workaround for sparse files issue on Windows vista

ChangeLog

@@ -1,3 +1,4 @@
+	* added workaround for sparse file issue on Windows Vista
 	* added new lt_trackers extension to exchange trackers between
 	  peers
 	* added support for BEP 17 http seeds

docs/manual.rst

@@ -2613,6 +2613,8 @@ It contains the following fields::
 		int last_scrape;
 
 		bool has_incoming;
+
+		int sparse_regions;
 	};
 
 ``progress`` is a value in the range [0, 1], that represents the progress of the
@@ -2800,6 +2802,9 @@ If it has never done that, this value is -1.
 ``has_incoming`` is true if there has ever been an incoming connection attempt
 to this torrent.'
 
+``sparse_regions`` the number of regions of non-downloaded pieces in the
+torrent. This is an interesting metric on windows vista, since there is
+a limit on the number of sparse regions in a single file there.
 
 peer_info
 =========
@@ -3217,6 +3222,8 @@ that will be sent to the tracker. The user-agent is a good way to identify your
 		bool strict_super_seeding;
 
 		int seeding_piece_quota;
+
+		int max_sparse_regions;
 	};
 
 ``user_agent`` this is the client identification to the tracker.
@@ -3526,6 +3533,16 @@ It defaults to 3 pieces, which means that when seeding, any peer we've
 sent more than this number of pieces to will be unchoked in favour of
 a choked peer.
 
+``max_sparse_regions`` is a limit of the number of *sparse regions* in
+a torrent. A sparse region is defined as a hole of pieces we have not
+yet downloaded, in between pieces that have been downloaded. This is
+used as a hack for windows vista which has a bug where you cannot
+write files with more than a certain number of sparse regions. This
+limit is not hard, it will be exceeded. Once it's exceeded, pieces
+that will maintain or decrease the number of sparse regions are
+prioritized. To disable this functionality, set this to 0. It defaults
+to 0 on all platforms except windows.
+
 pe_settings
 ===========
 

examples/client_test.cpp

@@ -1382,7 +1382,8 @@ int main(int ac, char* av[])
 				out << "  peers: " << esc("37") << s.num_peers << esc("0") << " (" << esc("37") << s.connect_candidates << esc("0") << ") "
 					<< "seeds: " << esc("37") << s.num_seeds << esc("0") << " "
 					<< "distributed copies: " << esc("37") << s.distributed_copies << esc("0")
-//					<< "  magnet-link: " << make_magnet_uri(h) << "\n"
+					<< " sparse regions: " << s.sparse_regions
+//					<< " magnet-link: " << make_magnet_uri(h) << "\n"
 					<< " download: " << esc("32") << (s.download_rate > 0 ? add_suffix(s.download_rate) + "/s ": "         ") << esc("0");
 				boost::posix_time::time_duration t = s.next_announce;
 				out << " next announce: " << esc("37")

include/libtorrent/piece_picker.hpp

@@ -182,6 +182,7 @@ namespace libtorrent
 
 		int cursor() const { return m_cursor; }
 		int reverse_cursor() const { return m_reverse_cursor; }
+		int sparse_regions() const { return m_sparse_regions; }
 
 		// sets all pieces to dont-have
 		void init(int blocks_per_piece, int total_num_blocks);
@@ -523,6 +524,9 @@ namespace libtorrent
 		// all the subsequent pieces
 		int m_reverse_cursor;
 
+		// the number of regions of pieces we don't have.
+		int m_sparse_regions;
+
 		// if this is set to true, it means update_pieces()
 		// has to be called before accessing m_pieces.
 		mutable bool m_dirty;

include/libtorrent/session_settings.hpp

@@ -148,6 +148,11 @@ namespace libtorrent
 			, prefer_udp_trackers(true)
 			, strict_super_seeding(false)
 			, seeding_piece_quota(3)
+#ifdef TORRENT_WINDOWS
+			, max_sparse_regions(30000)
+#else
+			, max_sparse_regions(0)
+#endif
 		{}
 
 		// this is the user agent that will be sent to the tracker
@@ -478,6 +483,17 @@ namespace libtorrent
 		// the number of pieces to send to each peer when seeding
 		// before rotating to a new peer
 		int seeding_piece_quota;
+
+		// the maximum number of sparse regions before starting
+		// to prioritize pieces close to other pieces (to maintain
+		// the number of sparse regions). This is set to 30000 on
+		// windows because windows vista has a new limit on the
+		// numbers of sparse regions one file may have
+		// if it is set to 0 this behavior is disabled
+		// this is a hack to avoid a terrible bug on windows
+		// don't use unless you have to, it screws with rarest-first
+		// piece selection, and reduces swarm performance
+		int max_sparse_regions;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

include/libtorrent/torrent.hpp

@@ -451,6 +451,8 @@ namespace libtorrent
 // --------------------------------------------
 		// PIECE MANAGEMENT
 
