fix race condition when cancelling requests after becoming a seed

ChangeLog

@@ -1,6 +1,7 @@
 
 2.0.11 not released
 
+	* fix race condition when cancelling requests after becoming a seed
 	* fix performance bug in the file pool, evicting MRU instead of LRU (HanabishiRecca)
 	* fix bug where file_progress could sometimes be reported as >100%
 	* don't hint FADV_RANDOM on posix systems. May improve seeding performance

src/peer_connection.cpp

@@ -3200,9 +3200,11 @@ namespace libtorrent {
 			{
 				// if any other peer has a busy request to this block, we need
 				// to cancel it too
-				t->cancel_block(block_finished);
 				if (t->has_picker())
+				{
+					t->cancel_block(block_finished);
 					t->picker().write_failed(block_finished);
+				}
 
 				if (t->has_storage())
 				{
@@ -3751,6 +3753,7 @@ namespace libtorrent {
 		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
 		TORRENT_ASSERT(block.piece_index < t->torrent_file().end_piece());
 		TORRENT_ASSERT(block.block_index < t->torrent_file().piece_size(block.piece_index));
+		TORRENT_ASSERT(t->has_picker());
 
 		// if all the peers that requested this block has been
 		// cancelled, then just ignore the cancel.

src/torrent.cpp

@@ -5094,6 +5094,10 @@ namespace {
 #ifndef TORRENT_DISABLE_STREAMING
 	void torrent::cancel_non_critical()
 	{
+		// if we don't have a piece picker, there's nothing to cancel.
+		// e.g. We may have become a seed already.
+		if (!has_picker()) return;
+
 		std::set<piece_index_t> time_critical;
 		for (auto const& p : m_time_critical_pieces)
 			time_critical.insert(p.piece);
@@ -5949,6 +5953,8 @@ namespace {
 	{
 		INVARIANT_CHECK;
 
+		TORRENT_ASSERT(has_picker());
+
 		TORRENT_ASSERT(!c.is_choked());
 		TORRENT_ASSERT(!c.ignore_unchoke_slots());
 		TORRENT_ASSERT(m_num_uploads > 0);