/usr/include/handlersocket/ is in libhsclient-dev 1.1.0-7-g1044a28-2+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/handlersocket/allocator.hpp
/usr/include/handlersocket/auto_addrinfo.hpp
/usr/include/handlersocket/auto_file.hpp
/usr/include/handlersocket/auto_ptrcontainer.hpp
/usr/include/handlersocket/config.hpp
/usr/include/handlersocket/escape.hpp
/usr/include/handlersocket/fatal.hpp
/usr/include/handlersocket/hstcpcli.hpp
/usr/include/handlersocket/mutex.hpp
/usr/include/handlersocket/socket.hpp
/usr/include/handlersocket/string_buffer.hpp
/usr/include/handlersocket/string_ref.hpp
/usr/include/handlersocket/string_util.hpp
/usr/include/handlersocket/thread.hpp
/usr/include/handlersocket/util.hpp