added a limit on how many bytes each connection can have pending in the disk write queue

include/libtorrent/peer_connection.hpp

@@ -74,10 +74,6 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/socket_type.hpp"
 #include "libtorrent/intrusive_ptr_base.hpp"
 
-// TODO: each time a block is 'taken over'
-// from another peer. That peer must be given
-// a chance to become not-interested.
-
 namespace libtorrent
 {
 	class torrent;
@@ -702,6 +698,10 @@ namespace libtorrent
 		// was last updated
 		ptime m_remote_dl_update;
 
+		// the number of bytes send to the disk-io
+		// thread that hasn't yet been completely written.
+		int m_outstanding_writing_bytes;
+		
 #ifndef NDEBUG
 	public:
 		bool m_in_constructor;

include/libtorrent/session_settings.hpp

@@ -108,6 +108,7 @@ namespace libtorrent
 			, unchoke_interval(20)
 			, num_want(200)
 			, initial_picker_threshold(4)
+			, max_outstanding_disk_bytes_per_connection(128 * 1024)
 #ifndef TORRENT_DISABLE_DHT
 			, use_dht_as_fallback(true)
 #endif
@@ -251,6 +252,14 @@ namespace libtorrent
 		// random pieces instead of rarest first.
 		int initial_picker_threshold;
 
+		// the maximum number of bytes a connection may have
+		// pending in the disk write queue before its download
+		// rate is being throttled. This prevents fast downloads
+		// to slow medias to allocate more and more memory
+		// indefinitely. This should be set to at least 32 kB
+		// to not completely disrupt normal downloads.
+		int max_outstanding_disk_bytes_per_connection;
+
 #ifndef TORRENT_DISABLE_DHT
 		// while this is true, the dht will note be used unless the
 		// tracker is online

src/peer_connection.cpp

@@ -116,6 +116,7 @@ namespace libtorrent
 		, m_remote_bytes_dled(0)
 		, m_remote_dl_rate(0)
 		, m_remote_dl_update(time_now())
+		, m_outstanding_writing_bytes(0)
 #ifndef NDEBUG
 		, m_in_constructor(true)
 #endif
@@ -189,6 +190,7 @@ namespace libtorrent
 		, m_remote_bytes_dled(0)
 		, m_remote_dl_rate(0)
 		, m_remote_dl_update(time_now())
+		, m_outstanding_writing_bytes(0)
 #ifndef NDEBUG
 		, m_in_constructor(true)
 #endif
@@ -1150,6 +1152,7 @@ namespace libtorrent
 		
 		fs.async_write(p, data, bind(&peer_connection::on_disk_write_complete
 			, self(), _1, _2, p, t));
+		m_outstanding_writing_bytes += p.length;
 		picker.mark_as_writing(block_finished, peer_info_struct());
 	}
 
@@ -1158,6 +1161,13 @@ namespace libtorrent
 	{
 		session_impl::mutex_t::scoped_lock l(m_ses.m_mutex);
 
+		m_outstanding_writing_bytes -= p.length;
+		assert(m_outstanding_writing_bytes >= 0);
+
+		// in case the outstanding bytes just dropped down
+		// to allow to receive more data
+		setup_receive();
+
 		if (ret == -1 || !t)
 		{
 			if (!t)
@@ -2324,7 +2334,9 @@ namespace libtorrent
 
 		return (m_bandwidth_limit[download_channel].quota_left() > 0
 				|| m_ignore_bandwidth_limits)
-			&& !m_connecting;
+			&& !m_connecting
+			&& m_outstanding_writing_bytes <
+				m_ses.settings().max_outstanding_disk_bytes_per_connection;
 	}
 
 	void peer_connection::connect(int ticket)