+		void update_sparse_piece_prio(int piece, int cursor, int reverse_cursor);
+
 		bool super_seeding() const
 		{ return m_super_seeding; }
 		

include/libtorrent/torrent_handle.hpp

@@ -120,6 +120,7 @@ namespace libtorrent
 			, seed_rank(0)
 			, last_scrape(0)
 			, has_incoming(false)
+			, sparse_regions(0)
 		{}
 
 		enum state_t
@@ -270,6 +271,9 @@ namespace libtorrent
 		// true if there are incoming connections to this
 		// torrent
 		bool has_incoming;
+
+		// the number of "holes" in the torrent
+		int sparse_regions;
 	};
 
 	struct TORRENT_EXPORT block_info

src/piece_picker.cpp

@@ -65,6 +65,7 @@ namespace libtorrent
 		, m_num_have(0)
 		, m_cursor(0)
 		, m_reverse_cursor(0)
+		, m_sparse_regions(1)
 		, m_dirty(false)
 	{
 #ifdef TORRENT_PICKER_LOG
@@ -1073,6 +1074,28 @@ namespace libtorrent
 			, has_index(index)) == m_downloads.end());
 
 		if (p.have()) return;
+
+// maintain sparse_regions
+		if (index == 0)
+		{
+			if (index == m_piece_map.size() - 1
+				|| m_piece_map[index + 1].have())
+				--m_sparse_regions;
+		}
+		else if (index == int(m_piece_map.size() - 1))
+		{
+			if (index == 0
+				|| m_piece_map[index - 1].have())
+				--m_sparse_regions;
+		}
+		else
+		{
+			bool have_before = m_piece_map[index-1].have();
+			bool have_after = m_piece_map[index+1].have();
+			if (have_after && have_before) --m_sparse_regions;
+			else if (!have_after && !have_before) ++m_sparse_regions;
+		}
+
 		if (p.filtered())
 		{
 			--m_num_filtered;

src/torrent.cpp

@@ -1547,6 +1547,19 @@ namespace libtorrent
 		}
 	}
 
+	void torrent::update_sparse_piece_prio(int i, int start, int end)
+	{
+		TORRENT_ASSERT(m_picker);
+		if (m_picker->have_piece(i) || m_picker->piece_priority(i) == 0)
+			return;
+		bool have_before = i == 0 || m_picker->have_piece(i - 1);
+		bool have_after = i == end - 1 || m_picker->have_piece(i + 1);
+		if (have_after && have_before)
+			m_picker->set_piece_priority(i, 7);
+		else if (have_after || have_before)
+			m_picker->set_piece_priority(i, 6);
+	}
+
 	void torrent::piece_passed(int index)
 	{
 //		INVARIANT_CHECK;
@@ -1591,6 +1604,19 @@ namespace libtorrent
 			if (p->connection) p->connection->received_valid_data(index);
 		}
 
+		if (settings().max_sparse_regions > 0
+			&& m_picker->sparse_regions() > settings().max_sparse_regions)
+		{
+			// we have too many sparse regions. Prioritize pieces
+			// that won't introduce new sparse regions
+			// prioritize pieces that will reduce the number of sparse
+			// regions even higher
+			int start = m_picker->cursor();
+			int end = m_picker->reverse_cursor();
+			if (index > start) update_sparse_piece_prio(index - 1, start, end);
+			if (index < end - 1) update_sparse_piece_prio(index + 1, start, end);
+		}
+
 #ifndef TORRENT_DISABLE_EXTENSIONS
 		for (extension_list_t::iterator i = m_extensions.begin()
 			, end(m_extensions.end()); i != end; ++i)
@@ -4654,6 +4680,20 @@ namespace libtorrent
 		if (m_time_scaler <= 0)
 		{
 			m_time_scaler = 10;
+
+			if (settings().max_sparse_regions > 0
+				&& m_picker
+				&& m_picker->sparse_regions() > settings().max_sparse_regions)
+			{
+				// we have too many sparse regions. Prioritize pieces
+				// that won't introduce new sparse regions
+				// prioritize pieces that will reduce the number of sparse
+				// regions even higher
+				int start = m_picker->cursor();
+				int end = m_picker->reverse_cursor();
+				for (int i = start; i < end; ++i)
+					update_sparse_piece_prio(i, start, end);
+			}
 			m_policy.pulse();
 		}
 	}
@@ -5059,6 +5099,7 @@ namespace libtorrent
 
 		if (has_picker())
 		{
+			st.sparse_regions = m_picker->sparse_regions();
 			int num_pieces = m_picker->num_pieces();
 			st.pieces.resize(num_pieces, false);
 			for (int i = 0; i < num_pieces; ++i)