The builtin http.server module does not support HTTP keep-alive when sending a response without a content-length. This causes any clients to hang waiting on more response data, while the server hangs waiting for the client to send another request. This is documented (https://docs.python.org/3/library/http.server.html#http.server.BaseHTTPRequestHandler.protocol_version), but it is confusing.

As far as I can tell the fix would be pretty simple: If no content-length header is set then close the connection regardless of the keep-alive header (Keep-alive is advisory and servers can close the connection at-will, regardless of what the client sends).

If a response contains an inaccurate content-length header there is nothing we can do, but if none is present the server (and clients) should not just hang.