more asserts around send buffers and fixes potential race condition

include/libtorrent/aux_/session_impl.hpp

@@ -382,6 +382,7 @@ namespace libtorrent
 			// this pool is used to allocate and recycle send
 			// buffers from.
 			boost::pool<> m_send_buffers;
+			boost::mutex m_send_buffer_mutex;
 
 			// the file pool that all storages in this session's
 			// torrents uses. It sets a limit on the number of

src/peer_connection.cpp

@@ -2607,6 +2607,7 @@ namespace libtorrent
 // return value is destructed
 	buffer::interval peer_connection::allocate_send_buffer(int size)
 	{
+		TORRENT_ASSERT(size > 0);
 		char* insert = m_send_buffer.allocate_appendix(size);
 		if (insert == 0)
 		{

src/session_impl.cpp

@@ -2388,7 +2388,11 @@ namespace detail
 	
 	std::pair<char*, int> session_impl::allocate_buffer(int size)
 	{
+		TORRENT_ASSERT(size > 0);
 		int num_buffers = (size + send_buffer_size - 1) / send_buffer_size;
+		TORRENT_ASSERT(num_buffers > 0);
+
+		boost::mutex::scoped_lock l(m_send_buffer_mutex);
 #ifdef TORRENT_STATS
 		m_buffer_allocations += num_buffers;
 		m_buffer_usage_logger << log_time() << " protocol_buffer: "
@@ -2400,8 +2404,12 @@ namespace detail
 
 	void session_impl::free_buffer(char* buf, int size)
 	{
+		TORRENT_ASSERT(size > 0);
 		TORRENT_ASSERT(size % send_buffer_size == 0);
 		int num_buffers = size / send_buffer_size;
+		TORRENT_ASSERT(num_buffers > 0);
+
+		boost::mutex::scoped_lock l(m_send_buffer_mutex);
 #ifdef TORRENT_STATS
 		m_buffer_allocations -= num_buffers;
 		TORRENT_ASSERT(m_buffer_allocations >= 0);