Fix 'compression' integration with 'fast_forward' sessions

- Compression flag is now propagated from client to backend connection
  when the backend connection is being created for a 'fast_forward'
  session. In case of client trying to connect without compression, to
  a server configured with compression, we fallback to an uncompressed
  connection.
- After a 'MySQL_Session' has obtained a connection for a 'fast_forward'
  session, i.e, the one-to-one relationship between the client and
  backend connection has been stablished, we completely disable the
  compression 'STATUS_MYSQL_CONNECTION_COMPRESSION' flag from the client
  'MySQL_Connection'.

lib/MySQL_Session.cpp

@@ -2864,6 +2864,19 @@ bool MySQL_Session::handler_again___status_CONNECTING_SERVER(int *_rc) {
 					myds->DSS=STATE_SLEEP;
 					myds->myconn->send_quit = false;
 					myds->myconn->reusable = false;
+					// In a 'fast_forward' session after we disable compression for the fronted connection
+					// after we have adquired a backend connection, this is, the 'FAST_FORWARD' session status
+					// is reached, and the 1-1 connection relationship is established. We can safely do this
+					// due two main reasons:
+					//   1. The client and backend have to agree on compression, i.e. if the client connected without
+					//   compression using fast-forward to a backend connections expected to have compression, it results
+					//   in a fallback to a connection without compression, as it's expected by protocol. In this case we do
+					//   not require to compress the data received from the backend.
+					//   2. The client and backend have agreed in using compression, in this case, the data received from
+					//   the backend is already compressed, so we are only required to forward the data to the client.
+					// In both cases, we do not require to perform any specials actions for the received data,
+					// so we completely disable the compression flag for the client connection.
+					client_myds->myconn->set_status(false, STATUS_MYSQL_CONNECTION_COMPRESSION);
 				}
 				NEXT_IMMEDIATE_NEW(st);
 				break;

lib/mysql_connection.cpp

@@ -795,6 +795,15 @@ void MySQL_Connection::connect_start() {
 						mysql->options.client_flag |= CLIENT_DEPRECATE_EOF;
 					}
 				}
+				// In case of 'fast_forward', we only enable compression if both, client and backend matches. Otherwise,
+				// we honor the behavior of a regular connection of when a connection doesn't agree on using compression
+				// during handshake, and we fallback to an uncompressed connection.
+				client_flags &= ~(CLIENT_COMPRESS); // we disable it by default
+				if (c->options.client_flag & CLIENT_COMPRESS) {
+					if (c->options.server_capabilities & CLIENT_COMPRESS) {
+						client_flags |= CLIENT_COMPRESS;
+					}
+				}
 			}
 		}
 	}