hide the block_cache_reference from disk_buffer_holder

include/libtorrent/disk_buffer_holder.hpp

@@ -102,7 +102,8 @@ namespace libtorrent
 			std::swap(h.m_ref, m_ref);
 		}
 
-		aux::block_cache_reference ref() const noexcept { return m_ref; }
+		// if this returns true, the buffer may not be modified in place
+		bool is_mutable() const noexcept { return m_ref.storage == nullptr; }
 
 		// implicitly convertible to true if the object is currently holding a
 		// buffer

include/libtorrent/disk_io_thread.hpp

@@ -521,8 +521,6 @@ namespace libtorrent
 		// the last time we expired write blocks from the cache
 		time_point m_last_cache_expiry = min_time();
 
-		time_point m_last_file_check;
-
 		// LRU cache of open files
 		file_pool m_file_pool{40};
 

src/bt_peer_connection.cpp

@@ -2463,7 +2463,7 @@ namespace libtorrent
 			send_buffer(msg, 13);
 		}
 
-		if (buffer.ref().storage == nullptr)
+		if (buffer.is_mutable())
 		{
 			append_send_buffer(std::move(buffer), r.length);
 		}

src/disk_io_thread.cpp

@@ -194,7 +194,6 @@ namespace libtorrent
 		, m_generic_threads(m_generic_io_jobs, ios)
 		, m_hash_io_jobs(*this, thread_type_t::hasher)
 		, m_hash_threads(m_hash_io_jobs, ios)
-		, m_last_file_check(clock_type::now())
 		, m_disk_cache(block_size, ios, std::bind(&disk_io_thread::trigger_cache_trim, this))
 		, m_stats_counters(cnt)
 		, m_ios(ios)