net: extend input_buffer_factory with passing back of unused buffers.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>

Author: rzarzynski

include/seastar/net/api.hh

@@ -131,13 +131,18 @@ public:
 
 class input_buffer_factory {
 public:
+  using buffer_t = temporary_buffer<char>;
+
   virtual ~input_buffer_factory() = default;
   /// Provide a rx buffer. Implementation is responsible for determining its size
   /// and memory. This is useful when a network stack implementation does not put
   /// extra requirements on these factors. The POSIX stack is the example here.
   /// \param allocator Memory allocator \c connected_socket implementation prefers.
   /// Maybe nullptr.
-  virtual temporary_buffer<char> create(compat::polymorphic_allocator<char>* allocator) = 0;
+  virtual buffer_t create(compat::polymorphic_allocator<char>* allocator) = 0;
+
+  // Give back to the factory unused part of a buffer obtained from it
+  virtual void return_unused(buffer_t&& buf) = 0;
 };
 
 } /* namespace net */

src/net/posix-stack.cc

@@ -119,9 +119,11 @@ class posix_connected_socket_impl final : public connected_socket_impl, posix_co
     virtual data_source source(net::input_buffer_factory* ibf) override {
         if (!ibf) {
             static struct final : input_buffer_factory {
-               temporary_buffer<char> create(compat::polymorphic_allocator<char>* const allocator) override {
+                buffer_t create(compat::polymorphic_allocator<char>* const allocator) override {
                     return make_temporary_buffer<char>(allocator, 8192);
                 }
+                void return_unused(buffer_t&&) override {
+                }
             } default_posix_inbuf_factory{};
             ibf = &default_posix_inbuf_factory;
         }
@@ -327,9 +329,11 @@ future<temporary_buffer<char>>
 posix_data_source_impl::get() {
     _buf = _buffer_factory->create(_buffer_allocator);
     return _fd->read_some(_buf.get_write(), _buf.size()).then([this] (size_t size) {
-        _buf.trim(size);
-        auto ret = std::move(_buf);
-        return make_ready_future<temporary_buffer<char>>(std::move(ret));
+        if (size < _buf.size()) {
+          _buffer_factory->return_unused(_buf.share(size, _buf.size() - size));
+          _buf.trim(size);
+        }
+        return make_ready_future<temporary_buffer<char>>(std::move(_buf));
     });
 }