+					, std::vector<address>()));
+			return;
+		}
 
 		// the port is ignored
 		tcp::resolver::query q(host, "80");

src/udp_tracker_connection.cpp

@@ -112,13 +112,12 @@ namespace libtorrent
 #if defined TORRENT_ASIO_DEBUGGING
 			add_outstanding_async("udp_tracker_connection::name_lookup");
 #endif
-			// when stopping, pass in the prefer cache flag, because we
+			// when stopping, pass in the cache-only flag, because we
 			// don't want to get stuck on DNS lookups when shutting down
-			// if we can avoid it
 			m_man.host_resolver().async_resolve(hostname
 				, tracker_req().event == tracker_request::stopped
-					? resolver_interface::prefer_cache
-					: resolver_interface::abort_on_shutdown
+					? resolver_interface::cache_only : 0
+					| resolver_interface::abort_on_shutdown
 				, boost::bind(&udp_tracker_connection::name_lookup
 					, shared_from_this(), _1, _2, port));