Merge "More dynamic fstream patches" from Paweł

"These are an additional fix (accounting for buffers droppped at stream
deconstruction) and a cleanup (history made private) for fstream dynamic
adjustments."

* 'pdziepak/dynamic-fstream-cleanup/v1' of github.com:cloudius-systems/seastar-dev:
  fstream: make members file_input_stream_history private
  fstream: account for buffers dropped while closing stream

core/fstream.cc

@@ -186,9 +186,12 @@ class file_data_source_impl : public data_source_impl {
             _done->set_value();
         }
         return _done->get_future().then([this] {
+            uint64_t dropped = 0;
             for (auto&& c : _read_buffers) {
+                dropped += c._size;
                 c._ready.ignore_ready_future();
             }
+            update_history_unused(dropped);
             return std::move(_dropped_reads);
         });
     }

core/fstream.hh

@@ -34,7 +34,7 @@
 #include "iostream.hh"
 #include "shared_ptr.hh"
 
-struct file_input_stream_history {
+class file_input_stream_history {
     static constexpr uint64_t window_size = 4 * 1024 * 1024;
     struct window {
         uint64_t total_read = 0;
@@ -43,6 +43,8 @@ struct file_input_stream_history {
     window current_window;
     window previous_window;
     unsigned read_ahead = 1;
+
+    friend class file_data_source_impl;
 };
 
 /// Data structure describing options for opening a file input stream