Implement full shutdown

WinTLS will now produce asio::error::eof when the underlying
stream gets closed without the peer sending its close notify.
This should be changed to a separate error code analogously to
Asio.SSL.

include/boost/wintls/detail/sspi_handshake.hpp

@@ -168,6 +168,12 @@ class sspi_handshake {
       return;
     }
     out_buffer_ = sspi_context_buffer{buffers[0].pvBuffer, buffers[0].cbBuffer};
+    // Setting SEC_I_CONTINUE_NEEDED here means that we will wait for the peer to send its close notify.
+    // Not doing so may open up the possibility of a truncation attack.
+    // If the underlying protocol is self-terminated (as is http) it would be okay not to wait for the close notify.
+    // We could implement such a shutdown as a "fast/unsafe_shutdown" and not change last_error_ here for that case.
+    // Cmp. https://security.stackexchange.com/questions/82028/ssl-tls-is-a-server-always-required-to-respond-to-a-close-notify
+    last_error_ = SEC_I_CONTINUE_NEEDED;
   }
 
   state operator()() {
@@ -245,7 +251,7 @@ class sspi_handshake {
     }
 
     bool has_buffer_output = out_buffers[0].cbBuffer != 0 && out_buffers[0].pvBuffer != nullptr;
-    if(has_buffer_output){
+    if (has_buffer_output) {
       out_buffer_ = sspi_context_buffer{out_buffers[0].pvBuffer, out_buffers[0].cbBuffer};
     }
 
@@ -270,6 +276,14 @@ class sspi_handshake {
         return has_buffer_output ? state::data_available : state::done;
       }
 
+      // If we get here during shutdown, this means we initiated the shutdown and now received the 
+      // close notify from the peer. That means that we are done.
+      // #TODO: could this happen during the initial handshake where it should be an error?
+      case SEC_I_CONTEXT_EXPIRED: {
+        last_error_ = SEC_E_OK; // #TODO: better solve this in the handshake loop?
+        return state::done;
+      }
+
       case SEC_I_INCOMPLETE_CREDENTIALS:
         BOOST_ASSERT_MSG(false, "client authentication not implemented");
 

test/shutdown_test.cpp

@@ -24,7 +24,7 @@ auto stream_truncated_error(const asio_ssl::stream<NextLayer>&) {
 
 template<typename NextLayer>
 auto stream_truncated_error(const boost::wintls::stream<NextLayer>&) {
-  return boost::system::errc::success; // #TODO: This should be an error
+  return net::error::eof; // #TODO: This should be a separate error code.
 }
 
 using TestTypes = std::tuple<std::tuple<asio_ssl_client_stream, asio_ssl_server_stream>,