Crow does not return "404 Not Found\r\n" when calling an invalid route because the socket is closed before the response can be asynchronously written to it

[GitGud711]

This issue affects crow v1.2.0 and higher.

In #499 the HTTP parser code was changed so that a route is searched for as soon as the HTTP method and URL are parsed from the request. If no route is found, the parsing is terminated with error CHPE_INVALID_EOF_STATE, causing the crow::Connection<>::do_read() function to terminate the socket early because of the error state. Before the socket was terminated though, crow::Connection<>::handle_url() called crow::Connection<>::complete_request(), which prepared the response buffers and was supposed to write them to the socket asynchronously (do_write_general() -> do_write()), but it never did.

The 2 problems I see are:

• Not finding a route is treated as a parsing error, when it in fact shouldn't be. It shouldn't end parsing with CHPE_INVALID_EOF_STATE.
• crow::Connection<>'s close_connection_ is not set in crow::Connection<>::handle_url() before calling crow::Connection<>::complete_request() when parsing terminates early because the route was not found. This means that the socket is not closed after the response buffers are written to it.

Overall, finding a route should be treated as a success case, but not as CHPE_OK. My suggestion would be to add another error, something like CHPE_OK_ROUTE_NOT_FOUND or something similar, and handle this case where required.

I tested this on Ubuntu 20.04.6 LTS on a x64 VirtualBox VM, using boost 1.76 and crow v1.2.0. I initiated a crow::SimpleApp and set multithreaded() on it.
For the REST request and socket logic I used:

• boost::beast::http::request<boost::beast::http::string_body> and boost::beast::http::response<boost::beast::http::string_body>
• boost::asio::io_context and boost::asio::ip::tcp::socket to create the socket
• boost::beast::http::write and boost::beast::http::read to write to and read from the socket.

If required I can provide a code sample.

I tested a hacky fix on my setup where I added a new error to the list (CROW_XX(OK_ROUTE_NOT_FOUND, "route not found for request")), and set parser_.http_errno to the error number and close_connection_ to true inside handle_url(), but this is not a proper way to do it. I don't know where and how to set this error using its name, not its index.